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2010 mechanical ventilation

CHAPTER

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22

Mechanical Ventilation
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Dean R. Hess
Neil
MacIntyre
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Jones
& Bartlett Learning, LLC

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OUTLINE

proportional assist
patient–ventilator
ventilation (PAV)
asynchrony
The Equation of Motion
spontaneous breathing
peak inspiratory
Indications for Mechanical Ventilation
trial (SBT)
pressure (PIP)
Complications of Mechanical Ventilation
synchronized
permissive
hypercapnia
© Jones & Ventilator
BartlettSettings
Learning, LLC
© Jones
& Bartlett
Learning,
LLC
intermittent
plateau
pressure
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ORtheDISTRIBUTION
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Monitoring
Mechanically Ventilated Patient
ventilation (SIMV)
positive end-expiratory
Choosing Ventilator Settings for Different Forms of
transpulmonary pressure
pressure (PEEP)
Respiratory Failure
ventilator-induced lung
pressure control
Ventilatory Support Involves Trade-Offs
injury (VILI)
ventilation (PCV)
Liberation from Mechanical Ventilation
control
pressure support
© Jones & Bartlett Learning, LLC
©volume
Jones
& Bartlett Learning, LLC
ventilation
ventilation
(PSV)
NOT FOR (VCV)
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OBJECTIVES NOT FOR SALE OR DISTRIBUTION
weaning parameters
pressure triggering

List the indications for and complications of
mechanical ventilation.
2. Discuss issues related to ventilator-associated
injury.
© lung
Jones
& Bartlett Learning, LLC
3. Select appropriate ventilator settings.
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4. List parameters that should be monitored during
mechanical ventilation.
5. Discuss issues related to liberation from
mechanical ventilation.
1.

INTRODUCTION

Mechanical ventilation is an important life support
technology ©
that
is an integral
component
of critical
Jones
& Bartlett
Learning,
LLC
care. Mechanical
ventilation
can
be
applied
as
negaNOT FOR SALE OR DISTRIBUTION
tive pressure to the outside of the thorax (e.g., the
iron lung) or, most often, as positive pressure to the
airway. The desired effect of positive pressure ventilation is to maintain adequate levels of PaO2 and PaCO2
while also
unloadingLearning,
the inspiratory
muscles. Mechani© Jones & KEY
Bartlett
Learning, LLC
© Jones
& Bartlett
LLC
TERMS
cal ventilation is a life-sustaining technology, but recNOT FOR SALE
DISTRIBUTION
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high-frequency
adaptiveOR
pressure
ognition is growing that when used incorrectly, it can
oscillatory ventilation
control
increase morbidity and mortality. Positive pressure
(HFOV)
adaptive support
ventilation is provided in intensive care units (ICUs),
intermittent mandatory
ventilation (ASV)
subacute facilities, long-term care facilities, and the
ventilation
airway pressure release
home. Positive pressure ventilation
can be
invasive Learning, LLC
& Bartlett ventilator
Learning, LLC
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& Bartlett
lung-protective
ventilation (APRV) © Jones
(i.e., with an endotracheal tube or tracheostomy tube)
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strategy
auto-PEEP
or noninvasive (e.g., with a face mask). This chapter
mean airway pressure
compressible volume
addresses invasive positive pressure ventilation as

(Paw)
continuous mandatory
it is applied in adults with acute respiratory failure.
neurally adjusted
ventilation (CMV)
Modern ventilators used in the intensive care unit are
ventilatory assist
continuous positive
microprocessor
controlled
and available
from several
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& Bartlett
LLC
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& Bartlett
Learning,
LLC
(NAVA)
airway
pressure
(CPAP) Learning,
manufacturers
(
Figure 22–1 and Figure 22–2).
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flow triggering

oxygen toxicity

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462
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Mechanical Ventilation

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FIGURE
Examples
of mechanical
ventilators commonly used in critical care in theNOT
United States.
NOT22–1
FOR
SALE
OR DISTRIBUTION
FOR

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463

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464

CHAPTER 22

Mechanical Ventilation

Equation
of Motion
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Bartlett
Learning,
LLC
Positive
pressure,
when
applied
NOT FOR SALE OR DISTRIBUTIONat the air-

Expiratory valve

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Bartlett Learning, LLC
NOT
FOR
SALE
OR DISTRIBUTION
Atmosphere

way opening, interacts with respiratory
system (lung and chest wall) compliance,
airways resistance, respiratory system
inertance, and tissue resistance to produce
gas flow into the lung.
andBartlett
tissue Electrical
©Inertance
Jones &
Learning, LLCMicroprocessor
(mode and breath delivery)
resistance are smallNOT
and their
effects
areORpower
FOR SALE
DISTRIBUTION
usually ignored. The interactions of airway
pressure (Paw), respiratory muscle pressure (Pmus), flow, and volume with respiFilter
Air
ratory system mechanics can be expressed
as the
equation&
of Bartlett
motion: Learning, LLC
© Jones
© Jones &
O2

FOR
SALE
OR DISTRIBUTION
PawNOT
ϩ Pmus
ϭ (Flow
ϫ Resistance)
ϩ (Volume/Compliance)

InspiratoryNOT
valve(s) FOR
(flow, volume, pressure, FIO2)

Filter

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Monitors & Bartlett Learning, LLC
Patient
and alarms
NOT
FOR SALE OR DISTRIBUTION

Bartlett Learning, LLC
SALEHumidifier
OR DISTRIBUTION

FIGURE 22–2 Modern ventilators are electronically and pneumatically controlled. The

For spontaneous breathing, Paw ϭ 0 and inspiratory valves control flow, pressure, and FIO2 to the patient. The expiratory valve
all of the pressure required for ventilation is closed during the inspiratory phase and the inspiratory valve is closed during the
phase. The expiratory valve controls positive end-expiratory pressure (PEEP).
© Jones & isBartlett
LLCmuscles. For expiratory
© Jones & Bartlett Learning, LLC
provided Learning,
by the respiratory
The inspiratory and expiratory valves are controlled by the microprocessor. Sensors
full
ventilatory
support,
Pmus
ϭ
0
and
all
NOT FOR SALE OR DISTRIBUTION
NOTand
FOR
SALE
OR DISTRIBUTION
measure pressure
flow, which
are displayed
as numeric and graphic data and
of the pressure required for ventilation determine when an alarm condition is generated.
is provided by the ventilator. For partial
ventilatory support, both the ventilator
and the respiratory muscles contribute to
© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
ventilation.
BOX 22–1
For full ventilatory
support,
the
ventilator
controls
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either the pressure or the flow and volume applied
Indications for Mechanical
to the airway. The equation of motion predicts that
Ventilation
Paw will vary for a given resistance and compliance
Apnea
if flow and volume are controlled (volume-targeted
Acute
ventilatory
failure (e.g.,
© JonesThe
& Bartlett
© Jones
& Bartlett
Learning, LLC
ventilation).
equation ofLearning,
motion alsoLLC
predicts that
rising
Paco
with
acidosis,
2
flow
and
volume
will
vary
for
a
given
resistance
and
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respiratory muscle dysfunccompliance if Paw is controlled (pressure-targeted
tion, excessive ventilatory
ventilation).
load, altered central ventilaAn important point to remember in considering the
tory drive)
equation of motion is that in the setting of high minute
Impending
ventilatory LLC
failure
ventilation,
long
inspiratory-to-expiratory
time
ratios,
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Severe oxygenation deficit
and
prolonged
expiratory
time
constants
(e.g.,
as
seen
in
NOT FOR SALE OR DISTRIBUTION
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obstructive lung disease), the lungs may not return to the
baseline circuit pressure during exhalation. This creates
auto-PEEP, which must be counteracted by Pmus and
Paw in the equation of motion to affect flow and volume
from drug overdose or from©anesthesia
with
delivery.
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Jones &involved
Bartlett
Learning, LLC
major
surgery
is
an
indication
that
does
not
involve
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primary respiratory system failure. In short, mechanical
Indications for Mechanical
ventilation is required when the patient’s capabilities to
Ventilation
ventilate the lung and/or effect gas transport across the
Mechanical ventilation is indicated in many situations
alveolocapillary interface is compromised to the point
1
(Box
22–1
).
Goals
of
mechanical
ventilation
are
shown
that the patient’s
life is threatened.
© Jones & Bartlett Learning, LLC
© Jones
& Bartlett Learning, LLC
in NOT
Box 22–2
.
Although
these
conditions
are
useful
in
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the determination of whether mechanical ventilation
Complications of Mechanical
is needed, clinical judgment is as important as strict
Ventilation
adherence to absolute guidelines. One indication for
Mechanical ventilation is not a benign therapy, and
mechanical ventilation is imminent acute respiratory
it can &
have
major effects
on theLLC
body’s homeostasis
failure;
in
such
cases,
initiating
mechanical
ventilation
© Jones & Bartlett Learning, LLC
© Jones
Bartlett
Learning,
2
(
Box
22–3
).
In
addition
to
the
serious
may
prevent
overt
respiratory
failure
and
respiratory
NOT FOR SALE OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTIONcomplications
reviewed here associated with positive pressure applied
arrest. On the other hand, depression of respiratory drive

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Complications of Mechanical Ventilation
to the lungs,3 intubated mechanically ventilated patients

465

bleeding5 and often are given antacids, proton pump

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& Bartlett
Learning,
LLC to prevent this
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Bartlett
Learning,
LLC associated with the use
are at risk
for complications
inhibitors,
or histamine
(H2) blockers
4
NOT
FOR
SALE
OR
DISTRIBUTION
NOT FOR SALE
ORairways,
DISTRIBUTION
of artificial
the most serious being accidental
complication. The nutritional needs of mechanically
disconnection and the development of pneumonia from
compromised natural airway defenses. Mechanically
ventilated patients are also at risk for gastrointestinal

ventilated patients play an important role in preventing
or promoting complications.6 Undernourished patients
are at risk for respiratory muscle weakness and pneumonia. An excessive caloric intake, on the other hand, may
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© Jones & Bartlett Learning, LLC
increase carbon dioxide (CO2) production, which can
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FOR
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OR
DISTRIBUTION
NOT
FOR requirements.
SALE OR DISTRIBUTION
markedly increase the patient’s
ventilatory
BOX 22–2
Sleep deprivation in mechanically ventilated patients has
recently become recognized.7
Goals of Mechanical Ventilation
Provide adequate oxygenation
Ventilator-Induced
Injury Learning, LLC
Provide
adequate alveolar
© Jones
& Bartlett
Learning, LLC
© Jones Lung
& Bartlett
The application
positive
pressure
the airways can
ventilation
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SALE OR DISTRIBUTION
NOTofFOR
SALE
ORtoDISTRIBUTION
create lung injury under a variety of circumstances.
Avoid alveolar overdistension
Pulmonary barotrauma (e.g., subcutaneous emphyMaintain alveolar recruitment
sema, pneumothorax, pneumomediastinum) is one of
Promote patient–ventilator
the most serious complications of excessive pressure
synchrony
and volume
deliveryLearning,
to the lung and
is a consequence of
auto-PEEP
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Learning,
LLC
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& Bartlett
LLC
alveolar
overdistention
Use
the
lowest
possible
Fio
2
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RESPIRATORY RECAP
to the point of rupture
When choosing appropriate goals
( Figure 22–3 ). 3 Howof mechanical ventilation for
Types of Ventilator-Induced
Lung Injury
ever, even when the lung
an individual patient, consider
» Volutrauma
is not distended to the
the risk of ventilator-induced
» Atelectrauma
point of rupture, exceslung injury.
© Jones & Bartlett Learning, LLC
© Jones
& Bartlett Learning, LLC
» Biotrauma
sive
transpulmonar
y
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NOT
FOR toxicity
SALE OR DISTRIBUTION
» Oxygen
stretching pressures

22–3
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Bartlett
Learning, LLC
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Complications of Mechanical Ventilation
Airway Complications
Cardiovascular Complications
Laryngeal edema
Reduced venous return
Tracheal
mucosal
trauma
cardiac
output
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© JonesReduced
& Bartlett
Learning,
LLC
Contamination of the lower respiratory tract
Hypotension
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Loss of humidifying function of the upper
Gastrointestinal and Nutritional
airway
Complications
Mechanical Complications
Gastrointestinal bleeding
Accidental disconnection
Malnutrition
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Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
Leaks in the
ventilator&circuit
Renal
Complications
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FOR
SALE
OR
DISTRIBUTION
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Loss of electrical power
Reduced
urine
output
Loss of gas pressure
Increase in antidiuretic hormone (ADH) and
Pulmonary Complications
decrease in atrial natriuretic peptide (ANP)
Ventilator-induced lung injury
Neuromuscular Complications
Barotrauma
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& Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
Sleep deprivation
Oxygen
toxicity
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OR DISTRIBUTION
NOT
FOR SALE
OR DISTRIBUTION
Increased
intracranial
pressure
Atelectasis
Critical illness weakness
Nosocomial pneumonia
Inflammation
Acid–Base Complications
Auto-PEEP
Respiratory acidosis
Asynchrony
alkalosis
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© JonesRespiratory
& Bartlett
Learning, LLC

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466

CHAPTER 22

Mechanical Ventilation

beyond the normal maximum (i.e., 30 to 35 cm H O)

Importantly, this approach may require acceptance of

2
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& Bartlett
Learning,
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Bartlett
LLC
produceLearning,
a parenchymal
lung injury not associated
less than
normal values
for pH andLLC
Pao2 in exchange for
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FOR
SALE
OR
DISTRIBUTION
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OR DISTRIBUTION
with extra-alveolar
air (ventilator-induced lung injury
lower (and safer) distending pressures.

