Electrophysiological Properties of the Human Diaphragm Assessed by Magnetic Phrenic Nerve Stimulation: Normal Values and Theoretical Considerations in Healthy Adults

PURPOSE: This study determined normal values for motor evoked potentials (MEPs) and compound muscle action potentials (CMAPs) of the diaphragm following cortical and cervical magnetic stimulation (COMS and CEMS) of the phrenic nerves in healthy adults. METHODS: Using surface electrodes, diaphragmatic MEP and CMAP were recorded in 70 subjects (34 ± 13 years, 25 men) following supramaximal cortical magnetic stimulation and CEMS at functional residual capacity and using a standardized inspiratory pressure trigger (-0.5 kPa). All healthy volunteers underwent standard spirometry and measurement of maximum inspiratory and expiratory pressure. RESULTS: At functional residual capacity, upper limit of normal for MEP latency was 25 ms in men and 23 ms in women (p < 0.05), and upper limit of normal for CMAP latency was 6 ms. In contrast to MEP and CMAP amplitude, corresponding latencies showed little interindividual and intraindividual variability. Use of an inspiratory pressure trigger enhanced reproducibility and amplitude of diaphragm MEP. Diaphragm responses to both cortical and cervical magnetic stimulation were symmetrical and independent of age (in our cohort), with higher values for latency and amplitude in men (each p < 0.05). Diaphragm CMAP amplitude showed weak-moderate correlations with forced vital capacity (r = 0.47; p < 0.01), maximum inspiratory pressure (r = 0.39; p < 0.01), and maximum expiratory pressure (r = 0.32; p < 0.01). CONCLUSIONS: Combination of cortical magnetic stimulation and CEMS of the phrenic nerves is feasible and allows noninvasive assessment of both central and peripheral conductivity of the diaphragm and the inspiratory pathway.