Quantification of the esophageal diaphragm electromyogram with magnetic phrenic nerve stimulation

Measurement of the amplitude of the esophageal diaphragm compound muscle ac tion potential (CMAP) could be useful for the diagnosis and monitoring of n euromuscular disease. However, quantification of the diaphragm CMAP has bee n hampered by difficulty in positioning the esophageal electrode at the dia phragm's electrically active center and many investigators report arbitrary units rather than voltage. To quantify the esophageal diaphragm CMAP we de signed a multipair electrode which we evaluated during unilateral magnetic stimulation. The esophageal catheter consisted of four sequential electrode pairs. Overall the electrode spanned 17 cm and covered the entire electric ally active region of the diaphragm. The diaphragm CMAP was simultaneously recorded from the four pairs at distances of 40, 39, 38, and 37 cm from the nose to the proximal electrode pair. Studies were undertaken in 10 normal subjects and 10 patients with diaphragm dysfunction. The amplitude of the C MAP (peak to peak) was defined as the average of five twitches recorded fro m the optimal pair of electrodes. The amplitude of the diaphragm CMAP elici ted by unilateral maximal magnetic stimulation was 1.45 +/- 0.35 mV (mean /- SD) for the right side and 1.68 +/- 0.47 mV for the left. When measured on different occasions the coefficient of variation (CV) was 8.6%. The ampl itude of the CMAP measured from dysfunctional hemidiaphragms was much less than that measured from normal subjects. This study suggests that the diaph ragm CMAP can be quantified using an appropriate esophageal electrode.