[VILI]).8 Importantly, it is the physical stretching and dis-

VILI also can result from the cyclical opening of an
tention of alveolar structures that causes the injury. This
alveolus during inhalation and closure during exhalation
concept has been demonstrated in numerous animal
(cyclical atelectasis producing atelectrauma).14,15 Indeed,
models in which limiting alveolar expansion (e.g., with
pressures at the junction between an open and a closed
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© Jones & Bartlett Learning, LLC
chest strapping) prevents lung injury even in the face of
alveolus may exceed 100 cm H2O during this process.16
8
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FOR
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OR
DISTRIBUTION
NOT
SALE
very high applied airway pressures.
This injury is reduced with the
useFOR
of smaller
tidalOR
vol-DISTRIBUTION
Clinical trials have confirmed these animal observaumes and may be ameliorated by optimal lung recruittions and indicate that ventilator strategies exposing
ment and an expiratory pressure that prevents alveolar
injured human lungs to transpulmonary pressures
derecruitment. Positive end-expiratory pressure (PEEP),
in excess of 30 to 35 cm H2O are associated with lung
however, can be a two-edged sword. If an increase in
© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
injury.9–12 Of note is that this injury may be more than
PEEP results in an increase in alveolar recruitment,
NOT
FOR
SALE
OR
DISTRIBUTION
NOT(distribution
FOR SALE
OR DISTRIBUTION
simply the result of excessive end-inspiratory alveothen the stress
of pressure)
in the lungs
lar stretch. Excessive tidal stretch (i.e., repetitive tidal
is reduced. If, on the other hand, an increase in PEEP
volumes greater than 9 mL/kg), even in the setting of
increases end-inspiratory transpulmonary pressure, then
maximal transpulmonary pressures less than 30 cm H2O,
the strain (change in size of the lungs during inflation)
may contribute to VILI.9,10,13 This provides the rationale
on the lungs is increased.17 Other ventilatory pattern
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Bartlett
Learning,
LLC
©
Jones
& Bartlett Learning, LLC
using lung-protective ventilator strategies that limit
factors may also be involved in the development of VILI.
NOT FOR SALE
OR DISTRIBUTION
NOTThese
FORinclude
SALEfrequency
OR DISTRIBUTION
tidal volume
and end-inspiratory distending pressures.
of stretch18 and the acceleration
19
or velocity of stretch. Vascular pressure elevations may
also contribute to VILI.20
VILI is manifest pathologically as diffuse alveolar
damage,7,8,15 and it increases inflammatory cytokines
© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
in the lungs (biotrauma).21–24 VILI is also associated
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OR DISTRIBUTION
with systemic cytokine release
andFOR
bacterial
transloca24
tion that are implicated in the systemic inflammatory
response with multiorgan dysfunction that increases
mortality. The way in which the lungs are ventilated
may therefore play a role in systemic inflammation
© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
(Figure 22–4).

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Oxygen Toxicity
Oxygen concentrations approaching 100% are known to
cause oxidant injuries in airways and lung parenchyma.25
Much &
of the
data supporting
the concept
© Jones
Bartlett
Learning,
LLC of oxygen toxicity, however, have come from animals that often have quite
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different tolerances to oxygen than humans. It is unclear
what the safe oxygen concentration or
duration of exposure is in sick humans,
MODS
such as those with acute lung injury
(ALI) or acute
respiratory
distress syn© Injury
Jones & Bartlett Learning, LLC
© Jones
& Bartlett
Learning, LLC
Biochemical
drome (ARDS). Many authorities have
Cytokines, complement,
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NOT
FOR
SALE
OR
DISTRIBUTION
Distal Organs
argued that a fraction of inspired oxygen
prostanoids, leukotrienes,
– Tissue injury secondary to
Bacteria
(Fio2) less than 0.4 is safe for prolonged
reactive oxygen species,
inflammatory mediators cells
proteases
periods of time and that a Fio2 greater
– Impaired oxygen delivery

Bacteremia
than 0.80 should be avoided. However,
Mechanical
VILI &
may
be moreLearning,
important cliniventilation
© Jones & Bartlett Learning,
LLC
© Jones
Bartlett
LLC
Neutrophils
cally
than
oxygen
toxicity.
In one large
Biophysical
NOT FOR
SALE Injury
OR DISTRIBUTION
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– Alveolar–capillary permeability
– Shear
study (ARDSnet), survival was greater
– Cardiac output
– Overdistention
in patients with ALI/ARDS who were
– Organ perfusion
– Cyclic stretch
ventilated with a lower tidal volume,
– Intrathoracic pressure
presumably avoiding significant VILI,
FIGURE 22–4 Mechanical ventilation can result in biochemical and biophysical injury to
despite
the fact LLC
that the required Fio2
© Jones & the
Bartlett
Learning,
LLC organ failure. MODS, multiple
© Jones
& Bartlett
Learning,
lungs, which
may result in multisystem
organ dysfunction
was
higher
in
the
group receiving the
syndrome.OR
Adapted
from Slutsky AS, Trembly L. Multiple system organ
failure:
is mechanical
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DISTRIBUTION
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FOR
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lower tidal volumes.
ventilation a contributing factor? Am J Respir Crit Care Med. 1998;157:1721–1725.
FIGURE 22–3 Computed tomography scan of the thorax
of a mechanically ventilated patient with severe barotrauma.
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Learning,
LLC pneumomediastinum,
Note the presence
of pneumothorax,
and subcutaneous
emphysema.
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Ventilator Settings

Ventilator-Associated Pneumonia

467

As intrathoracic pressure increases with positive

© Jones
& Bartlett
LLC filling decreases
© Jones & Bartlett Learning, LLC
pressure
ventilation,Learning,
right ventricular
The natural laryngeal mechanism that protects the lower
NOT
FOR
SALE
OR
DISTRIBUTION
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OR DISTRIBUTION
and cardiac output decreases. This is the rationale for
respiratory tract from aspiration is compromised by an

using volume repletion to maintain cardiac output in
endotracheal tube. This permits oropharyngeal debris
the setting of high intrathoracic pressure. The effect of
to leak into the airways. The endotracheal tube also
reduced cardiac filling on cardiac output may be parimpairs the cough reflex and serves as a potential portal
tially counteracted by better left ventricular function due
for pathogens to enter
lungs.&
The
underlying
disease LLC
© the
Jones
Bartlett
Learning,
© Jones & Bartlett Learning, LLC
to elevated intrathoracic pressures, which reduce left
process makes the lungs prone to infection. Finally, heavy
33
NOT FOR SALE OR DISTRIBUTION
NOTwith
FOR
OR DISTRIBUTION
ventricular afterload. In patients
leftSALE
heart failure,
antibiotic use in the ICU and the presence of very sick
the reduced cardiac fillpatients in close proximity to each other are risk factors
RESPIRATORY RECAP
ing and reduced left venfor antibiotic-resistant infection.
tricular afterload effects
Indications for and
Preventing ventilator-associated pneumonia (VAP)
Complications of Mechanical
of
elevated
intrathoracic
is important
it is associated
with
morbidity
© Jonesbecause
& Bartlett
Learning,
LLC
©
Jones
&
Bartlett
Learning, LLC
Ventilation
pressure may actually
and mortality.26 VAP prevention has become an impor»
Mechanical
ventilation
NOT FOR SALE OR DISTRIBUTION 26–29 improve cardiac
NOT FOR
SALE
OR
DISTRIBUTION
functant priority in the mechanically ventilated patient.
is indicated to support
tion such that intrathooxygenation and ventilation
Hand washing, elevating the head of the bed, and careracic pressure removal
of patients with acute
fully choosing antibiotic regimens can have important
respiratory failure.
may
produce
left
venpreventive effects. Circuit changes only when vis»
A number of complications
tricular
failure
if
positive
30
contaminated
appearLLC
to be helpful. Endotracheal
© Jones & ibly
Bartlett
Learning,
© Jones
&
Bartlett
Learning,
LLC
are possible with mechanical
pressure ventilation is
tubes that
continuous drainage of subglottic
ventilation, and efforts must
34
NOT FOR SALE
ORprovide
DISTRIBUTION
NOTremoved.
FOR SALE
OR DISTRIBUTION
secretions, endotracheal tubes with specialized cuff
be made to minimize these
Intrathoracic presconditions.
designs, and endotracheal tubes made with antimicrosure
can
also
influence
bial materials are other ways of reducing lung contamidistribution of perfusion, as described by the West model
nation with oropharyngeal material. However, these
of pulmonary perfusion. In the supine human lung,
tubes are more expensive
and their
cost-effectiveness
is
© Jones
& Bartlett
Learning,
LLC
© Jones & Bartlett Learning, LLC
blood flow is greatest in zone 3. As intra-alveolar pres31
controversial.
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FOR
OR DISTRIBUTION
sure rises, there is an increaseNOT
in zone
2 andSALE
zone 1 (dead
и)
space) regions, creating high ventilation-perfusion (Vи/Q

Auto-PEEP

units. Dyspnea, anxiety, and discomfort associated with
inadequate ventilatory support can lead to stress-related
catecholamine release, with increases in myocardial
© Jones & Bartlett Learning, LLC
oxygen demands and risk of dysrhythmias.34 In addition,
NOTvessel
FORoxygen
SALE
OR DISTRIBUTION
coronary blood
delivery
can be compromised by inadequate gas exchange from the lung injury
coupled with low mixed venous Po2 due to high oxygen
consumption demands by the inspiratory muscles.

Auto-PEEP (also known as intrinsic PEEP or air trapping)
is the result
of the lungsLearning,
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to the base© Jones
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line proximal airway pressure at end-exhalation. The
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determinants of auto-PEEP are high minute volume,
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intrathoracic
pressures,LLC
which can affect gas delivery,
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hemodynamics, end-inspiratory distention (and thus
NOT FOR SALE
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NOTVentilator
FOR SALE Settings
OR DISTRIBUTION
VILI), and patient breath triggering. Although someVolume Control Versus Pressure Control
times desired in long inspiratory time ventilatory strategies, auto-PEEP is generally to be avoided because it is
With volume control ventilation (VCV) , the ventiladifficult to recognize and to predict its effects.
tor controls the inspiratory flow (Figure 22–5). The
tidal
© Jones & Bartlett Learning, LLC volume is deter© Jones & Bartlett Learning, LLC
mined by the flow and
RESPIRATORY
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Hemodynamic Effects of Positive
the inspiratory time. In
Volume Control Versus
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practice, however, the
Pressure Control Ventilation
flow and tidal volume
Because positive pressure ventilation increases intratho» Volume control: Ventilation
are set on the ventilaracic pressure, it can reduce venous return, which may
remains constant with
in respiratory
tor. With VCV
the tidal
result
in
decreased
cardiac
output
and
a
drop
in
arterial
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mechanics, but airway
volume
is
delivere
d
blood
pressure.
Fluid
administration
and
drug
therapy
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and plateau pressures can
regardless of resistance
(such as with vasopressors and inotropes) may be necfluctuate.
or compliance, and the
essary to maintain cardiac output, blood pressure, and
» Pressure control: Ventilation
peak airway pressure
urine output under these circumstances. Mechanical
fluctuates with changes in
varies (Box 22–4). VCV
ventilation also can cause an increase in plasma antidirespiratory mechanics, but
pressure is limited to the
should
be
used
whenuretic
hormone
(ADH)
and
a
decrease
in
atrial
natri© Jones & Bartlett Learning, LLC
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peak pressure set on the
ever
a
constant
tidal
voluretic
peptide
(ANP),
which
may
reduce
urine
output
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ventilator.
32
ume is important in the
and promote fluid retention.

