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Neurocritical Care
Very few hospitals have dedicated neurointensive care units for management of critically ill neurologic
and neurosurgical patients. Frequently, these patients are managed in the medical or surgical ICU by
non-neurologists, who have a difficult time appreciating the delicate needs of these patients. This book
is a straightforward, practical reference for physicians who need to deal with a wide range of complex
conditions and associated medical problems. Although comprehensive in scope, this book is designed
for neurologists and physicians in the ICU who need a concise and thorough guide to management and
treatment. It emphasizes practical state-of-the-art suggestions for management and treatment and provides accessible, easy-to-follow, highly structured, and focused protocols for the assessment, day-to-day
management, and treatment of critically ill patients in various ICU divisions.
Michel T. Torbey, MD, MPH, FAHA, FCCM, is Associate Professor of Neurology and Neurosurgery,
Director of the Stroke Critical Care Program, and Director of the Neurointensive Care Unit at the Medical
College of Wisconsin, Milwaukee, Wisconsin. He is also coeditor of The Stroke Book with Dr. Magdy
H. Selim, published in 2007.



Neurocritical Care

Edited by

Michel T. Torbey
Medical College of Wisconsin
Milwaukee, Wisconsin


CAMBRIDGE UNIVERSITY PRESS

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São Paulo, Delhi, Dubai, Tokyo
Cambridge University Press
The Edinburgh Building, Cambridge CB2 8RU, UK
Published in the United States of America by Cambridge University Press, New York
www.cambridge.org
Information on this title: www.cambridge.org/9780521676892
Cambridge University Press 2010

This publication is in copyright. Subject to statutory exception and to the
provision of relevant collective licensing agreements, no reproduction of any part
may take place without the written permission of Cambridge University Press.
First published in print format 2009

ISBN-13

978-0-521-67689-2

Hardback

Cambridge University Press has no responsibility for the persistence or accuracy
of urls for external or third-party internet websites referred to in this publication,
and does not guarantee that any content on such websites is, or will remain,
accurate or appropriate.
Every effort has been made in preparing this book to provide accurate and up-todate information that is in accord with accepted standards and practice at the time
of publication. Although case histories are drawn from actual cases, every effort
has been made to disguise the identities of the individuals involved. Nevertheless,
the authors, editors, and publishers can make no warranties that the information
contained herein is totally free from error, not least because clinical standards are
constantly changing through research and regulation. The authors, editors, and
publishers therefore disclaim all liability for direct or consequential damages
resulting from the use of material contained in this book. Readers are strongly
advised to pay careful attention to information provided by the manufacturer of
any drugs or equipment that they plan to use.


Contents

Contributors
Foreword

Page vii
xi

Michael N. Diringer

Introduction

xiii

Michel T. Torbey
Section I:  Principles of Neurocritical Care

1 Cerebral Blood Flow Physiology and Metabolism

1

Marc Malkoff

2 Cerebral Edema and Intracranial Pressure

11

Ahmed Raslan and Anish Bhardwaj

3 Vasoactive Therapy

20

Santiago Ortega-Gutierrez, Kristin Santa, Marta Lopez-Vicente, and
Michel T. Torbey

4 Hypothermia: Physiology and Applications

38

Carmelo Graffagnino

5 Analgesia, Sedation, and Paralysis

49

Wendy C. Ziai and Rehan Sajjad

6 Mechanical Ventilation and Airway Management

68

Rahul Nanchal and Ahmed J. Khan

7 Neuropharmacology

81

Kristin Santa and Rebecca Schuman
Section II:  Neuromonitoring

8 Intracranial Pressure and Cerebral Blood Flow Monitoring

109

Andrew Beaumont

9 Hemodynamic and Electrophysiological Monitoring

119

Rahul Nanchal, Ahmed J. Khan, and Todd Gienapp
Section III: Management of Specific Disorders in the
Neurocritical Care Unit

10 Ischemic Stroke
May A. Kim, Justin Wagner, Clif Segil, Reed Levine, and
Gene Y. Sung

133
v


vi

Contents

11 Intracerebral Hemorrhage

143

Natalia Rost and Jonathan Rosand

12 Cerebral Venous Thrombosis

160

Magdy Selim

13 Subarachnoid Hemorrhage

167

Matthew Miller and Grant Sinson

14 Status Epilepticus

185

Marek A. Mirski and Panayiotis N. Varelas

15 Nerve and Muscle Disorders

199

Edward M. Manno

16 Head Trauma

209

Fernanda Tagliaferri, Christian Compagnone, and
Thomas A. Gennarelli

17 Encephalopathy

220

Angelos Katramados and Panayiotis N. Varelas

18 Coma and Brain Death

227

Jennifer L. Berkeley and Romergryko G. Geocadin

19 Neuroterrorism and Drug Overdose

242

Geoffrey S. F. Ling, Scott Marshall, and John J. Lewin III

20 Central Nervous System Infections

262

Ricardo Carhuapoma and H. Adrian Püttgen

21 Spinal Cord Injury

283

Melissa Y. Macias and Dennis J. Maiman

22 Postoperative Management in the Neurosurgical Critical Care Unit

294

Andy J. Redmond and Veronica L. Chiang

23 Ethical and Legal Considerations in Neuroscience Critical Care

308

Dan Larriviere and Michael A. Williams
Section IV: Management of Medical Disorders in the
Neurocritical Care Unit

