Expiratory activity of the abdominal muscles in man during general anesthesia

Abstract

The integrated electromyogram was recorded from the lateral abdominal muscles in 24 young normal male volunteers in the supine position, both during consciousness and during light general anesthesia, produced by 50 or 80% N₂O in O₂, or by 0.5–2% halothane in air after thiopental. No activity was observed during quiet breathing in the conscious state, but throughout anesthesia all subjects exhibited expiratory muscle activity. Activity gradually subsided as consciousness was regained, but suddenly disappeared on vocal arousal. Expiratory intragastric pressure increased up to 7.6 cm H₂O above control. End-expiratory P_co2, minute volume, and airway resistance (interrupter technique) did not change significantly. Forced inflation during inspiration, maintained for five to six respiratory cycles, depressed activity during the first three to four cycles; subsequent deflation restored activity. Inflation during expiration immediately reduced activity. Mechanisms that might lead to recruitment of the expiratory muscles are considered. Expiratory activity frequently observed in experimental animals might, at least in part, be due to anesthesia, and thus not represent the breathing pattern during consciousness. abdominal electromyogram; respiratory reflexes Submitted on November 4, 1963