Relationship between volume flow and velocity in the cerebral circulation.

The relationship between the velocity of the cerebral circulation and the cerebral blood flow was explored at varying levels of PaCO(2), systemic arterial pressure, intracranial pressure, and perfusion pressure, using radioisotope techniques in baboons. Only at low flow rates did velocity increase with flow, and then non-linearly; at high rates velocity increased progressively less. Changes in flow are reflected by changes in velocity in such restricted circumstances that mean circulation time is a very unreliable indication of cerebral blood flow.