CEREBROVASCULAR CARBON-DIOXIDE REACTIVITY DURING EXPOSURE TO EQUIPOTENT ISOFLURANE AND ISOFLURANE IN NITROUS-OXIDE ANESTHESIA

We have studied the effects of hypocapnia on cerebrovascular changes i n two MAC-equivalent anaesthetic regimens, using the transcranial Dopp ler technique as an index of cerebral blood flow (CBF) in 24 healthy A SA I patients undergoing spinal surgery. Eight of the patients were su bjected to carbon dioxide reactivity challenges in the awake state. Be fore 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 increas e in flow velocity per kPa change in PE'(CO2) (cm s-1 kPa-1). It was s ignificantly 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 chan ges 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 isoflu rane-nitrous oxide anaesthesia.