24 A Pulmonary Consult

319

Wendy Zouras and Kenneth Presberg

25 A Cardiology Consult

341

James Kleczka, Timothy Woods, and Lee Biblo

26 An Infectious Diseases Consult

364

Michael Frank and Mary Beth Graham

27 A Gastroenterology Consult

372

Yume Nguyen, Thomas Kerr, and Riad Azar

28 A Nephrology Consult

381

Jeffrey Wesson and Aaron Dall

29 An Endocrinology Consult

397

James A. Kruse

Index

411


Contributors

Riad Azar, MD
Division of Gastroenterology
Department of Internal Medicine
Washington University School of Medicine
St. Louis, Missouri
Andrew Beaumont, MD, PhD
Department of Neurosurgery
Medical College of Wisconsin
Milwaukee, Wisconsin
Jennifer L. Berkeley, MD, PhD
Neurosciences Critical Care Division
Johns Hopkins Medical Institutions
Baltimore, Maryland
Anish Bhardwaj, MD, FAHA, Fccm
Departments of Neurology, Neurological Surgery,
  Anesthesiology, and Peri-Operative Medicine
Oregon Health and Science University
Portland, Oregon
Lee Biblo, MD
Division of Cardiology
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Ricardo Carhuapoma, MD
Division of Neurocritical Care
Departments of Neurology and Anesthesia/Critical
  Care Medicine
Johns Hopkins University School of Medicine
Johns Hopkins Hospital
Baltimore, Maryland

Veronica L. Chiang, MD
Department of Neurosurgery
Yale University School of Medicine
New Haven, Connecticut
Christian Compagnone, MD
Neurosurgical and Trauma
  Intensive Care Unit
Maurizio Bufalini Hospital
Cesena, Italy
Aaron Dall, MD
Division of Nephrology
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Michael Frank, MD
Department of Medicine
Infectious Disease Clinic
Medical College of Wisconsin
Milwaukee, Wisconsin
Thomas A. Gennarelli, MD
Department of Neurosurgery
Neuroscience Center
Medical College of Wisconsin
Milwaukee, Wisconsin
Romergryko G. Geocadin, MD
Neurosciences Critical Care Division
Johns Hopkins Medical Institutions
Baltimore, Maryland

vii


viii

Todd Gienapp, MD
Division of Pulmonology
The Vancouver Clinic
Vancouver, Washington
Carmelo Graffagnino, MD, FRCPC
Departments of Medicine and Neurology
Duke Neuroscience Critical Care Unit
Duke University Medical Center
Durham, North Carolina
Mary Beth Graham, MD
Division of Infectious Disease
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Angelos Katramados, MD
Department of Neurology
Henry Ford Hospital
Detroit, Michigan
Thomas Kerr, MD, PhD
Division of Gastroenterology
Department of Internal Medicine
Washington University School of Medicine
St. Louis, Missouri
Ahmed J. Khan, MD
Division of Pulmonary Medicine
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
May A. Kim, MD
Department of Neurology
Keck School of Medicine
University of Southern California
Los Angeles, California

Contributors

Dan Larriviere, MD, JD
Department of Neurology
University of Virginia School of Medicine
Program in Health Law
University of Virginia School of Law
Charlottesville, Virginia
Reed Levine, MD
Department of Neurology
Keck School of Medicine
University of Southern California
Los Angeles, California
John J. Lewin III, PharmD, BCPS
Department of Pharmacy
Neurosciences Critical Care Unit
Johns Hopkins Hospital
Baltimore, Maryland
Geoffrey S. F. Ling, MD, PhD
Medical Corps, U.S. Army
Departments of Neurology and Critical Care
  Medicine for Anesthesiology and Surgery
Uniformed Services University of the Health
  Sciences
Bethesda, Maryland
Marta Lopez-Vicente, MD
Department of Family and Community Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Melissa Y. Macias, MD, PhD
Department of Neurosurgery
Medical College of Wisconsin
Milwaukee, Wisconsin
Dennis J. Maiman, MD, PhD
Spinal Cord Injury Center
Medical College of Wisconsin
Milwaukee, Wisconsin

James Kleczka, MD
Division of Cardiology
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin

Marc Malkoff, MD
Department of Neurology
Barrow Neurological Institute
Phoenix, Arizona

James A. Kruse, MD, FCCM
Critical Care Services
Bassett Healthcare
Cooperstown, New York

Edward M. Manno, MD
Department of Neurology
Mayo Clinic
Rochester, Minnesota


ix

Contributors

Scott Marshall, PharmD
US Army Medical Corps
Clinical Fellow, Neurosciences Critical Care
Johns Hopkins Hospital
Baltimore, Maryland
and Department of Neurology
Uniformed Services University of the Health
  Sciences
Bethesda, Maryland

Ahmed Raslan, MD
Division of Neurological Surgery
Oregon Health and Science University
Portland, Oregon

Matthew Miller, MD
Department of Neurosurgery
Medical College of Wisconsin
Milwaukee, Wisconsin

Jonathan Rosand, MD, MSc
Division of Vascular and Critical Care Neurology
Center for Human Genetic Research
Massachusetts General Hospital
Boston, Massachusetts

Marek A. Mirski, MD, PhD
Departments of Neurology, Anesthesiology and
  Critical Care Medicine, and Neurosurgery
Johns Hopkins University School of Medicine
Baltimore, Maryland
Rahul Nanchal, MD
Division of Pulmonary Critical Care
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Yume Nguyen, MD
Division of Gastroenterology
Department of Internal Medicine
Washington University School of Medicine
St. Louis, Missouri
Santiago Ortega-Gutierrez, MD
Department of Neurology
Medical College of Wisconsin
Milwaukee, Wisconsin
Kenneth Presberg, MD
Pulmonary and Critical Care Division
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
H. Adrian Püttgen, MD
Division of Neurocritical Care
Johns Hopkins Hospital
Baltimore, Maryland

Andy J. Redmond, MD
Department of Neurosurgery
Yale University School of Medicine
New Haven, Connecticut

Natalia Rost, MD
Division of Vascular and Critical Care Neurology
Center for Human Genetic Research
Massachusetts General Hospital
Boston, Massachusetts
Rehan Sajjad, MD
Department of Neurology
Medical College of Wisconsin
Milwaukee, Wisconsin
Kristin Santa, PharmD
Pharmacy Department
Froedtert Memorial Lutheran Hospital
Milwaukee, Wisconsin
Rebecca Schuman, PharmD
Pharmacy Department
Froedtert Memorial Lutheran Hospital
Milwaukee, Wisconsin
Clif Segil, MD
Department of Neurology
University of Southern California
Los Angeles, California
Magdy Selim, MD, PhD
Division of Cerebrovascular Diseases
Department of Neurology
Beth Israel Deaconess Medical Center
Harvard Medical School
Boston, Massachusetts


x

Contributors

Grant Sinson, MD
Department of Neurosurgery
Medical College of Wisconsin
Milwaukee, Wisconsin

