Depression of I Waves in Corticospinal Volleys by Sevoflurane, Thiopental, and Propofol

Abstract

Isoflurane depresses the number and amplitude of I waves of the motor-evoked potential produced by transcranial electrical stimulation of the motor cortex and thus affects components of the corticospinal volley that are believed to arise from Laminae III and V. This study extends these observations to sevoflurane (9 patients) and the two IV anesthetics, thiopental and propofol (10 sets of observations in 10 and 6 patients, respectively). The patients' ages ranged from 10 to 17 yr. Sevoflurane was administered to achieve end-tidal concentrations of 0.5%-3%. Thiopental and propofol were given as boluses of 5 mg/kg and 2 mg/kg, respectively, to patients anesthetized with nitrous oxide, fentanyl, midazolam, and a muscle relaxant. Sevoflurane had a depressant effect on I waves essentially similar to that of isoflurane; thiopental depressed I wave activity by an average of 33% (95% confidence interval: 20%-46%, P < 0.001) and propofol by 39% (95% confidence interval: 20%-40%, P < 0.001). With all three anesthetics, later I waves showed the most amplitude depression. The three anesthetics had qualitatively similar effects on I waves. Sevoflurane, thiopental, and propofol depress components of the corticospinal volley produced by transcranial electrical stimulation of motor cortex in a manner qualitatively similar to isoflurane. The findings indicate that anesthetics with primarily hypnotic actions suppress interneuronal activity in cerebral cortex.