Cerebral hemodynamics during cerebral ischemia induced by acute hypotension.

The extreme dependence of the brain upon its circulation for substrates essential for the maintenance of its metabolic activity is well recognized. A cessation of the cerebral circulation for only a few minutes, as occurs in cardiac arrest, results in irreversible brain damage. The brain, for the most part, is an aerobic organ and its large oxygen demands probably account for its unusual susceptibility to circulatory disturbances. The oxygen consumed by the normal adult brain (3.5 cc. per 100 Gm. per min.) is extracted from approximately 800 cc. of blood passing through it each minute. The minimum blood flow requirements of the brain, i.e., the rate at which signs of cerebral ischemia become manifest, have not been accurately determined. Previous efforts to estimate the critical rate have been made both in animals (1) and in man (2) but the procedures utilized for the determination of cerebral blood flow were not sufficiently quantitative to warrant definite conclusions. In order to study this problem, cerebral hemodynamic and metabolic changes were determined before and during acute reductions in arterial pressure induced by drug administration and/or postural adjustments.