HYPERVENTILATION REVERSES THE NITROUS OXIDE-INDUCED INCREASE IN CEREBRAL BLOOD-FLOW VELOCITY IN HUMAN VOLUNTEERS

Because hypocapnia is routine during general anaesthesia for intracran ial procedures, we have compared, in 13 healthy volunteers, the effect of normocapnia (PE'(CO2) 5.3 kPa) and hypocapnia (PE'(CO2) 3.3 kPa) o n mean blood flow velocity in the middle cerebral artery (Vmca) during normoventilation and hyperventilation with air and with 50% nitrous o xide in oxygen. After replacement of air with 50% nitrous oxide in oxy gen, there was an increase in mean Vmca during normoventilation (air: mean 68.23 (so 16.98) cm s(-1) vs nitrous oxide in oxygen: 90.69 (20.4 1) cm s(-1); P < 0.01), whereas during hyperventilation mean Vmca valu es were similar regardless of the inhaled gas mixture (air: 43.46 (9.9 7) cm s(-1) vs nitrous oxide in oxygen: 41.69 (8.08) cm s(-1). Our dat a suggest that the nitrous oxide-induced increase in mean Vmca can be blocked by hyperventilation.