Abdominal muscle activity and intraabdominal pressure after upper abdominal surgery.

To examine the influence of abdominal muscle activity on intraabdominal pressure, the integrated surface EMG from upper abdominal muscle (EAB) was related to changes in intragastric pressure (PGA) in six patients after upper abdominal surgery. A similar respiratory pattern of EMG activity was observed in all subjects. EAB increased rapidly at the onset of expiration, and thereafter more slowly throughout expiration. At the onset of inspiration EAB decreased abruptly, and was small during inspiration. Changes in intragastric pressures were closely related to changes in EAB. In five patients PGA and EAB waveforms were almost identical over the respiratory cycle. In one subject a biphasic change in PGA during inspiration was observed, suggesting the influence of other respiratory muscles. Abdominal muscle action results in changes in intraabdominal pressure previously attributed to diaphragmatic impairment.