Jeffrey Wesson, MD
Department of Veterans Affairs Medical Center
Medical College of Wisconsin
Milwaukee, Wisconsin

Gene Y. Sung, MD, MPH
Neurocritical Care and Stroke Program
LAC+USC Neurology
University of Southern California
Los Angeles, California

Michael A. Williams, MD, FAAN
Sandra and Malcolm Berman Brain and Spine
  Institute
and Adult Hydrocephalus Center
Sinai Hospital
Baltimore, Maryland

Fernanda Tagliaferri, MD
Department of Anesthesiology and
  Intensive Care
Maurizio Bufalini Hospital of Cesena
Cesena, Italy
Michel T. Torbey, MD, MPH, FAHA, FCCM
Department of Neurology and Neurosurgery
Medical College of Wisconsin
Milwaukee, Wisconsin
Panayiotis N. Varelas, MD, PhD
Department of Neurology and Neurosurgery
Henry Ford Hospital
Detroit, Michigan
Justin Wagner, MD
Keck School of Medicine
University of Southern California
Los Angeles, California

Timothy Woods, MD
Division of Cardiology
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin
Wendy C. Ziai, MD, MPH
Division of Neurosciences Critical Care
Departments of Neurology, Neurosurgery,
  Anesthesia, and Critical Care Medicine
Johns Hopkins University School of Medicine
Baltimore, Maryland
Wendy Zouras, MD
Division of Pulmonary/Critical Care Medicine
Department of Medicine
Medical College of Wisconsin
Milwaukee, Wisconsin


Foreword

Twenty-two years ago, I wandered into an eight-bed ICU dedicated to caring for critically ill neurologic and
neurosurgical patients to begin my fellowship training in Neurosciences Critical Care. Very few such units
existed; there were only two trained Fellows in the country, and this was the only one that actually paid the
Fellows a salary. Having just completed my residency in neurology, I was not particularly well prepared for
the task ahead, in terms of either knowledge or approach to patient care. I had much to learn not only about
the brain but also about how the heart, lungs, kidney, etc. affected the brain. More importantly, I had to
learn how to take care of “sick” patients, manage ventilators, insert Swan-Ganz catheters, feed patients, and
treat infections. Finally, I had to radically alter how I approached patients. No longer was the adage “time is
a neurologist’s best friend” an acceptable approach to diagnosis and treatment. No one had even considered
writing a textbook on neurocritical care. Most of my peers could not understand why I would want to pursue
neurocritical care.
Since then things have changed considerably. Most academic centers have or want a neuro ICU; some
have more than thirty beds. There is now board certification for neurointensivists, who are recognized by
Leapfrog and have a thriving subspecialty journal and a society with almost a thousand members. Equally
important is the growing appreciation by other intensivists of what they can offer to critically ill patients with
neurologic conditions. No longer do they see the brain as a “black box” that is best ignored, but rather they
are embracing brain-specific monitoring and interventions.
Although growth in the field has been explosive, the educational resources available for trainees have
lagged behind. A number of texts are available that emphasize different aspects of neurocritical care. They
often delve deeply into the neurologic aspects but give less attention to management of other organ systems.
Some are comprehensive and serve best as references; others are designed to be carried around and provide
limited depth. What has been missing is a practical guide for decision making that covers all aspects of care,
neurologic and systemic.
Neurocritical Care is designed to fill an important niche. It is comprehensive in scope but limited in depth,
providing a straightforward practical reference that focuses on clinical decision making and management
for critically ill neurologic and neurosurgical patients. The chapters provide practical state-of-the-art guidance for both neuro- and other intensivists caring for these patients and cover all aspects of their management, not just brain-specific ones. I certainly wish this book had been available when I was in training; it
would have helped prepare me for the task ahead.
Michael N. Diringer, MD
Immediate Past-President,
Neurocritical Care Society

xi



Introduction

Critically ill neurologic and neurosurgical patients can be difficult to manage, not necessarily owing to
­significant medical problems but rather as a result of the complexity and vulnerability of the brain vis-àvis physiologic changes that otherwise would be well tolerated by any other body organ. Hence it is very
important to approach these patients in a holistic way, combining the standard critical care approaches
with a neuro-focused approach. Most available textbooks in this field have either focused on neurocritical
care topics or general critical care topics. The “NICU Book” has been produced to provide all healthcare
professionals caring for critically ill neurologic and neurosurgical patients with a straightforward, concise,
and practical reference to assist them with management decisions.
The book is intended to be limited in depth but comprehensive in its scope. Emphasis has been placed on
selecting well-renowned contributors to provide a logical approach to the diagnosis and management of a
wide range of common conditions seen in the neurointensive care unit.
On completing the book the reader should be able to understand the nuances in neurocritical care
patients and determine the most effective therapy to limit secondary brain injury.
I am indebted to my wife and children, to the authors, neurocritical nurses, families, and particularly the
patients who continue to stimulate my passion for neurocritical care.
Michel T. Torbey, MD, MPH, FAHA, FCCM

xiii



Section I:  Principles of Neurocritical Care

1 Cerebral Blood Flow Physiology and Metabolism
Marc Malkoff, MD

 The brain comprises only 2% of total body weight;
however, under normal conditions, it receives
15–20% of the cardiac output and accounts for
20% of total body oxygen consumption.1 Because
energy reserves within the brain are negligible, adequate blood flow is essential for the provision of a
continuous supply of energy-producing substrates
and for the removal of the byproducts of cellular
metabolism.1 
Normal Physiology of Cerebral
Blood Flow