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468

CHAPTER 22

Mechanical Ventilation

maintenance of a desired Paco , such as with an acute

ramp. A descending ramp flow pattern produces a longer

overdistention in the lungs. Also, because the inspiratory
flow is fixed, VCV can cause patient–ventilator asynchrony, particularly if the inspiratory flow is set too low.
With VCV, the set flow can be constant or a descending

(Figure 22–6), the airway pressure is set and remains
constant despite changes in resistance and compliance.
Box 22–5 lists factors that affect the tidal volume with
PCV. The principal advantage of PCV is that it prevents

2
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The principalLLC
disadvantage of VCV is that
inspiratory
time unless
the peak flow
is increased.
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it can produce
a high peak alveolar pressure and areas of
With p r e s s u r e c o n t r o l v e n t i l a t i o n ( P C V )

Flow (L/min)

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0

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4

50

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6

8

10

12

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20
10
0

2

4

6

8

10

12

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600

Volume (mL)

400
200
0

Flow (L/min)

2
4
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8
Time (s)

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–40

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–80
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Pressure (cm H2O)

14

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800

Volume (mL)

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4

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40
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200
0
2

4

6

8

10

12

Time (s)

14

(B)
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FIGURE
22–5 (A) Constant-flow (square wave) NOT
volume control
Descending
ramp-flow
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Ventilator Settings
localized alveolar overdistention with changes in resis-

improve patient–ventilator synchrony.35,36 The choice

Because the flow can vary with PCV, this mode may

disadvantages (Table 22–1).37

469

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LLC
and compliance;
peak alveolar pressure canof VCV
or PCV often
is determined
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institutional bias, and both modes have advantages and

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BOX 22–4
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BOX 22–5
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Factors That Affect Tidal Volume (VT)
Factors That Affect Peak
with Pressure Control Ventilation
Inspiratory Pressure (PIP) with
Driving pressure: A higher driving presVolume Control Ventilation
sure (difference between peak inspiPeak&inspiratory
setting: A LLC
© Jones
Bartlettflow
Learning,
© ratory
Jones
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pressure and PEEP) increases
higher
flowOR
setting
increases
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the Vt.
the PIP.
Auto-PEEP: An increase in auto-PEEP
Inspiratory flow pattern: PIP is
reduces the Vt.
lower with descending ramp
Inspiratory time: An increase in inspiraflow.
tory time increases the Vt if inspiend-expiratory
© Jones & BartlettPositive
Learning,
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© Jones & Bartlett
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ratory flow is present; after flow
(PEEP):
An
increase
in
PEEP
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NOT FOR SALEdecreases
OR DISTRIBUTION
to zero, further increases in
increases the PIP.
the time do not affect the Vt.
Auto-PEEP: Auto-PEEP increases
Compliance: Decreased compliance
the PIP.
decreases the Vt.
Tidal volume (V): An increase in
Resistance: Increased resistance
Vt results
in a higher
PIP.
© Jones
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decreases the Vt; after flow decreases
Resistance:
Greater
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resisNOT FOR SALE OR DISTRIBUTION
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tance results in a higher PIP.
the delivered Vt.
Compliance: Decreased compliPatient effort: Greater inspiratory effort
ance results in a higher PIP.
by the patient increases the Vt.

Flow (L/min)

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40
0

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–40 LLC
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–80

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Pressure (cm H2O)

2

4

6

8

10

12

14

50

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20
10
0

2

4

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Volume (mL)

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6

8

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12

14

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400
200
0

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2

4

FIGURE 22–6 Pressure control ventilation.

10 Learning,
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Time (s)
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CHAPTER 22

470

Mechanical Ventilation
Pressure support ventilation (PSV) (Figure 22–9) is

Ventilator Mode

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a spontaneous
breathing
mode in LLC
which patient effort is
Options for breath delivery are referred to as modes
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augmented
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level of pressure
38–41
of ventilation.
Traditional modes include continu-

during inspiration.42 Although the clinician sets the level
ous mandatory ventilaof pressure support, the patient sets the respiratory rate,
tion (CMV), also called
RESPIRATORY RECAP
inspiratory flow, and inspiratory time. The Vt is deterassist/control (A/C),
Ventilator Modes
mined by the level of pressure support, the amount of
intermit- LLC
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tent mandator y venventilation (CMV)
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patient’s respiratory system. NOT FOR SALE OR DISTRIBUTION
tilation (SIMV ), and
» Synchronized intermittent
mandatory ventilation (SIMV)
pressure support venti» Pressure support ventilation
lation (PSV). The choice
TABLE 22–1 Advantages and Disadvantages of Volume
(PSV)
of mode often is based
Control
and Pressure Control Ventilation
» Continuous positive airway
on institutional policy
© Jones
© Jones
& Bartlett Learning,
LLC
pressure
(CPAP) & Bartlett Learning, LLC
Type
Advantages
Disadvantages
or the clinician’s bias.
» Adaptive
control OR DISTRIBUTION
NOTpressure
FOR SALE
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No one mode is clearly
(APC)
Volume
Constant tidal
Increased plateau
superior; each has its
control
volume (VT)
pressure (Pplat)
» Adaptive support ventilation
ventilation
with changes in
with decreasing
(ASV)
advantages and disadresistance and
compliance (alveolar
» Airway pressure release
vantages (Table 22–2).



compliance

overdistention)

clinicians

asynchrony

© Jones »& ventilation
Bartlett(APRV)
Learning, LLC Continuous manda© Jones & Bartlett
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Type ofLearning,
ventilation
Fixed inspiratory
Tube compensation (TC)
tory ventilation (CMV)
familiar
to most
flow may cause
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Proportional
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(or assist/control ven-

»
»

(PAV)
tilation) delivers a set
Neurally adjusted ventilatory
volume or pressure and
assist (NAVA)
a minimum respiratory
High-frequency oscillatory
ventilation (HFOV)
). The
(Figure 22–7
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Pressure
control
ventilation

Reduced risk of
Changes in VT with
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changes in resistance
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and compliance
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type of
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Variable flow
ventilation for most
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some patients

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Pressure (cm H2O)

Flow (L/min)

patient
canOR
trigger
addiNOT FOR
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tional breaths above the minimum rate, but the set
volume or pressure remains constant. When mechanical ventilation is begun, it often is best to use CMV
(assist/control) to produce nearly complete respiratory
rest&(i.e.,
full ventila©muscle
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tory
support).
Regardless
of
the
40
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20
mode used, the goal is to strike
0
a balance between excessive
–20
respiratory muscle rest, which
–40
promotes atrophy, and exces–60
2
4
6
8
10
12
14
respiratory
muscleLLC
activ© Jones & sive
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ity, which promotes fatigue—or,
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put more simply, to avoid the
20
extremes of too much rest and
16
too much exercise.
Patient-triggered
Ventilator-triggered
12
breath
Continuous positive airway
8
4
pressure (CPAP) is©a Jones
sponta-& Bartlett
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0
neous breathing mode ( Fig2
4
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ure  22–8). The airway pressure

breath

6

8

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OR

Volume (mL)

is usually but not necessarily
600
greater than atmospheric pres500
sure. CPAP is commonly used as
400
300
a means
of
maintaining
alveolar
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recruitment in mild to moderNOT FOR SALE OR DISTRIBUTION
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100
ate forms of pulmonary edema
0
and parenchymal lung injury.
2
4
6
8
10
12
14
CPAP often is used to evaluate a
Time (s)
patient’s ability to breathe spon- FIGURE 22–7 Continuous mandatory ventilation illustrating ventilator-triggered and patienttriggered breaths.
extubation.
& taneously
Bartlettbefore
Learning,
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Ventilator Settings
Pressure support ventilation is a frequently used

471

cycle to the expiratory phase without the need for active

© Jones
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Bartlett
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LLC However, because it is
of mechanical
ventilation.
exhalation.
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patient triggered,
PSV is not an appropriate mode for
The flow
at which
the ventilator cycles to the expirapatients who do not have an adequate respiratory drive.
tory phase during PSV can be a fixed absolute flow, a
PSV normally is flow cycled, with secondary cycling
flow based on the peak inspiratory flow, or a flow based
mechanisms of pressure and time. Although PSV often
on peak inspiratory flow and elapsed inspiratory time.
is considered a simple mode of ventilation, it can be
Several studies have reported asynchrony with PSV in
© Jones & Bartlett Learning, LLC
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quite complex (Figure 22–10). First, the ventilator must
individuals with airflow obstruction, such as chronic
43,44
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recognize the patient’s inspiratory effort, which depends
obstructive pulmonary disease
(COPD).
With airflow
on the ventilator’s trigger sensitivity and the amount
obstruction, the inspiratory flow decreases slowly during
of auto-PEEP. Second, the ventilator must deliver an
PSV, and the flow necessary to cycle may not be reached;
appropriate flow at the onset of inspiration. A flow that
this course of action stimulates active exhalation to presis too high can produce a pressure overshoot, and a flow
sure cycle the breath. The problem increases with higher
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that is too low can result in patient flow starvation and
levels of PSV and with higher levels of airflow obstrucNOT
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NOTventilators,
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asynchrony. Third, the ventilator must appropriately
tion. On newer
the OR
termination
flow can

■ TABLE 22–2

Advantages and Disadvantages of Common Modes of Mechanical Ventilation

© Jones & Bartlett Learning, LLC
Mode of Ventilation
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Continuous mandatory ventilation
(CMV)

© Jones & Bartlett Learning, LLC
Disadvantages
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Advantages

Guaranteed volume (or pressure) with each
breath
Low patient workload if sensitivity and
inspiratory flow set correctly

High mean airway pressure
Respiratory alkalosis and auto-PEEP if patient
triggers at rapid rate
Respiratory muscle atrophy possible

©mandatory
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mean airway
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ventilation (SIMV) NOT FOR SALE
Prevents respiratory
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Asynchrony if rate ©
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High work of breathing
withFOR
older ventilators
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Pressure support ventilation (PSV)

Requires spontaneous respiratory effort
Fatigue and tachypnea with PSV too low
Activation of expiratory muscles with PSV too high

Variable flow may improve synchrony in some
patients
Overcomes tube resistance
Prevents respiratory muscle atrophy

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Ventilator maintains tidal volume with changes
Does not precisely control tidal volume
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in respiratory system mechanics
Support
is taken
away if OR
the patient’s
tidal volume

Adaptive pressure control

Variable flow may improve synchrony in some
patients
Adaptive support ventilation (ASV)

consistently exceeds target

Ventilator adapts settings to patient’s
physiology

May not precisely control tidal volume

May improve ventilation to dependent lung
zones
May improve oxygenation in patients with ALI
or ARDS

Phigh–Plow difference
May be large transpulmonary pressure swings
during spontaneous breathing

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Airway pressure release ventilation
Allows spontaneous breathing at any time
May be uncomfortable for some patients
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(APRV)
during the ventilator cycle
May result in large tidal volumes, depending on

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through artificial airway
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Tube compensation (TC)

Proportional assist ventilation (PAV)

Pressure applied to the airway is determined
by respiratory drive and respiratory
mechanics

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DISTRIBUTION

Effect is usually small and may not affect patient
NOT FOR SALE OR
outcomes
Not useful with weak drive or weak respiratory
muscles
Clinician has little control over tidal volume or
respiratory rate

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Pressure applied to the airway is determined NOT
Requires
insertion
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tube to
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Neurally
NOTadjusted
FOR ventilatory
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OR
(NAVA)

by diaphragm activity

measure diaphragm EMG
Not useful with weak respiratory drive or motor
neuron disease

PEEP, positive end-expiratory pressure; Phigh, high airway pressure setting; Plow, pressure release level; ALI, acute lung injury; ARDS, acute
respiratory
distress
syndrome; EMG,
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472

CHAPTER 22

Mechanical Ventilation

Pressure (cm H2O)

Flow (L/min)

40
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30 LLC
20
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10

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NOT FOR SALE OR DISTRIBUTION

0
–10
–20
–30
–40
–50
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&2Bartlett4 Learning,
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6
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24

20
16
12
8
4
Bartlett
0

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Learning, LLC
2
4
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Volume (mL)

600
500
400
300
200
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0
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2

6

8

10

12

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14
NOT FOR SALE OR DISTRIBUTION

© Jones & Bartlett Learning, LLC
10 FOR12
14 DISTRIBUTION
NOT
SALE OR

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4

6

8

10

12

14

Time (s)

Flow (L/min)

FIGURE 22–8 Continuous positive airway pressure.

