Intracranial hypotension increases cerebral blood flow. In dogs the average increase in cortical blood flow was 30 ml./100 g/min (47%) when the intracranial pressure was lowered acutely from 100 to 40 mm CSF. Permanent intracranial hypotension was established in seven demented patients using a ventriculoatrial shunt. The mean post shunt pressure was 50 mm CSF. In this group, the cerebral vascular resistance decreased 32%, the cortical blood flow increased 37%, and the relative weight of functional grey matter increased 44%. The systemic blood pressure was 8% lower. The increase in cerebral blood flow is the result of an increase in the pressure differential between the precapillary arterioles and the veins. In addition, the vessels dilate in response to the decreased external pressure. This increase in cerebral blood flow may be the mechanism for improvement in patients with normal pressure hydrocephalus who are shunted.
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