Acoustic monitoring of intraoperative neuromuscular block

Standard methods for accurate intraoperative measurement of neuromuscular b lock are either expensive or inconvenient and are not used widely. We have evaluated a new method of monitoring neuromuscular block using a low-freque ncy microphone. The method is based on the phenomenon of low-frequency soun d emission by contracting skeletal muscle. Acoustic monitoring (MIC) with a n air-coupled microphone was used to evaluate intraoperative neuromuscular block in 25 anaesthetized patients. The MIC recorded the response of the ad ductor pollicis muscle to supramaximal electrical stimulation of the ulnar nerve with train-of-four stimuli. The ratios of the first response (TI) to control (T-C) were used for evaluation. Data obtained from the MIC were com pared with simultaneous recordings, from the same hand, of mechanomyography (FDT), electromyography (EMC) and accelerography (ACC). Throughout the ope rative procedure, TI/T-C ratios of the acoustic method correlated with the three reference devices: FDT, 12 patients, 262 data sets, r=0.86, bias (%MI C-%FDT)= mean -5.3 (SD 19.6)%; EMG, 18 patients, 490 data sets, r=0.85, bia s (%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.