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2003 waveform pocket guide BENNETT

a P-V loop during a mandatory
breath that displays the characteristic “figure eight”
often seen if inspiratory flow is set too low to meet
the patient’s demands, or a descending ramp flow
waveform results in inadequate end-inspiratory flow.
As the patient’s demand begins to outstrip the flow
delivery of the ventilator, the pressure starts to
decrease (A) while volume continues to increase.
Exhalation becomes slightly positive at the beginning
(B) and then assumes a normal configuration as the
lungs empty.
Detecting Air Leaks or Air Trapping
VT 1000
mL 900

Insp Area
0.000

800
700
600

500
3.0

cm
H2O

400
300
200
100
0
-100

PCIRC
-30

-20

-10

0

10

20

30

40

50

60 cmH2O

Figure 69. Air Leaks or Air Trapping

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Application

FLOW-VOLUME LOOP
3.0
2.5

EXH

A

2.0
1.5

.
V
L
sec

The flow-volume loop may be helpful in gauging the
effects of bronchodilators on patients.
Evaluating the Effect of Bronchodilators

1.0
0.5
VT
mL

0
0.5

PRE-BRONCHODILATOR
3.0
EXH

2.5

INSP

.
V

0

200

400

600

800

1000 1200 1400 1600

L
sec

1.5

1.0

.

B

0.5

VT
mL

0
0.5

Figure 70. Typical Flow-Volume Loop

55

D

1.0
0.5

VT
mL

0
1.0

1.5

1.5

2.0

Figure 70 shows a typical flow-volume loop. The
peak expiratory flow rate is noted at (A); peak
inspiratory flow rate is noted at (B). Flow is plotted
on the vertical axis and volume on the horizontal.
The lower half of the loop represents inspiration;
the upper half represents exhalation. The loop
arrangement resembles that of a pulmonary function
test, in which exhalation is plotted first followed by
the next inspiration.

V
L
sec

0.5

1.0

2.5

EXH

2.0

1.5

200

C

2.5

A

2.0

1.5
2.0
2.5

POST-BRONCHODILATOR

3.0

B

1.0

INSP

200

0

200

400

600

Figure 71.
Pre-Bronchodilator
Flow-Volume Loop

800

1000

2.0
2.5

INSP

200

0

200

400

600

800

1000

Figure 72.
Post-Bronchodilator
Flow-Volume Loop

Figures 71 and 72 show pre- and post-bronchodilator
flow-volume loops that indicate a positive response
to the bronchodilator administration. The flowvolume loop before bronchodilator administration
shows a low peak expiratory flow rate (A) and a
scalloped shape near the end of exhalation (B) that
is characteristic of poor airway conductivity. The
post-bronchodilator loop shows an improved peak
expiratory flow (C) as well as improved flow toward
the end of exhalation (D).

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