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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_885_Библиотеки_им_академика_М_И_Перельмана.pdf
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II: System-Based Management
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4. Central Nervous System
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Cerebral Blood Flow
Chapter 4-(i)
Yasuaki Harasaki, MD* and Kathryn Beauchamp, MD
* Assistant Professor of Neurosurgery, University of Colorado School of Medicine
†
Chief of Neurosurgery, Denver Health Medical Center
Take Home Points
• The brain is a unique organ in the body due to its susceptibility to lack of
oxygen and high metabolic requirements.
• The brain is supplied by four main arteries that form a circular interior artery
to help maintain consistent blood flow to the brain in times of occlusion and
hypoxia.
• Serious loss of blood flow to the brain can be caused by ischemic and
hemorrhagic stroke. Prolonged blood loss to the brain can result in tissue
death and altered mental status.
• Clinical signs of strokes should be assessed and a timely response can lessen
the long term effects of blood loss.
• Functional magnetic resonance imaging (fMRI) is used to measure cerebral
blood flow clinically. With improvement of this technology, physicians can
accurately track changes in rate of oxygen saturation in the cerebral blood.
†
Contact information: Denver Health Medical Center, University of Colorado Health
Sciences Center, 777 Bannock Street, MC 0206, Denver, CO 80204; Tel.: 303-436-5842
(Kathryn Beauchamp), email: Yasuaki.Harasaki@dhha.org; kathryn.beauchamp@dhha.org
43
44 Y. Harasaki and K. Beauchamp
• The manner in which drugs and other toxins interact with the brain is not
found in the body due to the blood brain barrier.
Background
• The brain uses 25% of the oxygen while only consisting 2.5% of the body’s
weight. Cerebral blood flow (CBF) is defined by cerebral perfusion pressure
(CPP)/Resistance (R). Blood for the brain is supplied by two internal carotid
arteries as well as two vertebral arteries.
• The brain stores little oxygen in its tissues compared to how much it
requires. The blood oxygenation level dependent (BOLD) effect is a dynamic
change in blood flow to the active parts of the brain.
• BOLD results in a 5 –10% increase in regional blood flow to the brain.
• The amount of oxygen the brain receives can vary between patients due to
cardiovascular issues such as high blood pressure and blockages in the vessel.
These can also lead to a cerebral vascular accident (or stroke).
{ Ischemic strokes are caused by an occlusion of blood to the brain. The
body can break some of these blockages up naturally and quickly which
results in a transient ischemic attack (TIA).
{ Symptoms of a TIA include partial paralysis or numbness to the face,
trouble in speaking or thinking clearly.
{ Hemorrhagic strokes can be caused by the rupture of weakened vessel
walls resulting in blood in the brain. High blood pressure and smoking
significantly increases your risk of this type of stroke.
• The blood brain barrier allows protection for the cerebrospinal fluid, of the brain
and spinal cord, from harmful drugs or toxins that may be present in the blood.
{ This is a highly coordinated exchange of limited molecules that limits
which molecules can enter the brain based on size and permeability.
{ Not all substances are toxic to the body. Some of these molecules are useful
for other organs; however they can be toxic to the neurons of the brain.
{ Certain circumstances, including ischmic stroke, can alter the selectivity of
the blood brain barrier and result in larger substances entering the space.
Main Body
• Classically, Lassen et al. (1959) described a range of autoregulation in which
cerebral blood flow remained constant through a range of cerebral perfusion
pressure between 50–150 mmHg.
Cerebral Blood Flow 45
• More contemporary data in healthy volunteers (Tan, 2012) suggests that the
autoregulatory cerebral blood flow plateau exists for a far narrower range of
MAP fluctuation across approximately 10 mmHg, and that CBF is more
passively determined by CPP.
• Normal white matter CBF 18–25 ml/100 g/min.
• Normal gray matter CBF 67–80 ml/100 g/min.
• The pressure gradient driving cerebral blood flow is determined by the mean
arterial pressure (MAP) and the intracranial pressure (ICP). The actual
regional cerebral blood flow is under further control of autoregulatory
mechanisms which respond to variables such as PaCO
and autonomic inputs.
2
• Cerebral perfusion pressure (CPP) = mean arterial pressure (MAP) —
intracranial pressure (ICP)
• Hypotension consisting of systolic blood pressure < 90 mmHg should be
avoided.
• In the setting of elevated ICP, both ICP directed and CPP directed manage-
ment have been described, with no clear superiority of either strategy.
• In CPP directed management, goal CPP is 50–70 mmHg.
• Cerebral perfusion pressure (CPP) provides the main pressure gradient
driving cerebral blood flow, and is defined as the difference between the mean
arterial pressure (MAP) and the intracranial pressure (ICP). This may be calculated in real-time in the presence of a fiber optic coupled intracranial
pressure monitor (see next chapter for ICP monitoring and management).
• Episodes of hypotension defined as systolic blood pressure <= 90 mmHg
have been associated with poor outcomes (Bratton, Chestnut et al., 2007).
• Cerebral blood flow is further modified by autoregulatory mechanisms which
act via local vasoconstriction/vasodilation (Willie, Tseng et al., 2014).
{ Increase in PaCO
{ Increase in sympathetic input leads to vasoconstriction.
leads to local vasodilation.
2
• Management strategies targeting ICP and CPP in the setting of elevated ICP
are both utilized. Current guidelines for management of severe traumatic brain
injury (Bratton, Chestnut et al., 2007) support goal CPP of 50–70 mmHg.
{ CPP < 50 mmHg associated with poor functional outcomes.
• CPP > 70 mmHg associated with five-fold increased risk of adult respiratory
distress syndrome (ARDS).
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Chapter 4-(ii)
Intracranial Hypertension
M. Dustin Richardson, MD* and Kathryn Beauchamp, MD
* Neurosurgical Resident, University of Colorado School of Medicine
†
Chief of Neurosurgery, Denver Health Medical Center
†
Take Home Points
• Intracranial hypertension is the pathologic elevation of pressure in the
intracranial space, which is defined as an intracranial pressure (ICP) greater
than 15–20 mmHg.
• Intracranial hypertension follows the Monroe-Kellie doctrine, which states
that within a rigidly fixed volume of space the intracranial pressure will
increase or decrease if the volume of the one or more of the intracranial
contents changes or if additional contents are added or subtracted.
• Brain function is determined by adequate cerebral blood flow to meet the
cerebral metabolic rate of oxygen consumption (CMRO2). The cerebral blood
flow is influenced by cerebral perfusion pressure, which is calculated by
subtracting the ICP from the mean arterial pressure.
Contact information: (M. Dustin Richardson) University of Colorado at Denver and Health
Sciences Center, 12631 E. 17
Bannock Street, MC 0206, Debnver, CO 80204; Tel.: 303-724-2305. Email: Dustin.
Richardson@ ucdenver.edu; kathryn.beauchamp@dhha.org
th
Ave., C307, Aurora CO 80045; (Kathryn Beauchamp) 777
47