Effects of midline laparotomy on expiratory muscle activation in anesthetized dogs

Abdominal surgery has a marked inhibitory influence on the diaphragm, but its effect on the expiratory muscles is not known. Therefore, we have recorded the electromyograms of the triangularis sterni, abdominal external oblique, and transversus abdominis before and after a midline laparotomy in 10 anesthetized, spontaneously breathing dogs. Measurements were obtained during quiet breathing in the supine posture, during breathing against expiratory threshold loads, during head-up tilting, and during hyperoxic hypercapnia. Expiratory activation of the transversus abdominis in all conditions was considerably reduced after laparotomy. This reduction was real, as no change in the compound muscle action potential during single pulse stimulation was observed. In contrast, expiratory recruitment of either the triangularis sterni or the abdominal external oblique was maintained or increased. We therefore conclude that laparotomy inhibits not only activation of the diaphragm during inspiration but also activation of the transversus abdominis during expiration. Visceral afferents thus affect in concert the two respiratory muscles lining the peritoneum. The present findings also emphasize the important fact that the pattern of activation of a particular abdominal muscle is not necessarily representative of the entire abdominal musculature.