Cerebral Hemodynamics during Cerebral Ischemia Induced by Acute Hypotension1

Abstract

Cerebral hypoxia was induced in a heterogeneous group of 46 patients by lowering the arterial pressure acutely with intravenous hexamethonium and /or by tilting. Despite the patients age, original level of arterial pressure, or original rate of cerebral blood flow, signs of cerebral hypoxia developed when the cerebral blood flow was reduced to 31.5 cc/minute/100 g of brain (standard error 0.98). The level of arterial pressure at which signs of cerebral hypoxia became manifest varied between 100.7 and 40.9 mm Hg. The cerebral vascular resistance fell significantly in all groups, except in subjects with spontaneous postural hypotension and malignant hypertension. In these 2 groups, cerebral hypoxia developed at comparatively normal levels of arterial pressure. Angina and ecg changes were absent during cerebral hypoxia. Data demonstrate that there is a critical rate of cerebral blood flow at which signs of cerebral hypoxia become manifest and suggest that the brain is more sensitive to hypoxia than the heart.