Differential Increases in Blood Flow Velocity in the Middle Cerebral Artery After Tourniquet Deflation During Sevoflurane, Isoflurane Or Propofol Anaesthesia (Report‪)‬

Pneumatic tourniquets are often used on the extremities during orthopaedic surgery to obtain a bloodless surgical field. Ischaemic metabolites released after tourniquet deflation provoke several physiological alterations (1,2). Decreases in arterial pH or increases in PaCO2 and lactate are known to occur immediately after tourniquet deflation (3). [P.sub.a][CO.sub.2] plays a central role in the regulation of cerebral vasomotor tone (4). Elevations of [P.sub.a]C[O.sub.2] result in dilation of cerebral arteries and consequently increased cerebral blood flow (CBF). The rapid elevation in [P.sub.a]C[O.sub.2] reported after tourniquet deflation (1-3) would thus be expected to result in a corresponding increase in CBF. There have been several reports examining the alterations in CBF or cerebral blood flow velocity after tourniquet deflation (2,3). The study by Hirst et al (3) and a previous report by our group2 showed that a transient increase in CBF does occur after tourniquet deflation. There are published data showing that the use of volatile anaesthetics, such as isoflurane or sevoflurane, may produce altered vasodilatory or vasoconstrictive responses in the cerebral arteries in response to changes in arterial carbon dioxide (C[O.sub.2])(5-7). In previous studies, we showed that cerebrovascular C[O.sub.2] reactivity was greater under isoflurane anaesthesia than under sevoflurane anaesthesia (6,7). In contrast, cerebrovascular C[O.sub.2] reactivity is preserved under propofol anaesthesia (8-13). These reports suggested that the degree of cerebral vasodilation after tourniquet deflation might be different with different anaesthetics. We hypothesised that cerebral vasodilation or vasoconstriction as indicated by changes in cerebral blood flow velocity after tourniquet deflation would have different degrees or time courses with different anaesthetics.