Effect of transdiaphragmatic pressure partitioning on activation and fatigue of the diaphragm.

Fatiguing contractions of the diaphragm during inspiratory resistive loading are accompanied by a predictable rate of decay in the high/low (H/L) frequency ratio of the diaphragmatic EMG (EMG-DI) when the esophageal and gastric pressure (Pes and Pga) components of transdiaphragmatic pressure (Pdi) are equal. However, Pes and Pga do not contribute equally to Pdi under a number of clinical and physiologic conditions. We therefore tested the effect of varying the Pes and Pga contribution to Pdi on the EMG-DI using an esophageal electrode in 5 normal men during fatiguing contractions. Falls in the H/L occurred in all subjects regardless of the relative contribution of Pes and Pga to Pdi at a tension-time index of 0.29 +/- 0.1 (mean +/- SE). However, the time constant of decay of the H/L varied widely among subjects when Pes predominated (173.9 +/- 45.7 s; coefficient of variation, 58.7%) or Pga predominated (78.4 +/- 19.4 s; coefficient of variation, 55.2%), whereas it was consistent among subjects when Pes and Pga were equal (72.3 +/- 3.8 s; coefficient of variation, 11.6%). In addition, a significant relationship was found between the mean integrated activity of the EMG-DI compared with maximum and the time constant of decay of the H/L (r = 0.57, p less than 0.03). We conclude that the differences in the rate of decay of the H/L was at least partly a result of differences in the relative activation of the crural diaphragm at the same Pdi.