Twitch transdiaphragmatic pressure depends critically on thoracoabdominal configuration

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Abstract
We measured the effect of thoracoabdominal configuration on twitch transdiaphragmatic pressure (Pdi, t) in response to supramaximal, transcutaneous, bilateral phrenic nerve shocks in three thin normal men. Pdi, t was measured as a function of lung volume (Vl) in the relaxation configuration, at functional residual capacity (FRC), and at the same end-tidal Vl 1) during relaxation; 2) with the abdomen (Ab) expanded and the rib cage (RC) in its relaxed FRC configuration; 3) with RC expanded and Ab in its relaxed FRC configuration; and 4) in configuration 3 with an active transdiaphragmatic pressure similar to that required to produce configuration 2. In increasing Vl from FRC to configuration 1, Pdi, t decreased by 3.6 cmH2O; to configuration 2 by 14.8 cmH2O; to configuration 3 by 3.7 cmH2O; and to configuration 4 by 2.7 cmH2O. We argue that changes in velocity of shortening and radius of curvature are unlikely to account for these effects and suggest that changes in diaphragmatic fiber length ( L di) are primarily responsible. If so, equivolume displacements of Ab and RC change L di in a ratio of ∼4:1. We conclude that Pdi, t is exquisitely sensitive to abdominal displacements that must be rigorously controlled if Pdi, t is to be used to assess diaphragmatic contractility.