Blood pressure regulation in ischemic cerebral vascular disease

The control of blood pressure in acute ischemic cerebrovascular disease has received little critical analysis. Most physicians are aware that precipitous drops in mean arterial blood pressure (MABP)should be avoided in patients with acute focalbrain ischemia, even if they are unfamiliar with the physiologic principle of cerebral blood flow (CBF) autoregulatory paralysis. However, specific antihypertensive regimens and guidelines for blood pressure regulation in acute stroke have not been critically evaluated. Autoregulation, or the ability to maintain blood flow over a wide range of perfusion pressures, is welldeveloped in the cerebral circulation. Normally, CBF is maintained at normal levels (approximately 50 mll min/lOO g) with mean arterial pressures between 50 and 150 mm Hg. In this range of arterial pressures, cerebral vessels constrict in response to elevations in pressure and dilate during decreases in pressure, and the vascular response is adequate to maintain blood flow. Autoregulation is not instantaneous and may require a few seconds for initiation. During gradual changes in perfusion pressure, blood flow is maintained, but during sudden changes in pressure, a substantial period of time occurs before blood flow returns to the basal level. Experimental studies in several species indicate that autoregulation of blood flow in the cerebrum results from vascular responses of both arteries and microvessels. In contrast, in the brainstem, large arteries apparently have little involvement in autoregulation. Microvascular pressure, therefore, is higher in the brainstem, and small arteries and arterioles must respond with greater changes in resistance to maintain blood flow.