Respiratory changes in abdominal configuration in supine dogs.

In an attempt to understand the respiratory changes in abdominal muscle length in supine dogs (Ninane et al., 1988), we have recorded the electromyographic (EMG) activity of the transversus abdominis, external oblique, and rectus abdominis in eight supine, lightly anesthetized animals, and we have measured the respiratory changes in anteroposterior (AP) and transverse (T) diameters of the abdomen. Five animals had phasic expiratory EMG activity in the transversus during room air breathing, while only two animals had expiratory activity in the external oblique; no animal had phasic expiratory activity in the rectus. Activation of the transversus during expiration was invariably associated with a decrease in the abdominal T diameter and a rise in gastric pressure. In contrast, the abdominal AP diameter tended to increase. These alterations in abdominal configuration remained unchanged after denervation of the triangularis sterni, but decreased in magnitude when activation of the transversus was reduced by supplemental anesthesia. Conversely, these alterations in abdominal configuration increased in magnitude when expiratory activation of the transversus was increased by hyperoxic hypercapnia. These observations indicate that in supine dogs: (1) Expiratory contraction of the transversus acts primarily to reduce the transverse diameter of the abdomen; (2) This reduction, in turn, promotes an increase in abdominal pressure which results secondarily in an outward motion of the ventral abdominal wall; and (3) The latter may explain why the rectus abdominis, although electrically silent, shortens during expiration below its in situ relaxation length. The present observations also establish that in supine dogs breathing at rest, the abdomen does not move with a single degree of freedom.