NITROUS-OXIDE ADDED TO ISOFLURANE INCREASES BRAIN ARTERY BLOOD-FLOW AND LOW-FREQUENCY BRAIN ELECTRICAL-ACTIVITY

Although changes in cerebral blood flow (CBF) and the electroencephalo gram (EEG) have been reported with nitrous oxide (N2O) administration, the interaction of these parameters is unclear. The purpose of this s tudy was to measure CBF and EEG during N2O administration in eight pat ients. A craniotomy was performed and CBF was measured in major brain arteries using a transit time Doppler flowmeter. EEG was recorded bila terally from frontooccipital leads. Power spectrum analysis was perfor med on the EEG and power for delta, theta, alpha, and beta frequency b ands analyzed over time. Arterial blood pressure was recorded continuo usly. N2O (66%) was added to the inspired gases during isoflurane anes thesia (0.8% end tidal) under hypocapnic (Paco2 = 29 mm Hg) and normoc apnic conditions (Paco2 = 39 mm Hg). During hypocapnia, N2O administra tion decreased alpha EEG activity and increased delta activity but did not change CBF. During normacapnia, N2O produced similar but greater changes in EEG and increased CBF 39%. In three patients, the isofluran e concentration was increased to 1.6% end tidal during normocapnia. N2 O administration in these patients also enhanced delta EEG activity an d increased CBF. The slowing of EEG activity with N2O is temporally re lated to increases in CBF during normocapnia. Hypocapnia abolished the increase in CBF during N2O and attenuated the shift of EEG to delta a ctivity.