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60
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FOR SALE OR DISTRIBUTION

40
20
0
–20
–40
Bartlett
–60

Pressure (cm H2O)

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Learning, LLC
2
4
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Volume (mL)

24
20
16
12
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8 LLC
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0

Trigger
2

4

6

8

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© Jones & Bartlett Learning, LLC
10 FOR 12
14 DISTRIBUTION
NOT
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6

8

600
500
400
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300
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200
100
0
2
4
6
8

10

12

14

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10

12

14

Time (s)

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Bartlett
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22–9
Pressure support
ventilation.
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© Jones & Bartlett Learning, LLC
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be adjusted to a level appropriate for the patient
exceeds the termination flow at which the ventilator
cycles, either active exhalation occurs to terminate inspi(Figure 22–11).
ration,&
orBartlett
a prolonged
inspiratory LLC
time is applied. With a
Another
concern
with
PSV
is
leaks
in
the
system,
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leak,
either
PCV
or
a
ventilator
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allows an adjustable
such
as
with
a
bronchopleural
fistula,
uncuffed
airway,
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termination flow should be used. Another option is to
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Ventilator Settings
set a maximum inspiratory time during PSV such that

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C
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Proximal Airway Pressure (cm H2O)

473

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the breath
can be time
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This secondary cycle typically has been fixed at

1

a prolonged time to prevent untoward effects of long
inspiratory times. Some new ventilators allow both the
2
B
flow cycle and time cycle to be set.
15
2
The flow at the onset of the inspiratory phase may also
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be important during PCV or PSV. This is called rise time
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10
the set pressure at the onset of inspiration. Flows that are
too high or too low at the onset of inspiration can cause
A
asynchrony. Most ventilators allow adjustment of the rise
5
time during PSV (Figure 22–12). The rise time should be
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adjusted to the patient’s comfort, and ventilator graphNOT FOR SALE OR DISTRIBUTION
SALE
OR setting.
DISTRIBUTION
ics may be NOT
usefulFOR
as a guide
to this
However,
Time
a high inspiratory flow at the onset of inspiration may
not be beneficial.45 If the flow is higher at the onset of
FIGURE 22–10 Design characteristics of a pressure-supported
breath. In this example, baseline pressure (i.e., PEEP) is set at 5 cm
inspiration, the inspiratory phase may be prematurely
H2O and pressure support is set at 15 cm H2O (PIP 20 cm H2O).
terminated during PSV if the ventilator cycles to the
inspiratory Learning,
pressure is triggered
at point A by a patient effort
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expiratory phase at a flow that is a fraction of the peak
resulting in an airway pressure decrease. Demand valve sensitivity and
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NOTinspiratory
FOR SALE
flow.OR DISTRIBUTION
responsiveness are characterized by the depth and duration of this
Sleep fragmentation may be more likely during PSV
negative pressure. The rise to pressure (line B) is provided by a fixed
high initial flow delivery into the airway. Note that if flows exceed patient
than during CMV because there is no backup rate.46 Cendemand, initial pressure exceeds set level (B1), whereas if flows are less
tral apnea during PSV results in an alarm, which awakens
than patient demand, a very slow (concave) rise to pressure can occur
the patient. The pattern of awakening and breathing
(B2). The plateau of pressure
(line&
C) is
maintained by
servo
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with sleeping and apnea results in periodic breathing
control of flow. A smooth plateau reflects appropriate responsiveness
NOT
FOR
OR DISTRIBUTION
NOT FORofSALE
ORbeDISTRIBUTION
and sleep disruption. This complication
PSV can
to patient demand; fluctuations
would
reflectSALE
less responsiveness
of the
servo mechanisms. Termination of pressure support occurs at point D
addressed by switching to CMV or by using a lower
and should coincide with the end of the spontaneous inspiratory effort.
level of pressure support. With CMV, there is a miniIf termination is delayed, the patient actively exhales (bump in pressure
mum respiratory rate set. With a lower level of pressure
above plateau) (D1); if termination is premature, the patient will have
support, Paco2 will likely be greater, and the associated
continued
inspiratory
efforts
(D2).
Modified
from
MacIntyre
N,
et
al.
The
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respiratory drive will decrease the risk of apnea.
Nagoya conference on system design and patient-ventilator interactions
1

D

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during
pressure
support
ventilation.
Chest.
1990;97:1463–1466.

Flow (L/min)

10%
100
80
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60
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40
20
0
–20
–40
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–80

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25%

100
80
60
40
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Bartlett
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50%
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Pressure (cm H2O)

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20

20

20

15

15

15

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10
10
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5

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10
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5

0

5

0
1

2

0
1

2

Time (s) &
Jones

1

2

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©
Bartlett Learning, LLC
FIGURE 22–11
Effect of changing the flow termination criteria (cycle
off flow
as a percentage
of peak
flow) during pressure support
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474

CHAPTER 22

Mechanical Ventilation
Fast

Moderate
Slow
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80
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40
20
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Flow (L/min)

100
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40
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Pressure (cm H2O)

20

20

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Time (s)
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2

FIGURE 22–12 Effect of changing rise time during pressure support ventilation. Note the effect on peak flow.

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Volume (mL)

Pressure (cm H2O)

Flow (L/min)

set rate, and the spontaneous
NOTmay
FOR
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breaths
be pressure
sup-DISTRIBUTION
ported ( Figure  22–14 ). The
intent is to provide respiratory
muscle rest during mandatory
breaths and respiratory mus& Bartlett Learning, LLC
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exercise with the inter2
4
6
8
10
12
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vening OR
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However, it
24
has been shown that con20
siderable inspiratory effort
Spontaneous
Mandatory
16
breath
occurs with both the mandabreath
12
8
tory breaths and the interven© Jones & Bartlett
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4
ing spontaneous
breaths. As
0
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the
level
of
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support is
2
4
6
8
10
12
14
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400
(Figure 22–15).47 This effect
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with the
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0
which results in unloading of
2
4
6
8
10
12
14
both mandatory and spontaTime (s)
neous breaths.48
FIGURE 22–13 Synchronized intermittent mandatory ventilation illustrating spontaneous and
On newer ventilators, a
mandatory
breaths.
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feedback
mechanism
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exists.49,50 This is called adaptive pressure control. The
Synchronized intermittent mandatory ventilation
desired tidal volume is set on the ventilator, but the
(SIMV) ( Figure 22–13 ) provides mandatory breaths
breath type is actually pressure control or pressure
(VCV or PCV) that are interspersed with spontaneous
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BartlettThe
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LLC the inspiratory
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The ventilator
then adjusts
mandatoryLLC
breaths are delivered at the
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40
20
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guaranteed (General Electric).

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VolumeLLC
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Esophageal Airway
Pressure Volume
Pressure

Flow

Volume (mL)

Pressure (cm H2O)

breath type is only pressure
support.50
Because breath delivery
during these volume feedback
2
4
6
8
10
12
14
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modes is pressure controlled,
24
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NOT
FORwill
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volume
vary OR
withDISTRIBUTION
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changes in respiratory system
Pressure-support
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breath
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12
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4
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volume to change, the
2
4
6
8
10
12
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ventilator adjusts
the pres800
sure setting in an attempt
600
to restore the tidal volume.
However, it is important to
400
realize that providing a vol200
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ume guarantee negates the
0 OR DISTRIBUTION
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pressure-limiting feature of a
2
4
6
8
10
12
14
clinician-set pressure control
Time (s)
level (i.e., worsening respiraFIGURE 22–14 Synchronized intermittent mandatory ventilation with pressure support of
tory system mechanics will
spontaneous breaths.
increase the applied pressure).
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Another potential problem
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Spontaneous
Mandatory
Spontaneous
with these approaches is that
if the
patient’s
demand
breath
breath
breaths
increases and produces a larger tidal volume, the pressure level will diminish, a change that may not be appropriate for a patient in respiratory failure.
Airway pressure release ventilation (APRV) is a
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time-cycled, pressure-controlled mode of ventilatory
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DISTRIBUTION
support.51 ItNOT
is a modification
SIMV
with an active
exhalation valve that allows the patient to breathe spontaneously throughout the ventilator-imposed pressures
(with or without PSV). Because APRV is often used
with a long inspiratory-to-expiratory timing pattern,
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will occur during the
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available under a variety of proprietary trade names:
APRV (Dräger), BiLevel (Puritan Bennett), BiVent (Siemens), BiPhasic (Avea), PCVϩ (Dräger), and DuoPAP
Time
(Hamilton).50
FIGURE 22–15 Synchronized intermittent mandatory
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APRV uses different terminology
to describe
breath
ventilation. Note that the esophageal (i.e., pleural) pressure
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change for the mandatory breath is nearly as great as that
delivery phases. Lung inflation depends on the high DISTRIBUTION
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airway pressure setting (Phigh). The duration of this
inflation is termed Thigh. Oxygenation is thus heavily
influenced by Phigh, Thigh, and Fio2. The magnitude and
pressure to deliver the set minimal target tidal volume
duration of lung deflation is determined by the pres(Figure  22–16). If tidal volume increases, the machine
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sure release©level
(Plow)&
and
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time (Tlow).LLC
The
decreases the inspiratory pressure, and if tidal volume
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decreases, the machine increases the inspiratory preson lung compliance, airways resistance, and the durasure. This mode goes by the following names: pressure
tion and timing of this pressure release maneuver. The
regulated volume control (Maquet Servo-i), AutoFlow
timing and magnitude of this tidal volume coupled
(Dräger), adaptive pressure ventilation (Hamilton Galiwith the patient’s spontaneous breathing determine
leo), volume control plus (Puritan Bennett), and volume
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noted earlier, Thigh is
pressure control or pressure controlled volume

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Mechanical Ventilation

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0

40 seconds

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Flow (L/min)

Flow (L/min)

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CHAPTER 22
Pressure (cm H2O)

476

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1000

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Volume (mL)

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40 seconds
1000

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0
40 seconds

40 seconds

(A)

(B)

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LLC
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FIGURE 22–16
(A) Effect of
adaptive pressure control with a compliance
increase
or respiratory
effort increase.LLC
(B) Effect of adaptive
pressure
control
with
a
compliance
decrease
or
respiratory
effort
decrease.
From
Branson
RD,
Johannigman
JA. The role of ventilator
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graphics when setting dual-control modes. Respir Care. 2005;50:187–201. Reprinted with permission.