 Cerebral blood flow (CBF) is normally approximately
50 mL/100 g per minute.2 Regionally it is greater for
gray matter than for white matter, 70 mL/100 g per
minute versus 20 mL/100 g per minute, respectively.
This rate is slightly increased in youth, and decreases
with age.3–5
◾◾ CBF less than 30 mL/100 g per minute can pro-

duce neurologic symptoms.2,3
◾◾ CBF between 15 and 20 mL/100 g per minute will
cause reversible damage or “electrical failure”1,6,7
◾◾ CBF rates of 10–15 mL/100 g per minute cause
irreversible neuronal damage.6,7
CBF is determined by blood viscosity,  cerebral
perfusion pressure (CPP),   and vessel radius. This
relationship is expressed with the Hagen–Poiselle
formula:
Q = P * Pi * r 4 / 8 * n * L
where P is CPP, r is the cumulative radii of cerebral regulatory resistance vessels, n is whole blood

viscosity, L is cerebral vessel length, and Q is CBF.
Vessel length is not a physiologic variable that
changes or can be manipulated. The brain has no
significant storage capacity, so metabolism and CBF
are tightly  coupled  , and this is called metabolicflow coupling. This relationship can be expressed
with  Fick’s equation :
CMRo2 = CBF * AVDo2.
◾◾ AVDo2 is the arteriovenous difference in oxygen

and CMRo2 is the central nervous system metabolic rate of oxygen consumption.
◾◾ Under normal conditions the brain maintains normal AVDo2 by responding to changes in metabolism, CPP, and blood viscosity with changes in
cerebral vessel caliber, that is, autoregulation.2,8,9
◾◾ As demonstrated with the  Hagen–Poiselle formula , vessel radius is the largest determinant
of CBF .

Autoregulation
◾◾  The differentiation between the terms cerebral

autoregulation and regulation of CBF is often confusing. The latter term broadly encompasses a variety of vasomotor regulatory mechanisms, of which
cerebral autoregulation is one type.10 Cerebral
autoregulation describes the intrinsic ability of the
cerebral circulation to maintain a constant blood
flow in the face of changing perfusion pressure.11
This is purely a pressure-related phenomenon.
The changes are subserved mainly by precapillary
resistance vessels.12 Arteries dilate in response to
1


2

M. Malkoff

decreased perfusion and constrict in response
to increased perfusion. The exact mechanism of
autoregulation is unknown. Proposed mechanisms include the myogenic hypothesis, the endothelial hypothesis, and the neurogenic hypothesis.
The myogenic theory is the most widely accepted
explanation for the mechanism of autoregulation.
◾◾ A metabolic hypothesis has also been described,
but is better suited as a description of metabolicflow coupling rather than autoregulation.
Bayliss first proposed the myogenic theory in
1902 after he observed the direct constriction and
relaxation of canine arteries in response to changes
in intravascular pressure. The myogenic theory
assumes that there is a basal tone of vascular smooth
muscle, which is affected by changes in transmural
pressure.1,13 This results in constriction of precapillary arterioles to rising intravascular pressure and
dilatation to falling intravascular pressure.8,11 Studies
suggest that there may be two myogenic mechanisms
involved in cerebral autoregulation: a rapid fast reaction to pressure pulsations and a slower reaction to
change in mean arterial pressure (MAP).14
Important evidence supporting the myogenic
theory is the existence of stretch-activated cation
channels (SACCs) in myocytes.10,15 SACC activation
is associated with an influx of cations, especially calcium and sodium, leading to cellular depolarization.
This, in turn, results in the opening of membrane
voltage-gated calcium channels (VGCCs) with the
end result of smooth muscle contraction.10,16 The
neurogenic hypothesis states that alterations in
transmural pressure trigger changes in neurotransmitter release from perivascular nerve fibers.10 The
hypothesis is supported by several facts:

◾◾ First, both intracranial and extracranial arteries

are endowed with a rich and active network of
nerves located throughout the adventitial space
as demonstrated by anatomic studies.10,17 There
is innervation from both extrinsic (remote) and
intrinsic (local) neurons. Origins of the extrinsic
neurons supplying blood vessels are the sympathetics from the superior cervical ganglion,
parasympathetics from the pterygopalatine,
sphenopalatine, and otic ganglion, trigeminal
connections from the gasserian ganglion, and
serotonergic neurons from the raphe nuclei.18

There are also intrinsic peptidergic neurons from
local adventitial neurons.18
◾◾ Second, the presence of specific neurotransmitter receptors has been demonstrated on vascular
endothelial and smooth muscle cells.10,18
◾◾ Third, nerve stimulation studies demonstrated
a correlation between altered vasomotor tone
after electrical stimulation of deendothelialized
arteries.10
Current literature suggests that some neurogenic
control comes into play only under conditions of
cerebrovascular stress.18–20 It may be that neurogenic
mechanisms lead to very fine-tuned modulation
or that they protect cerebral vessels during acute,
severe stress.20
The endothelial hypothesis suggests that the cerebral arterial endothelial cells may act as a mechanoreceptor that senses and transduces variations in
mechanical factors such as stretch and flow velocity
into altered vascular tone.10,21,22 It is known that the
endothelium releases substances that are vasoactive
such as endothelium-derived relaxing factors, nitric
oxide, endothelium-derived hyperpolarizing factor,
thromboxane, and endothelin-1.22,23 Observations
that increasing flow rate and shear stress without
increasing transmural pressure can induce endothelial vasodilatation suggest endothelial dependence of vasomotor activity.10,23 It is thought that the
changes in vasomotor tone are brought about by a
change in the endothelium’s liberation of relaxing
or contracting factors.22,23 This is corroborated by
an experiment wherein changes in vascular smooth
muscle tone were observed after smooth muscle
cells were exposed to perfusate from normal endothelium that had been exposed to changes in transmural pressure.23
The metabolic theory argues that autoregulation is determined by the release of vasoactive substances that regulate the resistance of the cerebral
vessels keeping CBF constant. This assumes that the
primary determinant of regional blood flow is local
cerebral metabolic activity, that is, metabolic-flow
coupling. Although no specific agent has been identified as the primary determinant of flow adenosine,
potassium, prostaglandins, and nitric oxide have
been proposed as metabolic coupling agents.
◾◾  Adenosine  is of particular interest, as it is known

