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We performed transcutaneous bilateral phrenic nerve stimulation at varying lung volumes between residual volume (RV) and total lung capacity (TLC) in six normal male volunteers. Peak twitch transdiaphragmatic pressure declined from 49.1 +/- 9.1 (SD) cmH2O at RV to 19.6 +/- 5.97 (SD) cmH2O at TLC. Twitch contraction time fell from 91.8 +/- 11.3 (SD) ms at RV to 57.7 +/- 7.4 (SD) ms at TLC. There was a good correlation between changes in contraction time and transdiaphragmatic pressure (r = 0.7). The fall in transdiaphragmatic pressure was almost all due to a fall in pleural pressure, with little change in gastric pressure between RV and TLC. At TLC the pleural pressure in response to phrenic nerve stimulation was -0.58 cmH2O. We conclude that, as lung volume increases and the diaphragm shortens, it becomes less effective as a pressure generator and that pressure it generates is less well converted into useful inspiratory pressure. At a lung volume close to TLC, the diaphragm ceases to act as an inspiratory muscle.
Smith et al. (Fri,) studied this question.