We have studied the effects of hypocapnia on cerebrovascular changes in two MAC-equivalent anaesthetic regimens, using the transcranial Doppler technique as an index of cerebral blood flow (CBF) in 24healthy ASA I patients undergoing spinal surgery. Eight of the patients were subjected to carbon dioxide reactivity challenges in the awake state. Before surgery, the other 16 patients received, in random order, either 1.15% isoflurane in oxygen or 0.5% isoflurane with 70% nitrous oxide. Carbon dioxide reactivity was calculated for each group as the increase in flow velocity per kPa change in PÉCO2 (cm s−1 kPa−1). It was significantly greater for the isoflurane group (14.09 (SD 2.44) cm s−1 kPa−1) and significantly less for the isoflurane—nitrous oxide group (7.95 (1.32) cm s-−1 kPa−1) compared with the awake group (11.24 (0.95) cm s−1 kPa−1). We conclude that cerebrovascular responsiveness to changes in arterial carbon dioxide concentration is influenced markedly by the anaesthetic procedure. Hyperventilation is more likely to affect CBF during isoflurane anaesthesia than during an MAC-equivalent isoflurane—nitrous oxide anaesthesi