to be a potent vasodilator. It is formed by the


3

Cerebral Blood Flow Physiology and Metabolism

breakdown of ATP, and is abundant when oxygen supply is not sufficient to meet metabolic
demands, that is, anaerobic metabolism.14
◾◾ Adenosine binds to A1 and A2 receptors located
on neurons and vascular smooth muscle cells
respectively.
▸▸ The A1 receptors inhibit neuronal activity, and
the A2 receptors activate a second messenger
cascade mediated by adenylate cyclase. These
events together are considered protective when
there is a flow-metabolism mismatch.
 Potassium   (K) is released during neuronal excitation. During periods of hypoxia, electrical stimulation,
and seizures, increases in perivascular K coincide
with increases in CBF.13 In the range of 2–10 mM,
extracellular K causes vasodilatation due to hyperpolarization of the vascular smooth muscle cells and
decreased cytosolic calcium ion levels.10 At concentrations above 10 mM, K acts as a vasoconstrictor.
 Arachidonic   acid metabolites, or eicosanoids,
affect cerebral arterial vasomotor tone, and likely
play a role in modulating CBF.10 Eicosanoids are
generated from three major enzyme systems:
cyclooxygenase (COX), lipoxygenase (LOX), and
epoxygenase (EPOX). Some are vasodilators such
as prostacyclin (PGI2) and others are vasoconstrictors such as thromboxane (TXA2). Also suggesting
the importance of prostaglandins is the fact that
the use of the nonsteroidal anti-inflammatory drug
(NSAID) indomethacin can block the ability of the
brain to maintain constant cerebral perfusion during arterial hypotension.13
 Nitric oxide (NO)   is a freely diffusible molecule
that regulates CBF. NO has a short half-life of approximately 6 seconds, and is produced from l-arginine
by a group of enzymes designated NO synthases
(NOS). NO usually works via a second messenger
pathway to stimulate vascular smooth muscle relaxation. Two constitutive forms of NOS, nNOS and
eNOS, and one inducible form, iNOS, are present in
the brain. The constitutive forms have a role in physiologic regulation of blood flow.
◾◾ nNos is found in glia and neurons near the vascula-

ture, and eNOS in the endothelial layers of large vessels and astrocytes in contact with blood vessels.10,24
◾◾ nNOS has an important role in the regulation of
CBF in response to metabolism, hypercapnia, and
ischemia.13

◾ ◾ eNOS is believed to play a role in blood flow

during ischemia. This is supported by the
observation that elevated levels of eNOS
were detected 4–6 hours after global ischemic
insult.13

Limits of Autoregulation
In a normotensive individual, the brain is able to
keep CBF stable between MAPs of 60–150 mm Hg.
This is sometimes called the autoregulatory plateau.
Below this level, vasodilation becomes insufficient,
and ischemia results. Above this level, increased
intraluminal pressure forcefully dilates the arterioles, causing luxury perfusion.2 This breakthrough
of autoregulation is accompanied by damage to the
endothelium, and disruption of the blood–brain
barrier. This results in extravasation of plasma proteins into the brain, neuronal dysfunction and damage, and development of edema.2,23
Autoregulation can be impaired by many pathologic insults including hypoxia, ischemia, head
injury, and aneurysmal subarachnoid hemorrhage.
Then CBF passively follows changes in arterial pressure. Loss of autoregulation can be a global, focal, or
multifocal process  .

CO2 Regulation of CBF
  Carbon dioxide has long been known to have an
effect on CBF. Between a Paco2 range of 25–60
mm Hg, a 1 mm Hg change in Paco2 changes CBF
by 3–4%.1,2,5 Decreases in Paco2 cause increased
in vasoconstriction, and increases in Paco2 cause
vasodilatation. CO2 is a rapidly diffusible gas that
readily crosses the blood–brain barrier into the
perivascular space and to the cerebral vascular
smooth muscle cells.14 CO2 is then broken down by
carbonic anhydrase into bicarbonate and hydrogen
ions. The change in CBF is not mediated by a direct
effect of carbon dioxide. Instead, the change is
mediated through one of two different mechanisms:
either pH changes in the extracellular fluid around
microvessels or the H+ ions affect vessels directly.
Effects occur within seconds after Paco2 is changed,
and complete equilibration occurs within 2 minutes.14 Decreased responsiveness to Paco2 variability is seen in severe carotid stenosis, head injury,
subarachnoid hemorrhage, cardiac failure, or where
vascular response is already exhausted. Carbon


4

dioxide–dependent vasoconstriction of arterioles is
also impaired at reduced hematocrit.25,26  

Oxygen
 In the normal physiologic range for Pao2, 60–100 mm
Hg, fluctuations in Pao2 do not affect CBF. However,
CBF does dramatically increase when Pao2 drops
below 50 mm Hg. Important mediators for this
enhanced blood flow may be increases in adenosine concentration and/or developing extracellular acidosis related to the anaerobic metabolism
of glucose.27 Molecular oxygen has been shown to
directly affect vascular smooth muscle tone, and
may be a mediator of the flow response to changes
in Pao2.1,28 

Shifts in CBF
  Chronic hypertension shifts the limits of autoregulation toward higher MAPs, that is, the lowest
and highest blood pressures tolerated are higher.
This inhibits the ability of chronically hypertensive
patients to maintain CBF and CMRo2 during acute
hypotensive stimuli. Pressures that would be tolerated in a nonhypertensive individual can be symptomatic in a hypertensive individual.29,30 Thickening
of the tunica media of arterial walls has been seen in
chronically hypertensive rats.31 Hypertrophy of the
vessel wall results in a decreased ability of the vasculature to dilate in response to a lower perfusion
pressure, and an increased ability to vasoconstrict
at higher perfusion pressures. These changes are
thought to protect the vascular tissue from increased
perfusion pressure.30–32 Experimental evidence suggests that these adaptive changes are reversible, and
that with effective treatment of the hypertension
one can shift the autoregulatory limits back toward
normal.30,33  

