The influence of propofol with and without nitrous oxide on cerebral blood flow velocity and CO2 reactivity in humans.

The cerebrovascular response to CO2 has been reported to be preserved during propofol anesthesia, but no comparison with awake control values has been made, and the additional influence of N2O has not been investigated. Using the noninvasive technique of transcranial Doppler ultrasonography, this study investigated the cerebrovascular response to varying levels of PaCO2 while awake and during anesthesia with propofol and propofol/N2O. Seven adults without systemic diseases undergoing nonneurologic surgery were studied. A pulsed-wave Doppler monitor was used to measure the mean middle cerebral artery flow velocity (Vmca) during varying levels of PaCO2 (25-55 mmHg) under the following conditions: 1) awake; 2) propofol 2.5 mg.kg-1 bolus followed by continuous infusion of 150 micrograms.kg-1.min-1; and 3) propofol as in the condition above plus 70% N2O. During the awake study condition, hypocapnia was induced by voluntary hyperventilation, and hypercapnia was induced with rebreathing of 7% CO2 in a closed circuit. During the anesthetized study conditions, hypocapnia and hypercapnia were induced by adjustment of minute ventilation. A minimum of five to six simultaneous Vmca and PaCO2 measurements were obtained under each of the study conditions. Systemic blood pressure was monitored via a radial arterial catheter, and phenylephrine was administered if mean arterial blood pressure decreased below 60 mmHg (phenylephrine was used in three of five patients in the propofol-N2O group). Linear regression and analysis of covariance were used for statistical analysis of Vmca-PaCO2 relationships.(ABSTRACT TRUNCATED AT 250 WORDS)