exchange, often with lower maximal set airway pressures
than CMV, has been demonstrated with APRV.51 How© Jones & Bartlett Learning, LLC
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ever, the end-inspiratory alveolar distention in APRV is
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Phigh
not necessarily less than that provided
during
other forms
of support, and it could be substantially higher, because
spontaneous tidal volumes can occur while the lung is fully
inflated with the APRV set pressure. Randomized controlled trials comparing APRV with other lung-protective
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strategies have shown no difference in outcome.52,53
Plow
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FORventilation
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AdaptiveNOT
support
(ASV)
automatically
selects tidal volume and frequency for mandatory breaths
Time
and the tidal volume for spontaneous breaths on the basis
of the respiratory system mechanics and target minute
Thigh
Tlow
ventilation. ASV delivers pressure-controlled breaths
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© Jones
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Bartlett
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22–17
Airway pressure
release ventilation.
using an
scheme,
in whichLLC
the mechanical work
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breathing is minimized. The ventilator selects a tidal
volume and frequency that the patient’s brain stem would
usually much greater than Tlow; thus, in the absence of
theoretically select. The ventilator calculates the required
spontaneous breathing, APRV is functionally the same
minute ventilation based on the patient’s ideal body
as pressure-controlled inverse ratio ventilation. To susweight and estimated dead space volume (2.2 mL/kg).
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tain optimal recruitment
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the greater
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The clinician sets a target percentage
venNOT
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the total time cycle (80% to 95%) usually occurs at Phigh,
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whereas in order to minimize derecruitment, the time
higher than 100% if the patient has increased ventilatory
spent at Plow is brief (0.2–0.8 second in adults). If Tlow is
requirements (e.g., because of sepsis or increased dead
too short, exhalation may be incomplete and intrinsic
space), or less than 100% during ventilator liberation.
PEEP may result.
The ventilator measures the expiratory time constant
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Spontaneous
during
APRV results
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determine NOT
an optimal
frequency
in terms of
of dependent alveoli, thus reducing shunt and improving
the work of breathing. The target tidal volume is calcuoxygenation. The spontaneous efforts also may enhance
lated as the minute ventilation divided by the frequency,
both recruitment and cardiac filling as compared with other
and the pressure limit is adjusted to achieve an average
controlled forms of support. The long inflation phase also
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more
slowly, fillingLLC
alveoli and raises mean airway
tor also adjusts the inspiration-to-expiration (I:E) ratio
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ASV has been shown to supply
Paw

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reasonable ventilatory support in a

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outcome stud-

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Ventilator Settings

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Proportional assist ventilation
(PAV) is a positive-feedback con-

Volume (L)

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ies in patients with acute respiratory
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0
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Tube compensation ( TC ) is
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75
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It measures the
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resistance of the artificial airway
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(e.g., 50% compensation rather than
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0

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Support

trol mode that provides ventilatory
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5 seconds
support in proportion
to the
neural
50
output of the respiratory center.
FIGURE 22–18 Proportional assist ventilation. From Marantz S, Patrick W, Webster K, et al.
The ventilator monitors respiratory Response of ventilator-dependent patients to different levels of proportional assist. J Appl Physiol.
drive as the inspiratory flow of the 1996;80:397–403. Reprinted with permission.
patient, integrates flow to volume,
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PAV/NAVA
measures elastance and resistance, and then calculates
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OR DISTRIBUTION
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theNOT
pressure
required
the equation of motion.
Using this calculated pressure and the tidal volume,
the ventilator calculates work of breathing (WoB):
WoB ϭ ∫P ϫ V. These calculations occur every 5 ms
APC/ASV
during breath delivery, and thus the applied pressure
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inspiratory
time varyLLC
breath by breath and within
PSV/PCV
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OR(Figure
DISTRIBUTION
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the breath
22–18 ). The ventilator estimates
resistance and elastance (or compliance) by applying
end-inspiratory and end-expiratory pause maneuvers of
300 ms every 4 to 10 seconds. The clinician adjusts the
percentage of support (from 5% to 95%), which allows
Volume
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the work to be partitioned
between
ventilator
and the
control
Effort/drive
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patient. Typically, the percentage of support is set so that
FIGURE 22–19 Effect of patient effort on the amount of support
the work of breathing is in the range of 0.5 to 1.0 joules
provided with various ventilator modes.
per liter. If the percentage of support is high, patient work
of breathing may be inappropriately low and excessive
volume and pressure may be applied (runaway phenom(Figure 22–19).62 The cycle criterion for PAV is flow and
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enon). If the percentage of support is too low, patient
is adjustable©byJones
the clinician,
similar to
pressure support
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work of breathing may be excessive.
ventilation. PAV requires the presence of an intact venPAV applies a pressure that will vary from breath to
tilatory drive and a functional neuromuscular system.
breath depending upon changes in the patient’s elasPAV is only available on one ventilator in the United
tance, resistance, and flow demand. This differs from
States (PAVϩ, Puritan Bennett 840) and cannot be
PSV or PCV, in which the level of applied pressure is
used with noninvasive ventilation because leaks prevent
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regardless
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accurate
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of respiratory
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478

CHAPTER 22

Mechanical Ventilation

and it may be associated with better patient–ventilator

Breath Triggering

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Learning,
LLC PAV improves clinical
sleep.64 Whether
pressure breaths can be either time triggered
NOTPositive
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outcomes
remains
to be determined.
(breaths delivered according to a clinician-set rate or

Neurally adjusted ventilatory assist (NAVA) is trigtimer) or patient triggered (breaths triggered by either a
gered, limited, and cycled by the electrical activity of the
change in circuit pressure or flow resulting from patient
diaphragm (diaphragmatic EMG). The neural drive is
effort). The patient effort required to trigger the ventitransformed into ventilatory output (neuro-ventilatory
lator is an imposed load for ©
theJones
patient.&Pressure
trig© Jones & Bartlett Learning, LLC
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coupling). The diaphragmatic EMG is measured by a
gering occurs because of a pressure drop in the system
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multiple-array esophageal electrode, which is ampli(Figure 22–20). The pressure level at which the ventilafied to determine the support level (NAVA gain). The
tor is triggered is set so that the trigger effort is minimal
cycle-off is commonly set at 80% of peak inspiratory
but auto-triggering is unlikely (typically this is 1 to 2 cm
activity. The level of assistance is adjusted in response to
H2O below the PEEP or CPAP). Flow triggering is an
changes in neural drive, respiratory system mechanics,
alternative
to pressure
triggering.
With
flow triggering
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inspiratory muscle function, and behavioral influences.
the ventilator responds to a change in flow rather than
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Because the trigger is based on diaphragmatic activity
a drop in pressure at the airway. With some ventilators,
rather than pressure or flow, triggering is not adversely
a pneumotachometer is placed between the ventilator
affected in patients with flow limitation and auto-PEEP.
circuit and the patient to measure inspiratory flow. In
NAVA is only available on the Servo-i ventilator. Small
other ventilators, a background or base flow and flow
clinical studies have demonstrated improved trigger and
sensitivity
are set. When
the flowLLC
in the expiratory cir© Jones & cycle
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synchrony with NAVA,65 but data demonstrating
cuit
decreases
by
the
amount
of
the
flow sensitivity, the
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improved
outcomes
are lacking. Another concern with
ventilator is triggered. For example, if the base flow is set
NAVA is the expense associated with the esophageal
at 10 L/min and
catheter and the invasive nature of its placement.
the flow sensitivRESPIRATORY RECAP
High-frequency oscillatory ventilation (HFOV) uses
ity
is
set
at

L/
Types of Ventilator Triggering
very high breathing frequencies66 (up to 900 breaths/min
min, the ventila©
Jones
&
Bartlett
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LLC
© Jones
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»
Ventilator
self-triggers
when Learning, LLC
in the adult) coupled with very small tidal volumes
tor
triggers
when
a
set
time
is
reached.
NOT inFOR
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OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
to provide gas exchange
the lungs.
literally
» Patient triggers the ventilator
the flow in the
vibrates a bias flow of gas delivered at the proximal end of
through changes in pressure
expiratory circuit
or flow.
the endotracheal tube and effects gas transport through
drops to 7 L/min
complex nonconvective gas transport mechanisms. At
(the assumption
the alveolar level, the substantial mean pressure funcis that the ©
patient
has &
inhaled
at 3  Learning,
L/min). Flow LLC
trig©
Jones
&
Bartlett
Learning,
LLC
Jones
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tions as high-level CPAP. The potential advantages to
gering
has
been
shown
to
reduce
the
work
of
breathNOT
SALE
ORvery
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
HFOV
areFOR
twofold.
First, the
small alveolar pressure
ing with CPAP.67 However, it may not be superior to
swings minimize overdistension and derecruitment. Secpressure triggering with pressure-supported breaths or
ond, the high mean airway pressure maintains alveolar
mandatory breaths.68 Neither pressure triggering nor
patency and prevents derecruitment. Experience with
flow triggering may be effective if significant auto-PEEP
HFOV in neonatal and pediatric respiratory failure is
is present.
Regardless
of whether LLC
pressure triggering or
© Jones & generally
Bartlettpositive,
Learning,
LLC in the adult is limited.
© Jones
& Bartlett
Learning,
but experience
flow triggering is used, the current generation of venNOT FOR SALE
DISTRIBUTION
NOTtilators
FOR is
SALE
OR DISTRIBUTION
Its use isOR
usually
reserved for refractory hypoxemic respimore responsive to patient effort, and differratory failure. Whether its use is associated with better
ences between pressure and flow triggering are minor.69
patient outcomes is yet to be determined.

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NOT FOR SALE OR DISTRIBUTION

© Jones & Bartlett Learning, LLC
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Flow

Flow
Flow
trigger

© Jones & Bartlett Learning, LLC
Beginning of
NOT FOR
SALE OR DISTRIBUTION
patient effort
Pressure

© Jones & Bartlett Learning, LLC
FOR SALE OR DISTRIBUTION

Beginning of
NOT
patient effort

Pressure

Pressure
trigger

© Jones & Bartlett Learning, LLCTime
(A) DISTRIBUTION
NOT FOR SALE OR

Time
© Jones & Bartlett Learning,
LLC
(B)
NOT FOR SALE OR DISTRIBUTION

FIGURE 22–20 (A) Pressure-triggered breath. (B) Flow-triggered breath.

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Ventilator Settings

479

to improve oxygenation,

Tidal Volume

RESPIRATORY
RECAP
© Jones
Bartlett
LLC
© Jones & Bartlett Learning, LLC
unless&
it is
coupled Learning,
with
Tidal volume is selected to provide an adequate Paco2
Settings for Tidal Volume,
NOTthe
FOR
SALE
OR DISTRIBUTION
NOT FOR SALE
OR DISTRIBUTION
ability
to spontanebut avoid alveolar overdistention, decreased cardiac
Respiratory Rate, and

ously breathe (see the
output, and auto-PEEP.70 Tidal volume is directly set
Inspiratory Time
discussion of APRV ear» Tidal volume: Set to avoid
in VCV but is determined by the driving pressure and
lier in this chapter), this
overdistention
inspiratory time in PCV and PSV. As noted earlier, large
extreme
(and
potentially
»
Respiratory rate: Set for
tidal volumes increase
mortality&inBartlett
patients with
ALI or LLC
© Jones
Learning,
© Jones
Bartlett
Learning, LLC
desired&partial
pressure
hazardous)
form
of
venARDS and increase the risk of developing ALI or ARDS
of
arterial
carbon
dioxide
NOT FOR SALE OR
DISTRIBUTION
NOT
FOR
SALE
OR
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tilation is seldom necesin patients with previously normal lungs.10,71 A tidal
(Pa CO 2)
sary to achieve adequate
volume should be chosen that maintains plateau pres» Inspiratory time: Set to avoid
oxygenation.
auto-PEEP and hemodynamic
sure (Pplat) below 30 cm H2O (assuming a near-normal
The I:E ratio is the
compromise
chest wall compliance), or perhaps higher if chest wall
relationship
between
compliance
is &
severely
reduced
(e.g., morbid
© Jones
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Learning,
LLC obesity, inspiratory ©
Jones & Bartlett Learning, LLC
time and expiratory time. For example, an
anasarca, ascites). Tidal volume should be selected
NOT FOR SALE OR DISTRIBUTION
NOT
SALE
DISTRIBUTION
inspiratory time ofFOR
2 seconds
withOR
an expiratory
time of
based on predicted body weight (PBW), which is deter4
seconds
produces
an
I:E
ratio
of
1:2
and
a
respiratory
mined by height and sex:
rate of 10 breaths/min. With VCV, the peak inspiratory
Male patients: PBW ϭ 50 ϩ 2.3 ϫ
flow, flow pattern, and tidal volume are the principal
[Height (inches) Ϫ 60]
determinants of inspiratory time and the I:E ratio.
© Jones & Bartlett Learning, LLC
© Jones
& Bartlett Learning, LLC
With PCV, the inspiratory time, I:E ratio, or percentFemale
patients:
PBW
ϭ
45.5
ϩ
2.3
ϫ
NOT FOR SALE OR DISTRIBUTION
NOTage
FOR
SALE time
OR DISTRIBUTION
inspiratory
are set directly. In both VCV and
[Height (inches) Ϫ 60]
PCV, the principal determinant of expiratory time is
A reasonable starting point for most patients with respithe respiratory rate.
ratory failure is 6 mL/kg PBW.

