Phonomyogram of the diaphragm during unilateral and bilateral phrenic nerve stimulation and changes with fatigue

As previously demonstrated in humans, contracting skeletal muscles vibrate laterally and generate sound. The amplitude of this acoustic signal, called the phonomyogram (PMG), is related to muscle force production. As in limb muscles, the diaphragm might produce mechanical tissue vibrations and a PMG. We recorded diaphragm twitches during phrenic nerve stimulation in normal subjects at FRC. Diaphragmatic compound motor action potentials (CMAP) were recorded on each side of the chest by surface electrodes over the eighth intercostal space. Condenser microphones were fixed to the skin close to the electrodes, and esophageal and gastric balloons employed to measure transdiaphragmatic pressure (Pdi). For unilateral and bilateral supramaximal single shocks (0.1 ms), PMG onset latencies from either side (12.4 +/- 0.6 ms) were intermediate between those of CMAP (7.3 +/- 0.7 ms) and of Pdi (20.3 +/- 2.8 ms). By varying stimulation intensity, significant linear relationships were obtained between CMAP and PMG amplitudes on each side. A significant linear relationship was also found between PMG for each side and Pdi peak values. PMG and Pdi both decreased with fatigue of the diaphragm. We conclude that PMG in response to single phrenic nerve shocks is a noninvasive index of electromechanical coupling, recruitment, and transdiaphragmatic pressure generation by the diaphragm.