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Nervous System
central venous pressure that is reflected back into the
internal jugular veins and then to the dural sinuses. This
briefly raises the ICP.
Regulation of Cerebral Blood Flow
The blood flow to the brain is maintained at approxi-
mately 750 to 900 mL/minute or 15% of the resting car-
diac output.
2
The regulation of blood flow to the brain is
controlled largely by autoregulatory or local mechanisms
that respond to the metabolic needs of the brain. Cerebral
autoregulation has been classically defined as the ability
of the brain to maintain constant cerebral blood flow
despite changes in systemic arterial pressure. This allows
the cerebral cortex to adjust cerebral blood flow locally to
satisfy its metabolic needs. The autoregulation of cerebral
blood flow is efficient within an MABP range of approxi-
mately 60 to 140 mm Hg.
2
Although total cerebral
blood flow remains relatively stable throughout marked
changes in cardiac output and arterial blood pressure,
regional blood flow may vary markedly in response to
local changes in metabolism. If blood pressure falls below
60 mm Hg, cerebral blood flow becomes severely com-
promised, and if it rises above the upper limit of auto-
regulation, blood flow increases rapidly and overstretches
the cerebral vessels. In persons with hypertension, this
autoregulatory range shifts to higher MABP levels.
Metabolic factors affecting cerebral blood flow
include an increase in carbon dioxide and hydrogen
ion concentrations. Increased carbon dioxide provides
a potent stimulus for vasodilation—a doubling of the
PCO
2
in the blood results in a doubling of cerebral
blood flow. Carbon dioxide is thought to increase cere-
bral blood flow by first combining with water to form
carbonic acid, with subsequent dissociation into hydro-
gen ions, which then causes vasodilation of the cere-
bral vessels. Other substances, such as lactic acid and
pyruvic acid, which increase the acidity of brain tissues,
will produce a similar increase in cerebral blood flow.
Oxygen deficiency also influences cerebral blood flow.
Except during periods of intense brain activity, the rate
of oxygen utilization by the brain remains within a nar-
row range. If blood flow to the brain becomes insuf-
ficient to supply this needed amount of oxygen, the
oxygen deficiency causes the cerebral vessels to dilate,
returning cerebral blood flow to near normal.
The sympathetic nervous system also contributes to
the control of blood flow in the large cerebral arteries
and the arteries that penetrate into the brain substance.
2
Under normal physiologic conditions, local regulatory
and autoregulatory mechanisms override the effects of
sympathetic stimulation. However, when local mecha-
nisms fail, sympathetic control of cerebral blood pres-
sure becomes important. For example, when the arterial
pressure rises to very high levels during strenuous exer-
cise or in other conditions, the sympathetic nervous
system constricts the large and intermediate-sized super-
ficial blood vessels as a means of protecting the smaller,
more easily damaged vessels. Sympathetic reflexes are
also thought to cause vasospasm in the intermediate and
large arteries in some types of brain damage, such as
that caused by rupture of a cerebral aneurysm.
Stroke (Brain Attack)
Stroke is the syndrome of acute focal neurologic deficit
resulting from a vascular induced disorder that injures
brain tissue. Stroke remains one of the leading causes
of morbidity and mortality in the United States.
18,19
The
term
brain attack
has been promoted to raise aware-
ness that time-dependent tissue damage occurs and that
rapid emergency treatment is necessary, similar to that
with heart attack.
There are two main types of strokes: ischemic and
hemorrhagic. Ischemic strokes reflect infarctions caused
by an interruption of blood flow in a cerebral vessel
and are the most common type of stroke, accounting
for about 87% of all strokes.
7
The less common hemor-
rhagic strokes, which have a much higher fatality rate
than ischemic strokes, are caused by spontaneous bleed-
ing into brain tissue. Intracerebral hemorrhage can also
result from ruptured cerebral aneurysms and bleeding
from arteriovenous malformations. Additionally, the
latest classifications define silent CNS infarction as isch-
emic lesions found incidentally on imaging, and tran-
sient ischemic attack (TIA) reflecting transient symptoms
without infarction on imaging.
20
Ischemic Stroke
Ischemic strokes result from a diverse set of causes of
cerebrovascular obstruction by thrombosis or emboli
(Fig. 37-14). Among the major risk factors for ischemic
Atrial
fibrillation
Cardiogenic
emboli
Carotid
plaque with
arteriogenic
emboli
Intracranial
atherosclerosis
Mitral
valve
disease
Left
ventricular
thrombi
FIGURE 37-14.
The most frequent sites of arterial and cardiac
abnormalities causing ischemic stroke.