Inspiratory Flow Pattern© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
For VCV, the inspiratory flowNOT
pattern
can be
constant
NOT FOR SALE OR DISTRIBUTION
FOR
SALE
ORorDISTRIBUTION

Respiratory Rate

A respiratory rate is chosen to provide an acceptable
descending ramp. For the same inspiratory time, the PIP
minute ventilation, as follows:
is greater with constant flow than with descending ramp

и
flow; the P aw is greater with ramp flow than with conVe ϭ Vt ϫ f
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tilation,
and
Vt
is
the
tidal
volume.
A
rate
of
15
to
beginning
of
inspiration,
patient–
ventilator
synchrony
NOT FOR SALE OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
25  breaths/min is used when mechanical ventilation
may be better with a descending ramp flow pattern.
is initiated. If a smaller tidal volume is selected to preAlthough the choice of flow pattern often is based on
vent alveolar overdistention, a higher respiratory rate
clinician bias or the capabilities of a specific ventilator,
may be required (25 to 35 breaths/min). The respiradescending ramp flow may be desirable compared with
tory
rate
may
be
limited
by
the
development
of
autoother &
inspiratory
patterns.LLC
An end-inspiratory
© Jones & Bartlett Learning, LLC
© Jones
Bartlett flow
Learning,
PEEP. The minute ventilation that produces a normal
pause can be set to improve distribution of ventilation,
NOT FOR SALE
OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
Paco2 without risk for lung injury or auto-PEEP may
but this prolongs inspiration and may have a deleterious
not be possible, and the Paco 2 thus is allowed to
effect on hemodynamics and auto-PEEP.
increase (permissive hypercapnia).
The inspiratory flow decreases exponentially with
PCV and PSV. The peak flow and rate of flow decrease
depend on the driving pressure,
airways resistance,
lung
Inspiratory Time © Jones & Bartlett Learning, LLC
© Jones
& Bartlett
Learning, LLC
compliance, and patient effort. With high resistance,
For patient-triggeredNOT
mandatory
inspiratory
FORbreaths,
SALEthe
OR
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
flow decreases slowly. With a low compliance and long
time should be short (1.5 seconds or less) to improve
inspiratory time, flow decreases more rapidly, and a
ventilator–patient synchrony. A shorter inspiratory time
period of zero flow may be present at end-inhalation
requires a higher inspiratory flow, which increases the
(Figure 22–21).
peak inspiratory pressure (PIP) but does not greatly
© Jones
Bartlettthe
Learning,
LLCincreases
© Jones & Bartlett Learning, LLC
affect
the Pplat.&Increasing
inspiratory time
Positive End-Expiratory
Pressure
SALE
OR(P–DISTRIBUTION
NOT FOR SALE
OR DISTRIBUTION
theNOT
meanFOR
airway
pressure
aw), which may improve
Because critical care patients are often immobile and
oxygenation in some patients with ARDS. When long
supine, with compromised cough ability, it is common
inspiratory times are used (over 1.5 seconds) and sponto use low-level PEEP (3 to 5 cm H2O) with all mechanitaneous breaths are not permitted, paralysis or sedation
cally ventilated patients to prevent atelectasis. In patients
(or both) often is required. Long inspiratory times also
© Jones & can
Bartlett
Learning,
© Jones
& Bartlett
Learning,
LLC
with ALI
or ARDS, more
substantial
levels of PEEP may
cause auto-PEEP
andLLC
may result in hemodynamic

NOT FOR SALE
OR
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NOTbeFOR
SALE
OR DISTRIBUTION
required
to maintain
alveolar recruitment. An approinstability
because
of the elevated Paw or the auto-PEEP.
priate PEEP level to maintain alveolar recruitment is
Although inverse ratio ventilation has been advocated
© Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION.


480

CHAPTER 22

Mechanical Ventilation

150

50

Flow (L/min)

Flow (L/min)

© Jones & Bartlett Learning, LLC
NOT FOR100
SALE OR DISTRIBUTION
0

Other uses of PEEP include preload and after-

150

© Jones &
Bartlett
Learning,
LLC
load
reduction
in the setting
of left heart failure,
NOT FORpneumatic
SALE OR
DISTRIBUTION
splinting in the setting of airway mala-

100

cia, and facilitation of leak speech with cuff deflation in patients with a tracheostomy.76

50
0

© Jones & Bartlett
Learning, LLC Mean Airway Pressure
© Jones & Bartlett Learning, LLC
–50
Across all modes, oxygenation
and cardiac
NOT FOR SALE
OR
DISTRIBUTION
NOT FOR
SALEeffects
OR DISTRIBUTION
–100

–50
–100
–150

–150
Time

Time

(A)

(B)
© Jones & Bartlett Learning,
LLC
FIGURE 22–21
waveforms
during
pressure
control ventilation: low resistance
NOTFlow
FOR
SALE
OR
DISTRIBUTION

and low compliance (A), and high resistance and high compliance (B).

© Jones & Bartlett Learning, LLC
NOT FOR SALE OR DISTRIBUTION
Sensitivity

PEEP
7 cm H2O

of mechanical ventilation often correlate best with


the mean airway pressure (P aw). Indeed, P aw is
a key component of the oxygenation index (OI ϭ

100 ϫ [P aw ϫ Fio2]/Pao2) that often is used
as a ©
more
accurate
reflectionLearning,
of gas transport
Jones
& Bartlett
LLC

impairment.
Factors
that
affect
the P aw during
NOT FOR SALE OR DISTRIBUTION
mechanical ventilation are the PIP, PEEP, I:E ratio,
respiratory rate, and inspiratory flow pattern.
Most patients can be managed with mean P values
less than 15 to 20 cm H2O.

© Jones & Bartlett Learning, LLC
Maneuvers
NOT FORRecruitment
SALE OR DISTRIBUTION
Sensitivity

A recruitment maneuver (RM) is an intentional transient increase in transpulmonary pressure to promote
reopening of unstable collapsed alveoli and thereby
improve gas exchange.77 However, although use of
© Jones & Bartlett Learning, LLC the maneuver is physiologically
© Jonesreasonable,
& Bartlett
Learning, LLC
there
PEEP
PEEP
NOT FOR SALE
OR
NOT
FOR SALE
OR DISTRIBUTION
have been no randomized
controlled
trials demon10 cm
H2O DISTRIBUTION
10 cm H2O
strating an outcome benefit from this improvement
in gas exchange. RMs are probably best reserved for
Trigger effort = 4 cm H2O
Trigger effort = 11 cm H2O
the setting of refractory hypoxemia in patients with
FIGURE 22–22 Trigger effort is increased when auto-PEEP is present. To trigger
ARDS.78 A variety of techniques have been described
the ventilator,
patient’s &
effort
must first overcome
the level
of auto-PEEP that is
©the
Jones
Bartlett
Learning,
LLC
© Jones
& Bartlett
Learning,
LLC
as recruitment
maneuvers
(Table 22–3
). It is uncerpresent. Increasing the set PEEP level may raise the trigger level closer to the total
NOT FOR SALE OR DISTRIBUTION
NOT FOR
SALE
OR isDISTRIBUTION
tain whether
any one
approach
superior to the
PEEP, thus improving the ability of the patient to trigger the ventilator. However, this
others.
After
performing
an
RM,
it
is important to
method should not be used if raising the set PEEP level results in an increase in the
total PEEP.
set PEEP to a level that retains recruitment. If the
lungs are already maximally recruited as the result
of PEEP, the benefits of an RM are likely minimal.
also part of a lung-protective
© Jones RESPIRATORY
& Bartlett Learning,
LLC
©
Jones
&
Bartlett Learning, LLC
RECAP
strategy. PEEP should be
NOT FORUses
SALE
OR
DISTRIBUTION
NOT
FOR
SALE
OR DISTRIBUTION
of Positive Endused cautiously in patients
TABLE 22–3 Different Lung Recruitment Maneuvers
Expiratory Pressure
with unilateral disease,
» Maintain alveolar
Recruitment
because it may overdistend
recruitment
Maneuver
Method
the more compliant lung,
» Counterbalance
Sustained highSustained inflation
delivered&
by Bartlett
increasing Learning, LLC
causing
shunting ofLearning,
blood to
© Jones
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© Jones
auto-PEEP
pressure inflation
PEEP to 30–50 cm H2O for 20–40 seconds
» Reduce preload and NOT FOR
the lessSALE
compliant
ORlung.
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
afterload
PEEP also may be useIntermittent sigh
Periodic sighs with a tidal volume reaching
» Pneumatic splinting of the ful to improve triggering by
Pplat of 45 cm H2O
airway
patients experiencing auto» Facilitation of leak speech
Extended sigh
Stepwise increase in PEEP by 5 cm H2O
PEEP.72–75 Auto-PEEP funcwith a simultaneous stepwise decrease in
© Jones & Bartletttions
Learning,
LLCpressure
© Jones
& Bartlett Learning, LLC
as a threshold
tidal volume over 2 minutes leading to a
that
mustFOR
be overcome
before
the pressure (or flow)
CPAP level
of 30 cm
H2ODISTRIBUTION
for 30 seconds
NOT
SALE OR
DISTRIBUTION
NOT FOR
SALE
OR
decreases at the airway to trigger the ventilator. IncreasIntermittent PEEP
Intermittent increase in PEEP from baseline
ing the set PEEP to a level near the auto-PEEP may
increase
to higher level
improve the patient’s ability to trigger the ventilator
(Figure 22–22). Whenever PEEP is used to overcome the
Pressure control
Pressure control ventilation of 10–15 cm
ϩ PEEP
© Jones & effect
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LLC PIP and Pplat must be
© Jones
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LLCcm H2O to reach a
of auto-PEEP
on triggering,
2O with PEEP of 25–30
peak
inspiratory
pressure
of 40–45 cm H2O
monitored
ensure that increasing the set PEEP does
NOT FOR SALE
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NOT FOR SALE OR
DISTRIBUTION
for 2 minutes
not contribute to further hyperinflation.
Auto- PEEP
10 cm H2O

–1 cm H2O

Auto- PEEP
3 cm H2O

–1 cm H2O



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Ventilator Settings

481

Importantly, an RM can produce injury in the form of

© Jones &hemodynamic
Bartlett Learning,
compromiseLLC
and barotrauma.
NOT FOR SALE OR DISTRIBUTION

Volume
© Jones & Bartlett
Learning,
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Expired gas
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Inspired Oxygen Concentration

