The staircase phenomenon at the corrugator supercilii muscle in comparison with the hand muscles

Phonomyography (PMG) is a novel method to monitor neuromuscular block. It is non-invasive and can be applied to any muscle. It can be used interchangeably with mechanomyography (MMG). The staircase phenomenon has not been investigated for this method or at the corrugator supercilii muscle. The purpose of this work was to determine the staircase effect at three different muscles using two different methods.

In 10 patients undergoing general anaesthesia with sevoflurane, using a laryngeal mask airway without the aid of neuromuscular block, one piezo-electric microphone each was applied to the corrugator supercilii muscle and the first dorsal interosseus muscle. In addition, a force transducer was attached to the tip of the thumb to determine the force of the adductor pollicis muscle. Supramaximal stimulation at 1 Hz was used at the ulnar and the facial nerve. All signals were simultaneously recorded for 30 min. Data are presented as means (SD).

The staircase effect was significantly positive for the first dorsal interosseus muscle and the adductor pollicis muscle. The signal potentiation was not significantly different between the first dorsal interosseus muscle with a maximum increase at 148 (19)% using PMG, and the adductor pollicis muscle at 154 (22)% using MMG. The evoked signals reached a plateau after 15-18 min at both muscles. There was only a small initial increase in signal height at the corrugator supercilii to a maximum of 117 (20)% at 7 min, after which the signals decreased to reach a plateau at 25 min. In comparison with the signal height of 105 (25)% at 30 min, there was no significant difference of signal heights throughout the observation period.

A positive staircase phenomenon is found equally at the first dorsal interosseus muscle and the adductor pollicis muscle. There is no significant staircase effect at the corrugator supercilii muscle.