michigan hypertension core curriculum - State of Michigan

34 Hypertension Core Curriculum
Table 5
8. Autoregulation of Cerebral Blood Flow
Under normal circumstances, cerebral blood flow (CBF) is ~ 50 ml/100g/min. CBF is
proportional to cerebral oxidative metabolism and is highly sensitive to pCO 2 ; higher levels of
carbon dioxide cause higher CBF. The cerebral circulation does, however, have a physiological
protective mechanism - cerebral autoregulation - that maintains CBF relatively constant across
a broad range of systemic perfusion pressures. In normal, non-hypertensive persons, cerebral
autoregulation keeps CBF constant between mean arterial pressures (MAP) of 50 - 150 mm Hg
(figure 6).
Figure 6
This is accomplished by dilatation and constriction of cerebral resistance vessels in
response to reductions and elevations, respectively, in systemic BP. In chronic hypertension,
when BP is poorly controlled, the entire autoregulatory curve is shifted to the right. The curve is
shifted rightward, in part, because the pressure-related hypertrophy of the cerebral resistance
vessels that diminish their capacity for maximum dilation (necessary to maintain blood flow when
systemic pressure falls). However, the hypertrophy of these same arterial resistance vessels
allows the arteriole to withstand higher than normal BP levels before its structural and functional
integrity is compromised. Accordingly, in chronic hypertension that is not well controlled, the
lower and upper limits of cerebral autoregulation move to higher BP levels.