Human diaphragmatic EMG: changes with lung volume and posture during supramaximal phrenic stimulation.

If esophageal and chest wall recordings of diaphragmatic electromyographic activity (EMG) accurately reflect neural drive to this muscle, then compound muscle action potentials (CMAPs) produced by supramaximal stimulation of the phrenic nerve should not alter with changes in diaphragmatic position. Maximal CMAPs were therefore recorded 1) during changes in lung volume from near residual volume to near total lung capacity, 2) during isovolume maneuvers at different lung volumes, and 3) while subjects were lying, sitting, and standing. The areas of maximal CMAPs recorded with the gastroesophageal catheter increased 5.1 +/- 3.6 times (mean +/- SD) between these volumes, increased 2.4 +/- 1.3 times as the diaphragm descended during an isovolume maneuver (at functional residual capacity), and increased 4.4 +/- 2.4 times between the lying and standing positions. Because the stimuli were supramaximal, these changes in EMG reflect changes in the relationship between the esophageal electrodes and the diaphragmatic muscle fibers. Artifactual changes were also documented for surface electrodes on the chest wall. Because of these positional changes in maximal CMAPs, previous studies, which used integrated diaphragmatic EMG to document "reflex" changes in neural drive, should be reevaluated.