Expiratory and inspiratory action of transversus abdominis during eupnea and hypercapnic ventilation.

ABSTRACT

BACKGROUND: Recently, there is interest in the clinical importance of monitoring abdominal muscles during respiratory failure. The clinical interpretation relies on the assumption that expiration is a passive physiologic process and, since diaphragm and abdomen are arranged in series, any inward motion of the abdominal wall represents a sign of diaphragm dysfunction. However, previous studies suggest transversus abdominis might be active even during eupnea and is preferentially recruited over the other abdominal muscles. OBJECTIVE: 1) Is transversus abdominis normally recruited during eupnea? 2) What is the degree of activation of transversus abdominis during hypercapnia? 3) Does the end-inspiratory length of transversus abdominis change during hypercapnia, while diaphragm function is normal? METHODS: In 30 spontaneously breathing canines, awake without confounding anesthetic, we measured directly both electrical activity and corresponding mechanical length and shortening of transversus abdominis during eupnea and hypercapnia. RESULTS: Transversus abdominis is consistently recruited during eupnea. During hypercapnia, transversus abdominis recruitment is progressive and significant. Throughout hypercapnia, transversus abdominis baseline end-inspiratory length is not constant baseline length decreases progressively throughout hypercapnia. After expiration, into early inspiration, transversus abdominis shows a consistent neural mechanical post -expiratory expiratory activity (PEEA) at rest, which progressively increases during hypercapnia. CONCLUSION: Transversus abdominis is an obligatory expiratory muscle, reinforcing the fundamental principle expiration is not a passive process. Beyond expiration, during hypercapnic ventilation, transversus abdominis contributes as an "accessory inspiratory muscle" into the early phase of inspiration. Clinical monitoring of abdominal wall motion during respiratory failure may be confounded by action of transversus abdominis.