Factors influencing the development of expiratory muscle activity during anaesthesia.

Expiratory muscle activity has been studied in the external oblique muscle during anaesthesia with spontaneous breathing in twenty-two patients. Expiratory muscle activity was absent in all subjects before induction but developed within 29 minutes of induction in twenty patients, remaining absent in the other two. Deepening the level of halothane anaesthesia reduced, but in only one case abolished, expiratory muscle activity. Endotracheal intubation and passage of a pharyngeal airway had no effect other than initial stimulation and relief of obstruction when present. Topical analgesia of larynx had no effect. Surgical stimulation and respiratory obstruction both caused marked increase in expiratory muscle activity.