Background: This study investigated the acoustic signal characteristics and best recording site of phonomyography at the corrugator supercilii muscle and compared phonomyography with acceleromyography.
Methods: In 12 patients (group I), after induction of anaesthesia and insertion of a laryngeal mask, a microphone (frequency range 2.5 Hz to 10 kHz) was placed on six different areas on the forehead and the peak-to-peak response after single-twitch stimulation of the facial nerve was measured. The microphone was placed where the response was largest and mivacurium 0.2 mg kg(-1) was administered. Fast Fourier transformation was applied to all signals to determine peak frequencies and the power-frequency relationship at different stages of neuromuscular block. In an additional 15 patients (group II), the same microphone and an acceleromyographic probe were placed above the middle portion of the left and right eyebrows respectively. Onset and offset of neuromuscular block were determined after mivacurium 0.2 mg kg(-1).
Results: In all seven women and all five men in group I, the best response was obtained just above the middle portion of the eyebrow. Peak frequency was 4.1 (SD 0.9) Hz without neuromuscular block and did not change significantly during onset and offset of neuromuscular block. Ninety per cent of the total signal power was below 40 Hz. In group II, mean onset time and maximum effect measured were 104 (20) s and 76 (10)% respectively using acceleromyography and 134 (30) s and 92 (4)% using phonomyography (P<0.04). Mean time to reach 25, 75 and 90% of control was 9.5 (2.8), 14 (5.1) and 15.1 (5.3) min respectively using acceleromyography and 6.9 (2.8), 12.5 (5.9) and 13.6 (4.9) min using phonomyography (P<0.04). Bland-Altman testing revealed significant bias (precision) for onset time, maximum effect and time to reach 25% of control (acceleromyography minus phonomyography) at -30 (38) s, -16 (11)% and 2.6 (2.8) min respectively.
Conclusions: Phonomyography can be used to determine neuromuscular block at the corrugator supercilii muscle. In comparison with acceleromyography, phonomyography tends to measure a longer onset with more pronounced maximum effect and shorter recovery of neuromuscular block.