REGIONAL CEREBRAL BLOOD-FLOW (SPECT) DURING ANESTHESIA WITH ISOFLURANE AND NITROUS-OXIDE IN HUMANS

Nitrous oxide and isoflurane have different effects on absolute cerebr al blood flow (CBF) and regional distribution of CBF in humans. In thi s study we examined the effects of isoflurane in combination with nitr ous oxide on CBF. We studied 10 patients (two groups of five patients, ASA I) anaesthetized with 50% nitrous oxide and either 0.5 or 1.0 MAC of isoflurane during normocapnia (Pa-CO2 5.7 kPa) using two-dimension al CBF measurement (CBFxenon) (i.v. (133)xenon washout technique) and a three-dimensional method for measurement of regional CBF (rCBF) dist ribution with SPECT (single photon emission computer-aided tomografy w ith Tc-99m-HM PAO). The results were compared with 1.0 MAC of isoflura ne from a previous study performed in exactly the same way as the pres ent investigation. During normocapnia, anaesthesia with 50% nitrous ox ide and 0.5 MAC of isoflurane resulted in a mean CBFxenon of 45 (SEM 5 ) ISI units. Increasing the isoflurane concentration to 7.0 MAC had no significant effect on mean CBFxenon (53 (5) ISI units). Both flow val ues were significantly (P=0.01) higher than the CBFxenon value obtaine d when 1 MAC of isoflurane was administered alone (33 (3) ISI units). There were no significant differences in rCBF distribution regardless of whether or not isoflurane was given alone or together with nitrous oxide at 0.5 or 1 MAC. In all situations there were higher relative fl ows in subcortical regions (thalamus and basal ganglia, 10%) and in th e pens (7-10% above average). rCBF in the cerebellum was approximately 10% greater than average. In summary, we have we have found that mean CBF was greater with combined nitrous oxide and isoflurane anaesthesi a than previously found with isoflurane alone; however, relative flow distribution was similar.