EFFECT OF 1-MAC OR 2-MAC ISOFLURANE WITH OR WITHOUT KETANSERIN ON CEREBRAL BLOOD-FLOW AUTOREGULATION IN MAN

We have studied the effect of 1 or 2 MAC isoflurane with or without ke tanserin on cerebral blood flow (CBF), cerebral oxygen metabolism (CMR o(2)) and CBF autoregulation in 20 adult patients undergoing lumbar di sc surgery. Ten patients received ketanserin and 10 isotonic saline. C BF measurements were started after Ih of infusion of saline or ketanse rin. The patients were anaesthetized with thiopentone 5 mg kg(-1) foll owed by isoflurane. During 1 MAC of isoflurane, baseline values were r ecorded and then CBF autoregulation was examined (mean arterial pressu re increased by about 30% with angiotensin). The sequence was repeated with 2 MAC of isoflurane. CBF was measured by the iv. xenon-133 techn ique. CMRo(2) was calculated as the product of CBF and the cerebral ar terio-venous oxygen content difference. Ketanserin had no effect on CB F, CMRo(2) or CBF autoregulation during isoflurane anaesthesia, theref ore all patients were pooled for evaluation of the effect of isofluran e. increasing isoflurane anaesthesia from 1 to 2 MAC increased mean CB F from 41 to 49 ml/100 g min(-1) (P < 0.01) and decreased mean CMRo(2) from 1.5 to 1.1 ml/100 g min(-1) (P < 0.001) and thus abolished the c oupling between flow and metabolism. The CBF autoregulation test indic ated that autoregulation was disrupted at 2 MAC, but not during 1 MAC isoflurane anaesthesia.