Temporal relation between acoustic and force responses at the adductor pollicis during nondepolarizing neuromuscular block

Background: Contracting muscle emits sounds. The purpose of this study was to compare the time course of muscular paralysis at the adductor pollicis m uscle (AP) with use of acoustic myography and mechanomyography. Methods: Thirteen elective surgery patients, American Society of Anesthesio logists physical status I, received rocuronium (0.6 mg/kg intravenously) as a bolus dose during general anesthesia. Force of AP was measured with use of a strain gauge, and sounds were recorded simultaneously with use of a sm all condenser microphone fixed on the palmar surface of the hand over the A P. Supramaximal stimulation was applied to the ulnar nerve at 0.1 Hz for 45 -60 min, In seven patients, the response to train-of-four stimulation was a lso recorded during recovery. Results: Force and sounds both were equally sensitive in measuring maximum block. The relation between sound and force was curvilinear, with good agre ement near 0 and 100% and acoustic response exceeding mechanical response a t intermediate levels of block The acoustic signal had a slower onset and a faster recovery than the force response. The fade response of sound to tra in-of-four stimulation also recovered faster than that of force. Conclusion: Acoustic myography is an alternative method to monitor muscular paralysis that is easy to set up and applicable to most superficial muscle s. However, the time course of relaxation at AP using acoustic myography di ffers from the time course of force relaxation, Therefore, these two method s are not equivalent when applied to AP.