Upper Limit of Autoregulation of Cerebral Blood Flow in the Baboon

The upper limit of autoregulation of cerebral blood flow was studied in ten young baboons. Blood pressure was increased by infusing angiotensin, and cerebral blood flow was measured by the intracarotid 133 xenon injection method. Autoregulation was maintained until blood pressure was 30-40% above resting values. At this blood pressure level, cerebrovascular resistance reached a maximum. Any additional increase in blood pressure resulted in an increase in cerebral blood flow and a decrease in cerebrovascular resistance; this situation is designated the "breakthrough of autoregulation." In four baboons subjected to unilateral sympathetic denervation, autoregulation of cerebral blood flow was studied bilaterally; no difference in the upper limit of autoregulation was found between the intact and the sympathectomized hemisphere. The breakthrough of autoregulation supposedly plays an important role in the pathogenesis of acute hypertensive encephalopathy. The old concept of hypertensive cerebral vasospasm has been revised in recent years, and it is now generally recognized that acute hypertensive encephalopathy is caused by focal overdistention of brain arterioles with lesions of the blood-brain barrier. However, whether this condition is associated with a high cerebral blood flow in the clinical syndrome has not been investigated.