Electrical and mechanical output of the expiratory muscles in anesthetized dogs

In anesthetized intact and vagotomized dogs chest wall diameters, expiratory muscles' (EM) electrical activity, work, mean pressure and volume displacement contributed by EM contraction were assessed in the supine and upright posture during rebreathing, and during continuous positive pressure breathing (CPAP) in the supine posture. Corresponding inspiratory mechanical output variables were related to diaphragm activity. During resting breathing triangularis sterni and internal interosseous were more easily recruited than transversus abdominis and external oblique. EM activity increased with tilting, CPAP and rebreathing. Vagotomy depressed or abolished abdominal EM activity, with lesser effects on rib cage EM. Expiratory mechanical output grossly paralleled EM activity: it markedly depended on rib cage and abdominal EM coactivation, besides lung inflation and chest wall shape. Upper rib cage configuration at end-expiration never departed from the relaxation one, suggesting trivial effects of the triangularis sterni contraction. Lower chest wall distortion occurred almost regularly, so that much of EM activity was not converted into external work. In contrast with expiratory electromechanical relations, those for the diaphragm were always highly significant and independent of EM activation.