PHONOMYOGRAM OF THE DIAPHRAGM DURING UNILATERAL AND BILATERAL PHRENIC-NERVE STIMULATION AND CHANGES WITH FATIGUE

As previously demonstrated in humans, contracting skeletal muscles vib rate laterally and generate sound. The amplitude of this acoustic sign al, called the phonomyogram (PMG), is related to muscle force producti on. As in limb muscles, the diaphragm might produce mechanical tissue vibrations and a PMG. We recorded diaphragm twitches during phrenic ne rve stimulation in normal subjects at FRC. Diaphragmatic compound moto r action potentials (CMAP) were recorded on each side of the chest by surface electrodes over the eighth intercostal space. Condenser microp hones were fixed to the skin close to the electrodes, and esophageal a nd gastric balloons employed to measure transdiaphragmatic pressure (P di). For unilateral and bilateral supramaximal single shocks (0.1 ms), PMG onset latencies from either side (12.4 +/- 0.6 ms) were intermedi ate 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 signific ant linear relationship was also found between PMG for each side and P di peak values. PMG and Pdi both decreased with fatigue of the diaphra gm. We conclude that PMG in response to single ph renic nerve shocks i s a noninvasive index of electromechanical coupling, recruitment, and transdiaphragmatic pressure generation by the diaphragm. (C) 1994 John Wiley and Sons, Inc.