Diaphragm EMG measured by cervical magnetic and electrical phrenic nerve stimulation

The purpose of the study was to compare electrical stimulation (ES) and cer vical magnetic stimulation (CMS) of the phrenic nerves for the measurement of the diaphragm compound muscle action potential (CMAP) and phrenic nerve conduction time. A specially designed esophageal catheter with three pairs of electrodes was used, with control of electrode positioning in 10 normal subjects. Pair A and pair B were close to the diaphragm (pair A lower than pair B); pair C was positioned 10 cm above the diaphragm to detect the elec tromyogram from extradiaphragmatic muscles. Electromyograms were also recor ded from upper and lower chest wall surface electrodes. The shape of the CM AP measured with CMS (CMS-CMAP) usually differed from that of the CMAP meas ured with ES (ES-CMAP). Moreover, the latency of the CMS-CMAP from pair B ( 5.3 +/- 0.4 ms) was significantly shorter than that from pair A (7.1 +/- 0. 7 ms). The amplitude of the CMS-CMAP (1.00 +/- 0.15 mV) was much higher tha n that of ES-CMAP (0.26 +/- 0.15 mV) when recorded from pair C. Good-qualit y CMS-CMAPs could be recorded in some subjects from an electrode positioned very low in the esophagus. The differences between ES-CMAP and CMS-CMAP re corded either from esophageal or chest wall electrodes make CMS unreliable for the measurement of phrenic nerve conduction time.