Evaluating diaphragm motor response variability in electric and magnetic phrenic nerve stimulations during passive expiration.

Objective: This study assessed variability in cervical electrical (CEPNS) and magnetic (CMPNS) phrenic nerve stimulation alongside transcranial magnetic stimulation (diTMS) of the diaphragm, focusing on motor response latency and amplitude at individually calculated suprathreshold intensities. Methods: Diaphragm motor responses were elicited via CEPNS and CMPNS (measuring compound muscle action potentials, CMAPs) and diTMS (measuring diaphragm motor evoked potentials, diMEPs). Latency and amplitude were recorded. Statistical analyses compared methods, evaluated variability using coefficients of variation (CV), and explored associations with height and central motor conduction time. Results: Among 25 participants (mean age 25 ± 4 years), CMPNS evoked CMAPs in 21 subjects, CEPNS in 16, with 12 responding to both. No significant latency or amplitude differences emerged between CEPNS and CMPNS responders. TMS evoked diMEPs in 24 participants. Latency was more consistent than amplitude across all methods; CMPNS exhibited the lowest amplitude variability. Height moderately correlated positively with CMAP latency and negatively with central motor conduction time. Conclusions: Individually adapted suprathreshold CEPNS and CMPNS assessed phrenic nerve conduction with varying success rates. Latency was a more reliable measure than amplitude. CMPNS and diTMS surpassed CEPNS in response reliability, positioning magnetic stimulations as preferable for assessing phrenic nerve conduction. CEPNS might not be suitable for examining motor response amplitudes when not applied at supramaximal intensity. Height-related anatomical factors influencing conduction merit further study. Significance: Latency of diaphragm motor responses elicited at individually estimated stimulation intensities offers a more consistent biomarker over amplitude for assessing phrenic nerve function.