Twitch transdiaphragmatic pressure depends critically on thoracoabdominal configuration

We measured the effect of thoracoabdominal configuration on twitch transdia phragmatic pressure (Pdi, t) in response to supramaximal, transcutaneous, b ilateral phrenic nerve shocks in three thin normal men. Pdi, t was measured as a function of lung volume (VL) in the relaxation configuration, at func tional residual capacity (FRC), and at the same end-tidal VL 1) during rela xation; 2) with the abdomen (Ab) expanded and the rib cage (RC) in its rela xed FRC configuration; 3) with RC expanded and Ab in its relaxed FRC config uration; and 4) in configuration 3 with an active transdiaphragmatic pressu re similar to that required to produce configuration 2. In increasing VL fr om FRC to configuration 1, Pdi,t decreased by 3.6 cmH(2)O; to configuration 2 by 14.8 cmH(2)O to configuration 3 by 3.7 cmH(2)O; and to configuration 4 by 2.7 cmH(2)O. We argue that changes in velocity of shortening and radiu s of curvature are unlikely to account for these effects and suggest that c hanges in diaphragmatic fiber length (L-di) are primarily responsible. If s o, equivolume displacements of Ab and RC change L-di in a ratio of similar to 4:1. We conclude that Pdi,t is exquisitely sensitive to abdominal displa cements that must be rigorously controlled if Pdi, t is to be used to asses s diaphragmatic contractility.