Blood Rheology

The viscosity of blood refers to its consistency or
“thickness,” and determines its internal frictional
resistance. This constitutes a determinant of flow.
The  Hagen–Poiselle  equation demonstrates that
blood flow is inversely related to blood viscosity.
Under normal conditions viscosity has little effect on
CBF, but in areas of the brain where autoregulation

M. Malkoff

is depressed or completely lost it assumes a greater
role.34,35 Factors that affect whole blood viscosity are
erythrocyte aggregation, deformability, shear rate,
plasma viscosity, and hematocrit.34
 Hematocrit  is the most important element
influencing whole blood viscosity.36 Increases in CBF
are known to occur during anemia. The increase is
attributable to both reduced arterial oxygen content
and blood viscosity.25
◾◾ Studies in animals and humans have demon-

strated an increase in CBF between 19% and
50% when hematocrit was reduced by 7–14%.36
The mechanism by which decreased hematocrit
changes CBF is not completely clear.
◾◾ Studies evaluating changes in CBF under conditions of low CPP show no change in CBF. This suggests that decreasing the hematocrit has a direct
vasodilatory effect, likely from the decreased
availability of oxygen.36,37 This implies that in areas
of decreased oxygen availability, that is, ischemia,
hemodilution would have no effect in increasing
blood flow.
◾◾ Plasma viscosity has also been shown to affect
CBF, and under conditions of increased CBF this
role is increased.38 Changes in CBF due to hemodilution are attributed to both improved rheology
of the blood as well as a compensatory response
to decreased oxygen delivery.14,25,38

Intracranial Pressure
  The intracranial space contains three incompressible elements: brain (80%), blood (10%), and CSF
(10%). The  Monro–Kellie doctrine  states that if
one changes the volume of any of those three elements there must be a compensatory change in the
other spaces to keep intracranial pressure (ICP)
the same. In pathologic states, when ICP increases
there is initially little change due to small volumes
of CSF shifting into distensible spinal subarachnoid
spaces. However, the exhaustion of this compensatory mechanism causes increases in ICP. The cerebral perfusion pressure (CPP) is defined as mean
arterial pressure  (MAP)   – ICP, and is maintained in
healthy individuals. However, as ICP rises, CPP will
decrease if MAP is not changed. Thus, changes in
ICP can have tremendous effects on CBF through
changes in CPP.   


Cerebral Blood Flow Physiology and Metabolism
Techniques For Measurement

 K ety–Schmidt Method.   Kety and Schmidt
first described their method in 1945.39 This process
assumes that the quantity of any inert substance
taken up by brain tissue is equal to the amount of
substance carried to the brain in the arteries minus
the amount removed by the venous system: the Fick
principle. This method requires the inhalation of
a highly diffusable inert gas, nitrous oxide. Jugular
and arterial concentrations are then monitored to
determine the quantity taken up by brain tissue.39
◾◾ This method has the advantage of proven reli-

ability and easy repeatability. The Kety–Schmidt
method also allows measurement of arteriovenous differences in oxygen, glucose, and lactate
useful for determining cerebral metabolic rate.
◾◾ Disadvantages include invasiveness, inability to
provide regional information, and the variability
of venous drainage.1 
Noninvasive
Xenon-133.   The noninvasive
xenon-133 clearance technique is also predicated
upon the  Fick  principle utilizing xenon-133 as the
inert substance. Xenon-133 is administrated either
intravenously or via inhalation. Detectors positioned
against the head monitor clearance of the isotope,
and arterial concentrations are estimated from the
analysis of end-tidal expired air. The elimination of
isotope can be divided into a fast compartment, representing flow to the gray matter, and a slow compartment, representing flow to the white matter.
◾◾ This method has the advantage of not being inva-

sive, and may be performed at the bedside.
◾◾ The disadvantages of this method are the inabil-

ity to obtain information about deep structures,
such as cerebellum or brain stem, and because
the method assumes a normal blood–brain
barrier.1,40 
Stable Xenon Computed Tomography.   Stable
xenon computed tomography relies on the fact that
xenon is a radiodense, lipid-soluble gas that acts as
a contrast agent during computed tomography. The
patient breathes a mixture of oxygen and 30–35%
stable xenon. Arterial levels of xenon can be calculated from end-tidal expiratory concentrations.
The relatively slow diffusion rate of xenon allows

5

­ igh-resolution imaging from serial tomograms
h
separated by approximately 1 minute.
◾◾ The advantage of this technique is that it provides

excellent anatomic resolution and the blood–
brain barrier partition coefficient can be determined for discrete regions.
◾◾ The disadvantages are that the patient must
remain still for the full study, 20–30 minutes and
must inhale high concentrations of xenon. Xenon
has been shown to function as a mild anesthetic
agent, and has been shown to directly increase
CBF.1,41 
Positron Emission Tomography.   Positron
emission tomography requires the intravenous or
inhalation administration of positron-emitting isotopes of carbon, fluorine, or oxygen. In the body, the
positrons combine with electrons, thus emitting γ
photons that are recorded by detectors surrounding
the body. The detectors are able to precisely locate
the source of the γ-ray emission with very good
resolution.
◾◾ This technique allows for ascertainment of the

metabolic rate of glucose, oxygen, protein synthesis, and CBF. 15O-labeled water is the most common tracer, and is used because it is inert, stable,
has a short half-life, and has few physiologic side
effects.
◾◾ The major drawback to this technique is cost.42 
Transcranial Doppler Ultrasound.   Tran­
scranial Doppler ultrasound (TCD) is a relatively
inexpensive and noninvasive method that allows
repeated measurements and continuous monitoring of CBF. It has a high temporal resolution, making
it ideal to study rapid changes in cerebral hemodynamics.43 Estimates of global CBF can be made
using TCD if the amount of flow through all vessels
is measured simultaneously. It has more common
applications for estimating regional CBF by analyzing the CBF velocity, resistance index, or pulsatility
index of intracranial or extracranial arteries such as
the middle cerebral artery. Increased flow velocities
in both the contralateral and ipsilateral middle cerebral arteries during motor tasks have been demonstrated.24 Other uses of this tool include monitoring
the development of vasospasm after subarachnoid


6

hemorrhage; pattern of collateral flow through the
circle of Willis; and state of artery patency before,
during, or after thrombolytic   therapy .

