Mechanics of the abdominal muscles in rabbits and dogs

In anesthetized, apneic rabbits and dogs, direct tetanic stimulations of the abdominal muscles (AMS) were performed at different tracheal pressures (Ptr) in the supine and upright posture. Lung volume (V), esophageal (Pes) and abdominal pressure (Pab), circumference of the upper and lower lung apposed rib cage (Crc, u and Crc, l) and of the abdomen (Cab), and transverse diameter of the rib cage facing the abdominal contents (Drc,ab) were measured. At Ptr = 0, Pab and Pes increased, and V decreased with increasing the strength of AMS; delta Pes and delta V eventually levelled off, while delta Pab was still increasing. Both delta Pes and delta V were larger in the upright posture, whereas delta Pab were similar. Relative to the expiratory reserve volume (ERV), maximal delta V in the supine and upright posture were 75.6 +/- 2.1 (mean +/- SE) and 86.1 +/- 2.2% in rabbits, and 56.5 +/- 3.4 and 75.2 +/- 3.7% in dogs. Maximal AMS decreased V and increased delta Pab the more so the larger the lung volume. In the volume range 10-70% VC, delta V were 3-4% VC larger in the upright posture, while delta Pab were similar in both postures. With AMS, Cab decreased, and Crc,u and Crc,l increased, while Drc,ab increased in dogs and decreased in rabbits. Hence, (a) the abdominal muscles can account for most of the ERV, particularly in the upright posture; (b) their maximal deflationary effects on the lung are already reached with submaximal activation; (c) their expiratory capacity is hindered by the expansion of the lung apposed rib cage and limited by diaphragmatic passive tension, and (d) their efficiency is reduced by paradoxical motion and distortion both between and within the lung apposed rib cage and abdominal compartments. Possible mechanisms for the dependency of delta V on species, volume and posture are discussed.