Effects of propofol and nitrous oxide on middle cerebral artery flow velocity and cerebral autoregulation

We studied the effects of adding 50% nitrous oxide to propofol anaesthesia administered by target‐controlled infusion on middle cerebral artery flow velocity and autoregulatory indices derived from transient hyperaemic response tests. Nine healthy (ASA 1) adult patients scheduled to undergo elective surgery were recruited. A standardised anaesthetic comprising alfentanil 10 µg.kg−1, propofol via a target‐controlled infusion pump and vecuronium 0.1 mg.kg−1 was used. Transcranial Doppler ultrasonography was used to measure middle cerebral artery (MCA) blood flow velocity and the transient hyperaemic response test was used to assess cerebral autoregulation. These measurements were performed while awake and then at an ‘induction’ target concentration of propofol (the target at which consciousness was lost, mean 6.2 (SD 1.1) µg.ml−1). The measurements were repeated after the addition of 50% nitrous oxide to the breathing gas mixture. Propofol caused a significant decrease in MCA flow velocity and a significant increase in the strength of autoregulation. The addition of nitrous oxide had no significant effect on MCA flow velocity or cerebral autoregulation. These results suggest that addition of 50% nitrous oxide does not influence propofol‐induced changes in cerebral haemodynamics.