Effect of brachial plexus co-activation on phrenic nerve conduction time

Background-Diaphragm function can be assessed by electromyography of the di aphragm during electrical phrenic nerve stimulation (ES). Whether phrenic n erve conduction time (PNCT) and diaphragm electrical activity can be reliab ly measured from chest wall electrodes with ES is uncertain. Methods-The diaphragm compound muscle action potential (CMAP) was recorded using an oesophageal electrode and lower chest wall electrodes during ES in six normal subjects. Two patients with bilateral diaphragm paralysis were also studied. Stimulations were deliberately given in a manner designed to avoid or incur co-activation of the brachial plexus. Results-For the oesophageal electrode the PNCT was similar with both stimul ation techniques with mean (SE) values of 7.1 (0.2) and 6.8 (0.2)ms, respec tively (pooled left and right values). However, for surface electrodes the PNCT was substantially shorter when the brachial plexus was activated (4.4 (0,1) ms) than when it was not (7.4 (0.2)ms) (mean difference 3.0 ms, 95% C I 2.7 to 3.4, p < 0.0001). A small short latency CMAP was recorded from the lower chest wall electrodes during stimulation of the brachial plexus alon e. Conclusions-The results of this study show that lower chest wall electrodes only accurately measure PNCT when care is taken to avoid stimulating the b rachial plexus. A false positive CMAP response to phrenic stimulation could be caused by inadvertent stimulation of the brachial plexus. This finding may further explain why the diaphragm CMAP recorded from chest wall electro des can be unreliable with cervical magnetic stimulation during which brach ial plexus activation occurs.