Comparison of supramaximal stimulation current determined by an electromyography-based neuromuscular monitor between obese and non-obese patients: A prospective observational study.

BACKGROUND

Electromyography (EMG)-based monitoring directly measures compound muscle action potential (cMAP); however, its performance in obese patients may be limited by inadequate stimulation and impaired signal detection. This study aimed to compare automatically determined supramaximal stimulation currents between obese and non-obese patients using an EMG-based neuromuscular monitor.

METHODS

In this prospective observational study, 60 adult patients undergoing general anesthesia were assigned to obese [body mass index (BMI) ≥30 kg/m², n = 30] or non-obese (BMI 18.5-24.9 kg/m², n = 30) groups. An EMG-based neuromuscular monitor was applied to the abductor digiti minimi (ADM) muscle. The primary outcome was the supramaximal stimulation current determined by device calibration. Secondary outcomes included baseline cMAP, train-of-four ratio, final train-of-four ratio, and ultrasound-measured distances from the skin surface to the ADM muscle and the ulnar nerve.

RESULTS

The supramaximal stimulation current was significantly higher in the obese group than in the non-obese group (43.5 [24-60] vs. 21 [18-33] mA, P < 0.001). The depths of the ADM muscle and the ulnar nerve were significantly greater in the obese group. Automatic calibration was successful in all patients; however, accurate determination of neuromuscular block onset and recovery was not achievable in three obese patients under the initial calibration settings. Manual increases in stimulation current restored reliable monitoring in two of these patients.

CONCLUSIONS

Obesity is associated with higher supramaximal stimulation currents in EMG-based neuromuscular monitoring, which may be attributable to increased tissue depth. In some patients, accurate EMG monitoring may require manual adjustment of stimulation current beyond automatic calibration.

REGISTRATION: University Hospital Medical Information Network (registration number UMIN000054036).