Volume

Volume
An Fio2 of 1.0 is commonly used when mechanical ventilation is initiated. Pulse oximetry (Spo2) is useful to guide
Inspired &
gasBartlett Learning, LLC
titration of the Fio2 (and
PEEP) provided
periodic
blood LLC
© Jones
& Bartlett
Learning,
© Jones
Patient
gas measurements NOT
are obtained
to
confirm
the
pulse
FOR SALE OR DISTRIBUTION
NOT FOR SALE
OR DISTRIBUTION
oximetry results.
RESPIRATORY RECAP
A target Spo 2 of
88% or higher
Setting the Fractional
Ventilator
Humidifer
Inspired Oxygen
usually provides
FIGURE 22–23 The ability to detect a leak depends on the site
a partial
©Concentration
& Bartlett Learning,
LLCpressure where volume ©
JonesIf&
Learning,
is measured.
theBartlett
volume on the
inspiratory limbLLC
is
» Jones
Initiate mechanical ventilation
greater than the volume on the expiratory limb, then there is a leak
of
arterial
oxygen
NOT
NOT
FOR
SALE
OR
DISTRIBUTION
withFOR
100% SALE
oxygen. OR DISTRIBUTION
in the system (circuit or patient). If the inspired volume at the patient
(Pao2) of 60  mm
» Titrate the F IO 2 to maintain an
is greater than the expired volume at the patient, there is a leak
H
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acceptable arterial oxygen
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pulse oximetry.
common practice
to wait 20 to 30
© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
minutes
after
the
Fio
is
changed
before
arterial
blood
2
NOT FOR SALE OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
400 mL is delivered to the patient. For patients ventilated
gas measurements are obtained, 10  minutes may be
with a small tidal volume, the compressible gas volume
adequate unless the patient has obstructive lung disease,
can greatly affect alveolar ventilation. Some ventilators
which requires a longer equilibration time.79
adjust for the effects of compressible volume such that
the volume chosen by the clinician
is the actual
delivered
©
Jones
&
Bartlett
Learning,
LLC
© Jones
& Bartlett
Learning, LLC
Sigh
Vt after correction for the effect of compressible volume.
FOR
SALE OR
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
Some ventilators areNOT
capable
of providing
periodic
sigh
The effects of compressible volume on the delivered
volumes.80 The rationale for use of sighs is that the
Vt, auto-PEEP, plateau pressure, and mixed exhaled
periodic hyperinflation reduces the risk of atelectasis.
–co ) are shown in
partial pressure of carbon dioxide (Pe
2
Indeed, a sigh is actually a very brief RM. For many
Equation 22–1.
years the use of sighs during mechanical ventilation was
The mechanical
dead&space
of the Learning,
circuit shouldLLC
also
Jones &important.
Bartlett However,
Learning,
LLC
© Jones
Bartlett
not©considered
several
studies of
be considered. Mechanical dead space is that part of the
NOT with
FORARDS
SALE
OR
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
patients
have
reported
improved alveolar
ventilator circuit through which the patient rebreathes
recruitment with the use of sighs.81,82
and thus becomes an extension of the patient’s anatomic
dead space. Alveolar ventilation is zero if the sum of the
Alarms
volume loss in the circuit and the mechanical dead space
It is particularly important that all alarms be correctly
is greater
than the Vt
set on the ventilator.
© Jones & Bartlett Learning, LLC
© Jones
& Bartlett
Learning,
LLC
set on the ventilator. The most important alarm is the
NOT FOR SALE
OR
DISTRIBUTION
NOT
FOR
SALE
OR
DISTRIBUTION
patient-disconnect alarm, which can be a low presHumidification
sure alarm or a low exhaled volume alarm (or both). A
Because the function of the upper airway is bypassed
sensitive alarm should detect not only disconnection
when endotracheal and tracheostomy tubes are used,
but also leaks in the system. The ability to detect a leak
the inspired gas must be filtered, warmed, and humididepends on the site where
the volume
is measured
(Fig© Jones
& Bartlett
Learning,
LLC
© Jones
& Bartlett
Learning, LLC
fied before delivery to the patient.
All ventilator
circuits
ure 22–23). Other alarms set on the ventilator include
include a filter in the inspiratory
limb
andSALE
an active
or DISTRIBUTION
NOT
FOR
SALE
OR
DISTRIBUTION
NOT
FOR
OR
those for high pressure, I:E ratio, Fio2, and loss of PEEP.
passive humidifier. An active humidifier typically humidTo detect changes in resistance and compliance, the peak
ifies the inspired gas by passing it over or bubbling it
airway pressure alarm is important with VCV, and the
through a heated water bath. When an active humidifier
low exhaled volume alarm with PCV or PSV.
is used, the ventilator circuit may be heated to prevent
© Jones & Bartlett Learning, LLC
© Jones &
LLC
excessive condensation
in Bartlett
the circuit.Learning,
A passive humidiCircuit
fier uses anNOT
artificial
noseSALE
(heat and
moisture
exchanger)
NOT FOR SALE OR DISTRIBUTION
FOR
OR
DISTRIBUTION
Because of the gas compression in the ventilator circuit
to collect heat and humidity from the patient’s exhaled
and the compliance of the ventilator circuit tubing, as
gas and returns that to the patient on the next inhalamuch as 3 to 5 mL/cm H2O can be compressed in the
tion. Regardless of the humidification technique used,
ventilator circuit. In other words, at an airway prescondensation should be seen in the inspiratory ventila© Jones & sure
Bartlett
Learning,
LLC
© Jones
& Bartlett
Learning,
LLC tube or both,
of 25 cm
H2O above
PEEP, about 100 mL of the
tor circuit
or the proximal
endotracheal
gas delivered
from the ventilator is not delivered to the
that DISTRIBUTION
the inspired gas is fully saturated
NOT FOR SALE
OR DISTRIBUTION
NOTwhich
FORindicates
SALE OR
patient. If the ventilator is set to deliver 500 mL, only
with water vapor.

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482

CHAPTER 22

Mechanical Ventilation

22–1
© Jones & BartlettEQUATION
Learning, LLC
NOT FOR SALE OR DISTRIBUTION

Effects of Compressible Volume
The effect of compressible volume on
the delivered tidal volume (Vt) can be
expressed as follows:
VTpt =

Monitoring
Mechanically
© Jones
& Bartlettthe
Learning,
LLC
Patient
NOTVentilated
FOR SALE OR
DISTRIBUTION
It is important to monitor the function of the mechanical
ventilator frequently, including checking the ventilator
settings and alarm systems, the humidifier and circuitry,
and the patient’s airway.

© Jones & Bartlett Learning, LLC
OR DISTRIBUTION
Physical Assessment

1
×V
Tvent SALE
NOT
FOR
1 + (Cpc/Crs)

© Jones & Bartlett Learning, LLC
NOT FOR SALE OR DISTRIBUTION

Asymmetric chest motion may indic ate m a i n s te m
(endobronchial) intubation, pneumothorax, or atelectasis. Paradoxical chest motion may be seen with flail
chest or respiratory
dysfunction.
Retractions
© Jones & Bartlett Learning, LLC
© Jonesmuscle
& Bartlett
Learning,
LLC
may
occur
if
the
inspiratory
flow
or
sensitivity
is inapCpc
ϭ Compliance
of the
ventilator
circuit
NOT
FOR SALE
OR
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
propriately set or
Crs ϭ Compliance of the respiratory
RESPIRATORY RECAP
if the airway is
system
obstructed. If
Methods of Humidification
the
patient
is
not
with Mechanical Ventilation
Vtvent ϭ Tidal volume from the ventilator
»
ActiveLLC
humidification: Heated
breathing
in
syn© Jones & Bartlett
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LLC
©
Jones
&
Bartlett
Learning,
circuit
humidifier
chrony with the
NOT FOR SALE
DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
» Passive humidification:
TheOR
effect
of compressible volume on
ventilator (i.e., is
Artificial nose
auto-PEEP (positive end-expiratory presbucking the venti»
The presence of condensate
sure) can be expressed as follows:
lator), the settings
in the inspiratory circuit
on the ventilator
near the patient indicates
Crs + Cpc
Auto-PEEP =
× Measured
Pplat Learning, LLC
adequate
humidification.
may not be appro©
Jones
&
Bartlett
© Jones
& Bartlett Learning, LLC
Crs
priate or the paNOT FOR SALE OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
tient may need sedation or analgesia or both. A patient
where auto-PEEP is the patient’s actual
respiratory rate greater than the trigger rate on the
auto-PEEP (positive end-expiratory
ventilator may indicate the presence of auto-PEEP compressure).
promising triggering. In conjunction with inspection,
The effect of compressible volume
the chest can
be palpated
to assessLearning,
the symmetry
of
on
Pplat (plateau
pressure)
can be LLC
©the
Jones
& Bartlett
Learning,
© Jones
& Bartlett
LLC
chest movement. Palpation of the tracheal position can
expressed
as
follows:
NOT FOR SALE OR DISTRIBUTION
NOT FOR SALE OR DISTRIBUTION
help detect pneumothorax. Crepitation indicates subCrs + Cpc
Pplat =
× Measured auto-PEEP
cutaneous emphysema. Percussion can be useful in the
Crs
detection of unilateral hyperresonance or tympany with
where Pplat is the patient’s actual plateau
a pneumothorax. Unilateral decreased breath sounds
pressure.
may indicate
bronchial
intubation,LLC
pneumothorax, atel© Jones & Bartlett Learning, LLC
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The effect of compressible volume on
ectasis, or pleural effusion. An end-inspiratory squeak
NOT FOR SALE
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–co ) can be expressed as
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Vtpt ϭ Tidal volume delivered to the
patient

VTvent
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–co
Pe
ϫJones
2
2(vent)©
VTpt

Blood Gas Measurements

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The earliest indicators of hypoxemia
often
are changes
in the patient’s clinical statusNOT
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and OR
con- DISTRIBUTION
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FOR SALE

where:
–co ϭ Patient’s actual Pe
–co
Pe
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2


Pe co
ϭ Pe co from the ventilator
2(vent)

2

circuit

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Vtvent
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volumeOR
fromDISTRIBUTION
the ventilator
NOT ϭ
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circuit
Vtpt ϭ Tidal volume delivered to the
patient

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fusion, changes in level of consciousness, tachycardia
or bradycardia, changes in blood pressure, tachypnea,
bucking the venRESPIRATORY RECAP
tilator, c y anoJones & Monitoring
Bartlett Required
Learning,
LLC
sis). The ©most
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commonlyNOT
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» Physical examination
assessment of
» Blood gas measurements
oxygenation is the
» Lung mechanics
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arterial oxygen.
» Patient–ventilator
synchrony
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A low&Pao
2 indi»
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PIP
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Resistance
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PaO2
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FIO2

483

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Pplat

Lung
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Paw

Compliance
tidal volume

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PEEP
ratio

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PEEP

Total-PEEP

Auto-PEEP

FIGURE 22–24 Factors affecting PaO2 during
mechanical ventilation.

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PaCO2
VCO2

VA = VE – VD

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VT
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FIGURE 22–26 Airway pressure waveform during volume
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An end-inspiratory
and an
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plateau
and
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determined by the tidal volume setting on the ventilator and
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the &
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(including auto-PEEP).

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f

has become the standard of care in mechanically ventilated patients. End-tidal Pco2 is used to monitor carbon
dioxide levels noninvasively. In patients with normal
FIGURE 22–25 Factors affecting PaCO2
lungs, end-tidal Pco2 closely©approximates
the PacoLearning,
2.
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during mechanical
ventilation. & Bartlett Learning, LLC
However, in patients with an elevated Vd/Vt, there can
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be a large and inconsistent gradient between the Paco2
and the end-tidal Pco2. For this reason, monitoring endand a dysfunction in the lungs’ ability to oxygenate artetidal Pco2 is of limited value for the assessment of the
rial blood. The Pao2 must always be interpreted in relaPaco2 during mechanical ventilation. End-tidal Pco2 is
tion to the Fio2 (and often the mean airway pressure). In
useful
to differentiate
intubation
from esopha©
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&
Bartlett
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LLC
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mechanically ventilated patients, a number of factors can
geal
intubation.
affect the Pao , such as a change in the Fio , the PEEP
Tl

TE

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2

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level, or the patient’s lung function (Figure 22–24).
–o or Sv
–o ) is
Lung Mechanics
The mixed venous oxygenation (Pv
2
2

a better indicator of tissue oxygenation. A Pvo2 less
Monitoring of the peak pressure, Pplat, and auto-PEEP
–o less than 70%) indicates tissue
than 35 mm Hg (or Sv
is particularly important. Pplat is measured by applica2
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Paco2 is determined
by carbon dioxide
tion of&anBartlett
end-inspiratory
pause LLC
of 0.5 to 1.5 seconds,
и co ) and the alveolar ventilation (Vи a). If
production
(V
and
auto-PEEP
is
determined
by
application
of an endNOT FOR SALE
OR
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OR
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2
the Vи co2 is constant, the Paco2 varies inversely with the
expiratory pause of 0.5 to 1.5 seconds (Figure 22–26).
Vи a. The minute ventilation (Vи e) affects the Paco2 indiDuring PCV the inspiratory flow often decreases to a
rectly because of the relationship between the Vи e and
no-flow period at end-inspiration. In this case, the peak
и e decreases the Paco , and a
the Vи a. An increase in the V
pressure and Pplat are equivalent. Both Pplat and auto2
и e increases
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decrease in the V
the Paco
.
This
is
illustrated
PEEP can be accurately measured
only when
the patient
2
by the following relationship:
is not exerting effort.
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To avoid overdistention and lung injury, the goal is to
и co ϫ 0.863) / (V
и e ϫ [1 Ϫ Vd/Vt])
Paco2 ϭ (V
2
maintain Pplat below 30 cm H2O (and lower if possible).
where Paco2 is the partial pressure of arterial carbon
To assist in this and to minimize unnecessary cardiac
и e is minute
dioxide, Vи co2 is carbon dioxide production, V
effects and triggering difficulties, auto-PEEP should be
© Jones
Bartlett
© Jones
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ventilation,
and&Vd/Vt
is theLearning,
ratio of deadLLC
space to tidal
as low as possible,
preferably
zero. Learning,
Importantly, LLC
these
factors that determine
volume.
circuit measurements
respiratory
system pressures
NOTFigure
FOR 22–25
SALEshows
OR the
DISTRIBUTION
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the Paco2 during mechanical ventilation.
all assume normal chest wall compliance in order for
The use of noninvasive monitors may reduce the
them to be a reasonable estimate of transpulmonary
need for arterial blood gas determinations, because
pressures (i.e., a normal chest wall compliance will
they allow continuous assessment between blood gas
have little effect on the measured airway pressures).
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LLC can be used to titrate an
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Pulse oximetry
In the&setting
of abnormal,
veryLLC
low chest wall comappropriate
and
PEEP.
Continuous
pulse
oximetry
pliance
(e.g.,
obesity,
ascites),
these
airway pressure
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484

