MECHANICAL SIGNIFICANCE OF RESPIRATORY MUSCLE-ACTIVITY IN HUMANS DURING HALOTHANE ANESTHESIA

Background: Prior human studies have shown that halothane attenuates a ctivity in the parasternal intercostal muscle and enhances phasic acti vity in respiratory muscles with expiratory actions. This expiratory m uscle activity could contribute to reductions in the functional residu al capacity produced by anesthesia. Termination of this activity could contribute to the maintenance of inspiratory rib cage expansion. The purpose of this study was to estimate in humans the mechanical signifi cance of expiratory muscle activity during halothane anesthesia and to search for the presence of scalene muscle activity during halothane a nesthesia that might contribute to inspiratory rib cage expansion. Met hods: Six subjects (3 males, 3 females) were studied while awake and d uring 1.2 MAC halothane anesthesia, both during quiet breathing and du ring carbon dioxide rebreathing. Respiratory muscle activity was measu red using fine-wire electromyography electrodes. Chest wall configurat ion was determined using images of the thorax obtained by three-dimens ional, fast computed tomography and respiratory impedance plethysmogra phy. Functional residual capacity was measured by a nitrogen dilution technique. Measurements were obtained after paralysis with 0.1 mg/kg v ecuronium and mechanical ventilation. Results: Phasic inspiratory acti vity was present in the scalene muscle of four anesthetized subjects d uring quiet breathing and all anesthetized subjects during rebreathing . Phasic inspiratory activity was present in the parasternal intercost al muscle during halothane anesthesia in only the three female subject s and was enhanced by rebreathing; parasternal intercostal muscle acti vity was never present in anesthetized males. During anesthesia with q uiet breathing, phasic expiratory activity was observed in the transve rsus abdominis muscles of only the three male subjects. Despite these differences in the pattern of respiratory muscle use, the pattern of c hest wall responses to rebreathing was similar between males and femal es. When expiratory muscle activity was present, paralysis increased t he end-expiratory thoracic volume by expanding the rib cage, demonstra ting that this activity reduced thoracic volume in these subjects. Cha nges in thoracic blood volume were significant determinants of the cha nge in functional residual capacity produced by paralysis. Conclusions : In humans anesthetized with 1.2 MAC end-tidal halothane, there are m arked interindividual differences in respiratory muscle use during qui et breathing that may be related to sex; phasic inspiratory scalene mu scle and parasternal intercostal muscle activity may contribute to ins piratory rib cage expansion in some subjects; and when present, expira tory muscle activity significantly constricts the rib cage and contrib utes to reductions in functional residual capacity caused by halothane anesthesia.