Salutary effect of fall in abdominal pressure during diaphragm paralysis

To examine the mechanical effects of the fall in abdominal pressure (Pab) that occurs during inspiration in diaphragmatic paralysis, we studied lung inflation and rib cage expansion before and after the abdomen was opened in nine spontaneously breathing dogs with bilateral phrenicotomy . We measured Pab, tidal volume, and parasternal electromyographic (EMG) activity during quiet breathing and CO2-induced hyperpnea. In six dogs, we also measured changes in anteroposterior and transverse rib cage diameters, the resting length of the parasternal intercostal muscles, and the amount of shortening of these muscles during inspiration. Opening the abdomen caused a marked reduction in the fall in Pab during inspiration and invariably resulted in a decrease in tidal volume (mean decrease, 13%), which contrasted with marked increases in inspiratory rib cage expansion and in the amount of parasternal intercostal shortening. The procedure, however, did not affect the resting length or inspiratory EMG activity of the parasternals . These findings indicate that although the fall in Pab, which occurs during inspiration in diaphragmatic paralysis, causes paradoxical inward displacement of the ventral abdominal wall, it has a salutary effect on tidal volume. This phenomenon is probably due to the fact that the diaphragm is part of the abdominal wall.