Cerebral Metabolism

 Cerebral metabolism is a term used to denote the
multitude of biochemical pathways in the brain
collectively geared toward enzyme-mediated use
of substrate to carry out cellular work.10 Because
there is no significant source of energy storage, the
brain is highly dependent on a continuous supply
of energy. As noted previously, the brain receives
15–20% of the total cardiac output, and CBF is
meticulously maintained across a wide variety of
pressures. This ensures adequate substrate delivery. The main energy substrates are high-energy
triphosphates, that is, adenosine triphosphate
(ATP). The central nervous system has small stores
of glycogen, and is almost entirely dependent on the
glucose for production of energy.3,12 As in other tissues, glucose can be either anaerobically degraded
to lactic acid through glycolysis, or oxidized, aerobically degraded, to CO2 and water via oxidative phosphorylation.12 Because the energy yield of glycolysis
is small compared to that with oxidative phosphorylation, the brain relies for its continuing function on
oxidative metabolism.12 Oxygen is delivered to the
tissue where it is involved in a variety of reactions
in the cell, but the majority of oxygen is used in the
generation of energy by the aerobic metabolism of
glucose. In view of the fact that the need for energy
is dependent on oxygen, it is not surprising that the
brain’s metabolic rate of oxygen (CMRo2) in a normal conscious human is approximately 150–160
μmol/100 g per minute or approximately 20% of
the resting body oxygen consumption.13 The global
rate of glucose utilization, also known as the cerebral metabolic rate of glucose (CMRGlu), is 35–30
μmol/100 g per minute. If one assumes normal CBF,
then the extraction fraction (i.e., the proportion
of substance extracted by the brain relative to the
amount delivered to it in arterial blood) is 50% for
oxygen and 10% for glucose.3,34 Most of the brain’s
energy is used for the maintenance and restoration
of ion gradients across the cell membrane. However,
rapid synthesis, degradation, molecular transport, and synaptic transmission are also significant
energy-consuming processes.1,3,12 

M. Malkoff

Oxidative Phosphorylation
 Glucose is transported into the cells of the central
nervous system through two different glucose transports, GLUT-1 and 3. GLUT-1 is located on glia and
endothelial cells and GLUT-3, on neuronal surfaces.
◾◾ Glucose is brought into the cell, and then phos-

phorylated by the enzyme hexokinase into glucose6-phosphate. Through the initial steps of the glycolytic
pathway, the glucose is metabolized into pyruvate.
◾◾ Another critical enzyme and site of regulation is
 phosphofructokinase (PFK)  . This enzyme is inhibited in high-energy states, that is, excess ATP, and
activated in low-energy states. In the presence of
oxygen, the newly generated pyruvate then enters
the citric acid cycle, or Krebs cycle. The pyruvate
is completely oxidized, with the products being
the reduced forms of nicotinamide-adenine dinucleotide (NADH), guanosine triphosphate (GTP),
and flavin adenine dinucleotide (FADH2).
◾◾ For pyruvate to enter the Krebs cycle it is irreversibly decarboxylated into acetyl-coenzyme A
(acetyl-CoA) by the enzyme pyruvate dehydrogenase (PDH). This is another major regulation
point. The PDH is inhibited by NADH and ATP,
both indicators of a high-energy state.
◾◾ The reduced NADH and FADH2 then act as electron donors within the mitochondria to a series of
proton pump complexes designated the electron
transport chain. The final electron donor is oxygen, which is converted to water. The proton gradient that is made by the electron transport chain
is used to generate ATP. This final step is termed
oxidative phosphorylation.
◾◾ This entire process is summarized by the
­equation: Glucose + 6 O2 + 38 ADP + 38 Pi yields
6 CO2 + 44 H2O + 38 ATP. In conclusion, 1 mole of
­glucose yields 38  moles of ATP though oxidative
metabolism.  

Glycolysis
 In the absence of oxygen, anaerobic glycolysis
occurs. During periods of ischemic stress, experimental evidence shows the upregulation of the
GLUT glucose transporters so that more glucose is
imported into the cell for energy production.3,34,44
Glucose is metabolized into pyruvate, as occurs in
the steps before entry into the citric acid cycle.


7

Cerebral Blood Flow Physiology and Metabolism
◾◾ In a low oxygen state, there is depletion of NADH,

and the enzyme PDH is inhibited, impairing the ability of pyruvate to enter the Krebs
cycle. Pyruvate is then transformed into lactate
through a reversible reaction catalyzed by lactate
dehydrogenase.
◾◾ The end result of glycolysis is the production of
lactate and ATP summarized by this equation:
Glucose + 2 ADP + 2 Pi yields 2 lactate + 2 ATP. In
summary, through glycolysis 1 mole of glucose
yields 2 moles of ATP. The accumulation of lactate
is potentially neurotoxic from lactic acidosis  .

Pentose Shunt Pathway
 The main role of the pentose shunt pathway is to
maintain the production of ribose 5-phosphate and
reduced nicotinamide adenine dinucleotide phosphate. This is achieved through the use of phosphorylated glucose. The ribose 5-phosphate and
its derivatives are incorporated into many biomolecules including ATP, NAD, FAD, RNA, and DNA.
This metabolic shunt pathway is critical for maintaining their synthesis. Of the total amount of glucose entering the glycolytic pathway under normal
conditions, about 85% enters the Krebs cycle, 5–10%
is converted to lactate through anaerobic glycolysis,
and the last 5% is metabolized in the pentose shunt
pathway  .

