Effect of lung volume on the oesophageal diaphragm EMG assessed by magnetic phrenic nerve stimulation

Previous studies have shown conflicting results on the effect of lung volum e on the diaphragm compound muscle action potential (CMAP). Consequently, t he ability to quantify the oesophageal diaphragm electromyography (EMG) has been questioned. If lung volume changes have little effect on the diaphrag m CMAP the accurate measurement of voluntary EMC, as an index of respirator y drive, may be possible. Furthermore, the measurement of CMAP could provid e useful clinical information when evaluating patients with neuromuscular d isease. To reassess the effect of lung volume on the oesophageal diaphragm CMAP, si x normal subjects were studied using an oesophageal catheter incorporating seven electrodes (number one being proximal and seven distal) that were 1 c m in length and 1 cm apart. Electrode number three was positioned at the ce ntre of the electrically active region of the diaphragm (EARdi) at function al residual capacity (FRC), The diaphragm CMAP elicited by bilateral magnet ic stimulation of the phrenic nerves was simultaneously recorded from four electrode pairs. Pair one was created from electrodes one and three, pair t wo from electrodes two and four, pair three from electrodes three and five, and pair four from electrodes five and seven. Phrenic nerve stimulation wa s at residual volume (RV), FRC, FRC+1.0 L, FRC+2.0 L, and total lung capaci ty (TLC). The CMAP recorded from pair one was least influenced by changes in lung vol ume and the amplitude was 2.41+/-0.39 (mean+/-SD), 2.60+/-0.27, 2.64+/-0.29 , and 2.71+/-0.45 mV at RV, FRC, FRC+1.0 L and FRC+2.0 L, respectively. At TLC the CMAP was more variable. The CMAP amplitude recorded from pair two i ncreased with increasing long volume and at FRC+2.0 L was 3.7 times larger than that at FRC, Pair four usually recorded substantially smaller CMAPs at all lung volumes. This study shows that the diaphragm compound muscle action potential record ed from an oesophageal electrode just above the diaphragm is relatively sta ble over the lung volume range residual volume to functional residual capac ity+2.0 L.