CHAPTER 22

Mechanical Ventilation

1.6

1000

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900
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OR
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800

1.2

Volume (mL)

Volume Above FRC (L)

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0.8

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Upper
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OR point
DISTRIBUTION
inflection

0.4

Lower
inflection point

700
600
500
400
300
200
100
0

0.0
10

20

30

© Jones & Bartlett
Learning,
Airway Pressure
(cm H2O) LLC
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FIGURE 22–27 Pressure–volume curve for normal lungs (solid

40

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10
20
30
Pressure (cm H2O)

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Pressure
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manometer

circles) and ARDS (open circles). Note the presence of a lower and
upper inflection point on the pressure–volume curve for ARDS.
Calibrated

may be profoundly
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LLC affected by chest wall
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stiffness
and
these
effects
need
to
be
subtracted
from
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Patient

the airway pressure to determine true transpulmonary
Filter
pressure. This can be done directly with an esophageal
FIGURE
22–28
Super
syringe
technique
to measure
pressure measurement or estimated by an experienced
pressure–volume curve.
clinician. The use of esophageal pressures for both
estimating true transpulmonary
assess© Jones &pressures
Bartlettand
Learning,
LLC
© Jones & Bartlett Learning, LLC
ing the triggering loads
from
auto-PEEP
is
described
in
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the effects of resistance on the pressure measurement.
more detail next.
The role of the PV curve in setting the ventilator curAuto-PEEP has other manifestations that can be monrently is unclear. Although its use is physiologically
itored. The patient’s breathing pattern can be observed;
attractive, more experience is needed with these meaif exhalation is still occurring when the next breath is
surements before
the PV
can beLearning,
recommended
for
delivered,
auto-PEEP
is present.
Inspiratory
efforts that
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© Jones
&curve
Bartlett
LLC
routine
use
in
setting
the
ventilator.
do NOT
not trigger
the
ventilator
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the
presence
of
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The stress index is a method to assess the level of
PEEP. From the flow graphics on the ventilator, it can be
PEEP to avoid overdistension and underrecruitment
observed that expiratory flow does not return to zero
(Figure 22–29).84 This approach uses the shape of the
before the subsequent breath is delivered when autopressure–time curve during constant-flow tidal volume
PEEP is present.
delivery.
If the compliance
is worsening
The
inflation
pressure–volume
(PV)
curve
of
the
© Jones & Bartlett Learning, LLC
© Jones
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LLC as the lungs
are inflated (upward concavity, stress index Ͼ 1), this
respiratory system can be used to set the ventilator.83
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For patients with ARDS, the PV curve is sigmoidal
suggests overdistension, and the recommendation is
(Figure 22–27). A lower inflection point presumably
to decrease PEEP or tidal volume. If the compliance is
represents the pressure at which a large number of
improving as the lungs are inflated (downward concavalveoli are recruited, or opened. An upper inflection
ity, stress index Ͻ 1), this suggests further potential for
point presumably represents
the
pressure
at
which
a
recruitment,
and the recommendation
to increase
© Jones & Bartlett Learning, LLC
© Jones &is Bartlett
Learning, LLC
large number of alveoli
are
overdistended.
Therefore,
it
PEEP.
The
ideal
stress
index
is
1,
in
which
there
is a linear
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OR DISTRIBUTION
would seem reasonable to set the PEEP above the lower
increase in pressure with constant-flow inflation of the
inflection point and the Pplat below the upper inflection
lungs.
point. Because there is hysteresis in the PV curve (i.e.,
Esophageal pressure is measured from a thin-walled
it is shifted leftward during deflation), some argue that
balloon, which contains a small volume of air (Ͻ1 mL),
the©ideal
PEEP&
setting
shouldLearning,
be determined
during the
placed into© the
lower&esophagus
( Figure 22–30
). 85
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Bartlett
LLC
Jones
Bartlett Learning,
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deflation
the PVOR
plot.DISTRIBUTION
EsophagealNOT
pressure
changes
reflect
in pleural
NOT phase
FOR ofSALE
FOR
SALE
ORchanges
DISTRIBUTION
The traditional PV curve is measured as a series of
pressure, but the absolute esophageal pressure does not
plateau pressures during small incremental changes in
reflect absolute pleural pressure. Changes in esophageal
inspiratory and expiratory volumes from a super syringe
pressure can be used to assess respiratory effort and
(Figure 22–28). The inflation PV curve can also be meapatient work of breathing during spontaneous breathsured
when
a
slow
(e.g.,
5
to
10
L/min),
constant
flow
is
ing and
modes of
ventilation, to assess
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© Jones
&patient-triggered
Bartlett Learning,
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set
on
the
ventilator
and
the
ventilator
display
of
the
PV
chest
wall
compliance
during
full
ventilatory support,
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curve is observed. This slow flow effectively eliminates
and to assess auto-PEEP during spontaneous breathing

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Monitoring the Mechanically Ventilated Patient
Tidal Recruitment

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485

Overdistention

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Flow

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Airway
openingOR DISTRIBUTION
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pressure

Stress index < 1

Stress index = 1

Stress index > 1

FIGURELLC
22–29 Stress index concept during©constant-flow
volume
control ventilation.
For a LLC
© Jones & Bartlett Learning,
Jones &
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stress index less than 1, the airway pressure curve presents a downward concavity, suggesting a
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continuous decrease in elastance and tidal recruitment. For a stress index higher than 1, the curve
presents an upward concavity, suggesting a continuous increase in elastance and overdistention.
For a stress index equal to 1, the curve is straight, suggesting the absence of tidal variations in
elastance. Reprinted with permission of the American Thoracic Society. Copyright © American
Thoracic Society. Grasso S, Stripoli T, De Michele M, et al. ARDSnet ventilatory protocol and
alveolar
hyperinflation:
role of positive
end-expiratoryLLC
pressure. Am J Respir Crit Care Med.
© Jones
& Bartlett
Learning,
© Jones
2007;176:761–767. Official Journal of the American Thoracic Society; Diane Gern, Publisher.

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Volume (L)
Pressure
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Flow (L/s)

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1
0
0.8

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0
30

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Pes
(cm H2O)

20
Esophageal
balloon

0

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FIGURE 22–30 Position
of esophageal
balloon OR
to measure
changes in intrapleural pressure.

Estimation of
auto-PEEP

Missed
© Jones
trigger
effort

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NOTTime
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FIGURE 22–31 Auto-PEEP. Note the amount of effort required to trigger
the ventilator, represented by the amount of decrease in esophageal pressure
required for triggering. Also note the presence of an inspiratory effort that does
not trigger the ventilator.

and©patient-triggered
modes
of ventilation.
If exhalaJones & Bartlett
Learning,
LLC
© Jones & Bartlett Learning, LLC
tion
is
passive,
the
change
in
esophageal
(i.e.,
pleural)
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DISTRIBUTION
recognizedNOT
as a patient
respiratory
(observed by
pressure required to reverse flow at the proximal airway
inspecting chest wall movement) that is greater than the
(i.e., to trigger the ventilator) reflects the amount of
trigger rate on the ventilator.
auto-PEEP. Negative esophageal pressure changes that
The increase in esophageal pressure (⌬Pes) during
produce no flow at the airway indicate failed trigger
passive inflation of the lungs can be used to calculate
in other
words, the
patient’s inspiratory efforts
© Jones & efforts;
Bartlett
Learning,
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© Jones
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):
chest wall
compliance
(Figure 22–32
are
insufficient
to
overcome
the level of auto-PEEP and
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Ccw ϭ Vt/⌬Pes
trigger the ventilator (Figure 22–31). Clinically, this is

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486

CHAPTER 22

Mechanical Ventilation

Changes in esophageal pressure, relative to changes in

positive (i.e., PEEP is greater than esophageal pressure)

© Jones
Bartlett
Learning,
LLC
© Jones &alveolar
Bartlett
Learning,
pressure,
can beLLC
used to calculate transpul(Figure&22–33
). This
is most likely
with a decrease in
NOT
FOR
SALE
OR
DISTRIBUTION
NOT FOR SALE
DISTRIBUTION
monary OR
pressure
(lung stress). This may allow more
chest wall compliance, such as occurs with abdominal

precise setting of tidal volume (and Pplat) in patients
compartment syndrome, pleural effusion, or obesity. In
with reduced chest wall compliance. In this case, transthis case, it is desirable to keep PEEP greater than pleural
pulmonary pressure (difference between Pplat and Pes)
pressure. Unfortunately, artifacts in esophageal pressure,
is targeted at less than 27 cm H2O.
especially in supine critically ill patients, make it very diffi© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
The use of an esophageal balloon has been advocated
cult to measure absolute pleural pressure accurately.87,88 In
86
NOT
FOR
SALE
OR
DISTRIBUTION
NOT FOR
SALEbladder
OR DISTRIBUTION
to allow more precise setting of PEEP. If pleural pressure
patients with abdominal compartment
syndrome,
is high relative to alveolar pressure (i.e., PEEP), then there
pressure may be useful to assess intra-abdominal presmay be a potential for derecruitment. With this approach,
sure, the potential collapsing effect on the lungs, and the
PEEP is increased until the transpulmonary pressure is
amount of PEEP necessary to counterbalance this effect.89

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60 FOR SALE OR DISTRIBUTION
NOT

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NOT FOR SALE OR DISTRIBUTION

Flow (L/min)

40
20
0
–20
–40
Bartlett
–60

Paw (cm H2O)
Pes (cm H2O)

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Learning, LLC
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1

2

© Jones & Bartlett Learning, LLC
NOT FOR SALE
OR DISTRIBUTION
3
4

32
Esophageal pressure

16
8
0

Ccw = VT/⌬
⌬Pes
= 350 mL/5 cm H2O
= 70 mL/cm H2O

Airway pressure

24

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NOT FOR SALE OR DISTRIBUTION
1

5

2

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3

4

5

Volume (mL)

400
300
200

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100
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NOT FOR SALE OR DISTRIBUTION

0

1

2

3

4

5

Time (s)
FIGURE 22–32 Calculation of chest wall compliance. The esophageal pressure increases by 5 cm H2O with a tidal volume of 350 mL in a
passively ventilated patient.

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60
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OR DISTRIBUTION

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Flow (L/min)

40
20
0
–20
–40

Paw (cm H2O)
Pes (cm H2O)

40

© Jones & Bartlett Learning, LLC
NOT FOR SALE OR DISTRIBUTION
2

4

6

8

© Jones & Bartlett Learning, LLC
NOT FOR SALE OR DISTRIBUTION
10

12

14

30

©
Jones & Bartlett Learning, LLC
20
NOT FOR SALE OR DISTRIBUTION

© Jones & Bartlett Learning, LLC
NOT FOR SALE OR DISTRIBUTION

10
0

2

4

6

8

10

Time (s)

12

14

© Jones & Bartlett Learning, LLC
© Jones & Bartlett Learning, LLC
FIGURE 22–33 Airway and esophageal pressures in a passively ventilated patient. In this case, the transpulmonary pressure during exhalation is
NOT FOR
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positive because PEEP is greater than the esophageal pressure.

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