Ketosis
 Metabolism utilizing ketone bodies occurs in circumstances such as starvation or diabetes when
glucose is not available for use by the cell. Adipose
tissue is catabolized, and the products are brought
to the liver. There β-hydroxybutyrate and acetoacetate are generated and transported in the plasma
to the brain. In the brain they are metabolized into 2
molecules of acetyl-CoA, which are able to enter the
citric acid cycle. Under conditions of hypoglycemia,
oxidation of ketone bodies may provide up to 75% of
the total cerebral energy supply.45 

Metabolic Contributions
Neurons

 Gray matter has a metabolic rate that is 3–4 times
greater than that of white matter, and is closely
linked to the functional activity of neurons.3 Most

of the brain’s energy is used for the maintenance
and restoration of ion gradients across cell membrane.1,12 Neurons are the major site of ATP utilization mainly for maintenance of large numbers
of Na+, K+-ATPase pumps located on axonal membranes as noted above. ATP production is also used
for neurotransmitter metabolism and biosynthetic
work such as protein chaperoning and axonal transport. Neurons are also involved in ATP production
through oxidative metabolism of glucose, and under
certain conditions through ketone body metabolism.
Byproducts of neuronal metabolism are responsible for the flow-metabolic coupling between brain
metabolism and CBF  .
Astrocytes

 Glial cells occupy almost half of the volume of the
brain, and there are 20–50 times as many glial cells
as neurons.3,12 However, they consume less than
10% of total cerebral energy due to low metabolic
demands.12 They are instrumental in regulating the
composition of the perineuronal fluid environment.
This occurs in three important ways:
1. Buffering extracellular potassium concen­
trations
2. The glutamate-glutamine cycle
3. The lactate shuttle.
After neuronal activity the extracellular fluid (ECF)
has an increased concentration of potassium ions
(K+). The K+ enters astrocytes through both passive
and active means. The K+ continues to spread along
osmotic gradients within the astrocytes through gap
junctions. This is called K spatial buffering, and is
important because excessive accumulation of K+ in
the ECF can affect membrane polarity.
Glutamate can also accumulate in the ECF after
neuronal activity. In addition to reuptake by neurons, glutamate is also sequestered by astrocytes.
The glutamate is enzymatically changed to glutamine through glutamine synthase, and is subsequently released into the ECF, where it can be taken
up by neurons. This is important for limiting the
action of this excitatory neurotransmitter through
decreasing its presence in the synaptic cleft, and the
transfer of glutamine across the ECF from astrocytes
to neurons has the advantage of being a non-neuroexcitatory process.46 This is termed the glutamateglutamine cycle.


8

Glucose is also taken up by astrocytes, and can be
shunted into one of two pathways:
1. It may enter anaerobic glycolysis, and is
metabolized into lactate.
2. It may be converted into glycogen.47
Lactate produced by astrocytes can then be transported to neurons, where it enters the Krebs cycle and
subsequent oxidative phosphorylation. Energy production from oxidative metabolism of lactate is about
half as effective when compared with glucose.47 
Blood–Brain Barrier

 The blood–brain barrier (BBB) isolates the brain
from variations in body fluid composition, thereby
providing a stable environment for neural–neural
and neural–glial interactions.10 It does this by first
acting as an ionic and molecular sieve through its
involvement in ionic transport and selective transport of small molecules and proteins.26 Large molecules, polar molecules are generally excluded by the
BBB except for metabolically important molecules
such as glucose, amino acids, lactate, and neurotransmitter precursors. The movement of these
molecules into the CBF depends on special transport mechanisms.10,26 For example, the movement
of glucose depends on the transporter GLUT-1.
During times of stress, such as hypoglycemia, the
BBB has been shown to have adaptive responses to
a changing metabolic environment by increasing
the transport of lactate and ketone bodies into the
CBF.48 Second, the endothelial cells contain a host
of enzymes that protect the brain from circulating
neurochemicals and toxins. For example, amino
acid decarboxylase (MAO), pseudocholinesterase,
γ-aminobutyric acid (GABA) transaminase, aminopeptidases, and alkaline phosphatase are present in
the brain capillaries.10 This prevents the unrestricted
entry of potential toxins into the brain  .

Effects of Temperature on Metabolism
 In hypothermic conditions, the flux of glucose going
through glycolysis and the Krebs cycle declines. The
energy state of the brain, as measured by the ATP/
Pi ratio, increases, suggesting that energy-consuming reactions are reduced more than ATP synthesis.49 Measurements of the brain’s metabolic rate,
for example, CMRo2, is reduced 2-to 4-fold by a
10-­degree decrease in temperature.49 On the other

M. Malkoff

hand, in the setting of hyperthermia several studies using different animal models have observed
a rise in CMRO2, supporting the idea that hyperthermia itself leads to a rise in whole brain energy
turnover.50 

Suggested Readings
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Chillon JM, Baumbach GL. Autoregulation of cerebral blood
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Gonzalez C, Estrada C. Nitric oxide mediates the neurogenic
vasodilation of bovine cerebral arteries. J Cereb Blood Flow
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Harrison MJ, Pollock S, Thomas D, Marshall J. Haematocrit,
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Khurana VG, Smith LA, Weiler DA, et al. Adenovirusmediated gene transfer to human cerebral arteries. J Cereb
Blood Flow Metab 2000; 20(9): 1360–71.
Rebel A, Ulatowski JA, Kwansa H, Bucci E, Koehler RC.
Cerebrovascular response to decreased hematocrit: effect
of cell-free hemoglobin, plasma viscosity, and CO2. Am J
Physiol Heart Circ Physiol 2003; 285(4): H1600–8.
Symon L, Branston NM, Strong AJ. Autoregulation in acute
focal ischemia. An experimental study. Stroke 1976; 7(6):
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Wade JP, Taylor DW, Barnett HJ, Hachinski VC. Hemoglobin
concentration and prognosis in symptomatic obstructive
cerebrovascular disease. Stroke 1987; 18(1): 68–71.
Winn HR. Adenosine and its receptors: Influence on cerebral
blood flow. In Welch KMA, Caplan LR, Reis DJ, et al. (eds),
Primer on Cerebrovascular Diseases. San Diego: Academic
Press 1997, pp 77–70.

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