Reduced response of cerebral blood flow to hypercapnia: restoration by extracranial-intracranial bypass.

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.