michigan hypertension core curriculum - State of Michigan

Clinical Implications: In the setting of chronic, poorly controlled hypertension rapid reductions in
BP can lead to cerebral ischemia attributable to reductions in cerebral blood flow at BP levels that
are within the hypertensive range.
A. Reductions in MAP below the Lower Limit of CBF Autoregulation
As systemic perfusion pressure falls below the lower limits of CBF autoregulation, cerebral
resistance vessels have dilated maximally but are no longer able to maintain cerebral blood flow.
As CBF falls, cerebral oxidative metabolism is supported by augmentation of oxygen extraction
from the declining cerebral blood flow. At this point symptoms of cerebral hypoperfusion such as
lethargy, confusion, somnolence, etc. can appear. When CBF falls below ~10 ml/100g/min, the
ionic gradient across neuronal cell membranes becomes disrupted leading to calcium influx and
potassium efflux and neuronal cell injury/death occur. Unlike the heart where oxygen extraction
is maximal at rest, the brain can extract greater amounts of oxygen from blood traversing it
when cerebral blood flow falls. This provides at least some measure of cushion against cerebral
ischemia.
B. Increases in MAP above the Upper Limit of CBF Autoregulation
As systemic perfusion pressure rises cerebral resistance vessels normally constrict.
When systemic perfusion pressure rises above the upper limits of CBF autoregulation then CBF
increases dramatically and there is increased permeability of the cerebral vasculature leading
to cerebral edema and increased intracranial pressure. Raised intracranial pressure has two
important physiological effects. First, systemic BP increases further. Secondly CBF may fall
though this tendency is counterbalanced by the reflex rise in systemic perfusion pressure.
Symptoms of CNS dysfunction again can occur such as seizures, lethargy, stupor, coma, etc.
The clinical term for this life-threatening clinical situation is hypertensive encephalopathy. Table
6 displays clinical symptoms that characterize hypertensive encephalopathy.
Table 6
9. Blood Pressure and Cerebral Blood Flow Regulation during Acute Brain Ischemia
Hypertension is the major risk factor for stroke. Approximately 80% of patients with acute
stroke have elevated BP at the time of hospital admission. The observation of elevated BP at the
time of admission has also been made in persons without antecedent hypertension.
Blood pressure reflexively rises during acute cerebral ischemia and brain trauma. During
acute cerebral ischemia, cerebral blood flow autoregulation is disrupted in the ischemic areas
of the brain surrounding the already infarcted area. Disruption of CBF autoregulation is greater
for brainstem ischemic lesions than for hemispheric lesions; severe hemispheric lesions cause
greater disruption of autoregulation than minor hemispheric lesions; and subcoritcal lesions
NKFM & MDCH 35