Insertional action of the abdominal muscles in rabbits and dogs

The insertional action of the abdominal muscles was studied in supine anesthetized, apneic rabbits and dogs by comparing the changes in esophageal pressure (Pes), upper and lower rib cage circumference (Crc,u and Crc,l) and lung volume (VL) in response to electrical stimulation of all abdominal muscles before and after evisceration. In eviscerated animals, abdominal muscle contraction increased Pes and decreased both VL and Crc,l, but had no effect on Crc,u. Maximal responses were obtained at submaximal intensities of stimulation, and became larger with increasing lung volume. Relative to the vital capacity in intact animals, maximal ΔVL for stimulation performed at FRC and TLC were 7.2 ± 2.9 (SD) and 39.5 ± 7% in rabbits, and 6.3 ± 0.8 and 18.3 ± 5.9% in dogs, respectively. Relative to the changes in lung volume occurring with maximal contraction of the abdominal muscles in intact animals, the values of ΔVL observed in the eviscerated animals amounted to ≃35 and ≃45% for stimulation performed at FRC and TLC, respectively. Hence, abdominal muscles exert substantial insertional action on the lower rib cage that can result in appreciable lung deflationary effects, particularly at elevated lung volumes.