Reduction of tonic ribcage muscle activity by anesthesia with thiopental

Surface electromyograms were recorded from the scalene, sternomastoid, and parasternal intercostal muscles in 16 supine premedicated adult patients before, during, and after the induction of anesthesia with thiopental and after the administration of succinylcholine. Muscle activity was measured as the rectified, integrated signal, and expressed as a percentage of the activity detected during a maximal inspiration. Activity was found in the scalene and sternomastoid muscles of all patients, and in the intercostal muscles of 13. This activity was more commonly tonic, and, at end-expiration, it was 7%, 6%, and 5% of the activity that could be generated by a maximal inspiration, for the scalene, sternomastoid, and intercostal muscles, respectively. In all subjects, this activity was markedly reduced or abolished by the administration of thiopental, to 0.7%, 0.6%, and 0.6%, respectively. There was no tonic activity in these muscles during spontaneous respiration after induction of anesthesia. It is suggested that the reduction of tonic inspiratory activity in these muscles may contribute to the reduction in functional residual capacity associated with anesthesia.