Reduced response of cerebral blood flow to hypercapnia: Restoration by extracranial—intracranial bypass

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Abstract
Cerebral blood flow, estimated by the xenon clearance technique, has been used to study eight patients before and after extracranial—intracranial (EC—IC) bypass surgery. Response of cerebral blood flow to hypercapnia was also measured to estimate cerebral reactivity, an indicator of cerebral vasodilator reserve capacity. Measurements in all patients were repeated 3 months after surgery. Resting cerebral blood flow was not increased by the operation but cerebral reactivity in the ipsilateral hemisphere was significantly increased in all patients (P = 0·002). Reactivity also increased in the contralateral hemisphere in six of the eight patients (P = 0·065). The response of cerebral blood flow to hypercapnia may prove useful in the selection of symptomatic patients with carotid occlusions or inaccessible stenoses for revascularization by EC—IC bypass.