Acoustic monitoring of intraoperative neuromuscular block

Standard methods for accurate intraoperative measurement of neuromuscular block are either expensive or inconvenient and are not used widely. We have evaluated a new method of monitoring neuromuscular block using a low-frequency microphone. The method is based on the phenomenon of low-frequency sound emission by contracting skeletal muscle. Acoustic monitoring (MIC) with an air-coupled microphone was used to evaluate intraoperative neuromuscular block in 25 anaesthetized patients. The MIC recorded the response of the adductor pollicis muscle to supramaximal electrical stimulation of the ulnar nerve with train-of-four stimuli. The ratios of the first response (TI) to control (TC) were used for evaluation. Data obtained from the MIC were compared with simultaneous recordings, from the same hand, of mechanomyography (FDT), electromyography (EMG) and accelerography (ACC). Throughout the operative procedure, TI/TC ratios of the acoustic method correlated with the three reference devices: FDT, 12 patients, 262 data sets, r = 0.86, bias (%MIC-%FDT) = mean -5.3 (SD 19.6)%; EMG, 18 patients, 490 data sets, r = 0.85, bias (%MIC-%EMG) = -0.39 (20.29)%; and ACC, 13 patients, 328 data sets, r = 0.91, bias (%MIC-%ACC) = -3.0 (15.6)%. We conclude that monitoring intraoperative neuromuscular block by a microphone which transduces low-frequency muscle sounds is clinically feasible.