michigan hypertension core curriculum - State of Michigan

cause greater disruption of CBF autoregulation than cortical lesions. This disruption of CBF
autoregulation can last for weeks. The disruption of autoregulation in the ischemic areas of the
brain leads to a dependence of systemic perfusion pressure for blood flow and therefore oxygen
delivery into these areas (figure 7).
Figure 7. Relation of Systemic Perfusion Pressure to Cerebral Blood Flow
On the other hand, excessive blood flow as can occur with very elevated systemic BP
can facilitate formation of cerebral edema. Effective cranial perfusion pressure (CPP) is the
difference between MAP and intracranial pressure (ICP). A significant proportion of persons with
acute stroke have elevated ICP; therefore, the rise in systemic perfusion pressure might also
help maintain CBF in the ischemic area of the brain when ICP is elevated. The highest levels
of systemic BP appear to be associated with intracranial hematomas. Interestingly, this is also
the one stroke subtype where autoregulation of CBF may not be impaired, particularly when the
intracranial hematoma is small (< 45 ml).
Blood pressure perceptibly falls during the first 4 days after acute stroke and continues to
fall spontaneously through 7 - 10 days post-stroke. In one large series of acute stroke patients,
the fall in BP by day 10 averaged 20/10 mm Hg. Diurnal variation in BP is abnormal during acute
stroke. The normal nocturnal declines in BP, both systolic and diastolic, are markedly attenuated
to absent.
The therapeutic and clinical implications of the abnormal autoregulation of CBF in acute
cerebral ischemia are profound. Though elevated BP is a risk factor for stroke and, in the setting of
acute stroke, might facilitate cerebral edema formation, the dependence of CBF in ischemic brain
areas on systemic perfusion pressure makes antihypertensive treatment risky. Even moderate
reductions in BP might be associated with worsening cerebral ischemia and infarct extension,
particularly in the ischemic penumbra (the damaged but still viable brain tissue surrounding the
infarct).
36 Hypertension Core Curriculum