Effect of diaphragm fatigue on neural respiratory drive.

To test the hypothesis that diaphragm fatigue leads to an increase in neural respiratory drive, we measured the esophageal diaphragm electromyogram (EMG) during CO(2) rebreathing before and after diaphragm fatigue in six normal subjects. The electrode catheter was positioned on the basis of the amplitude and polarity of the diaphragm compound muscle action potential recorded simultaneously from four pairs of electrodes during bilateral anterior magnetic phrenic nerve stimulation (BAMPS) at functional residual capacity. Two minutes of maximum isocapnic voluntary ventilation (MIVV) were performed in six subjects to induce diaphragm fatigue. A maximal voluntary breathing against an inspiratory resistive loading (IRL) was also performed in four subjects. The reduction of transdiaphragmatic pressure elicited by BAMPS was 22% (range 13-27%) after 2 min of MIVV and was similar, 20% (range 13-26%), after IRL. There was a linear relationship between minute ventilation (VE) and the root mean square (RMS) of the EMG during CO(2) rebreathing before and after fatigue. The mean slope of the linear regression of RMS on VE was similar before and after diaphragm fatigue: 2.80 +/- 1.31 vs. 3.29 +/- 1.40 for MIVV and 1.51 +/- 0.31 vs 1.55 +/- 0.31 for IRL, respectively. We conclude that the esophageal diaphragm EMG can be used to assess neural drive and that diaphragm fatigue of the intensity observed in this study does not affect respiratory drive.