TRANSVERSE ABDOMINIS LENGTH CHANGES DURING EUPNEA, HYPERCAPNIA, AND AIRWAY OCCLUSION

The abdominal muscles accelerate airflow during expiration and may also influence the end-expiratory volume and configuration of the thorax. Although much is known about their electrical activity, the degree to which they change length during the respiratory cycle has not been previously assessed. In the present study we measured respiratory changes in transverse abdominis length using sonomicrometry in 14 pentobarbital sodium-anesthetized supine dogs and compared length changes to simultaneously recorded tidal volume and trnasverse abdominis electromyograms (EMG). To determine muscle resting length at passive functional residual capacity (LFRC), the animals were hyperventilated to apnea. The transverse abdominis was electrically active in all animals during resting O2 breathing (eupnea). During inspiration the transverse abdominis lengthened above resting length in all 14 dogs by a mean of 3.7 .+-. 1.1% LFRC; during expiration the trnasverse abdominis shortened below resting length in 13 of 14 dogs by a mean of 4.2 .+-. 0.9% LFRC. Increasing hyperoxic hypercapnia (produced in 9 animals) progressively heightened transverse abdominis EMG and progressively increased the extent of muscle shortening below resting length (to 12.6 .+-. 3.2% LFRC at a PCO2 of 90 Torr). During single-breath airway occlusion substantial inspiratory lengthening of the transverse abdominis occurred, both during O2 breathing and during CO2 rebreathing. These results demonstrate that 1) the transverse abdominis operates at lengths both above and below its resting length; 2) hypercapnia increases the magnitude of transverse abdominis expiratory shortening below resting length; and 3) the transverse abdominis lengthens passively during occluded breaths.