Anterior and posterior parts of diaphragm: A search for the best recording sites and the relevant normative data with surface electrodes

Background and purpose Phrenic nerve conduction studies, including comprehensive recording of compound muscle action potentials (CMAP) from all regions of the dia­ phragm, can be useful in both neurolo­gical conditions affecting the phrenic nerve or diaphragm and non-neurological conditions such as COPD and GERD, and may have potential diagnostic value. It was aimed (a) to determine the most convenient stimulation and recording sites for phrenic nerve conduction studies (NCS), (b) to record CMAPs reflecting the activity of the posterior part of the diaphragm with surface recording electrodes, (c) to define normative values for these studies. Methods Sixty-three healthy subjects were included in the study and CMAPs were recorded simultaneously with two anterior and two posterior electrode derivations while stimulating the phrenic nerve behind the sternocleidomastoid muscle. Results Among the four different electrode placements, the recording electrode pairs providing the highest amplitude CMAPs were those placed 5 cm above the xiphoid process (active) and 16 cm laterally on the costal margin (reference). With this placement method, the upper limit of CMAP latency was 7.70 ms, the lower limit of base-topeak amplitude was 0.30 mV and the lower limit of peak-to-peak amplitude was 0.45 mV. Diaphragmatic CMAPs have been obtained from the majority of volunteers with surface electrodes placed in the posterior regions of the diaphragm, albeit with lower amplitude. Age is the only independent factor affecting the CMAP latency in this study. Conclusion Normative values with the best recording sites for phrenic NCSs were determined. The posterior aspect of the diaphragm could be recorded with surface electrodes despite the lower amplitude and reliability compared to the anterior part.