A new method of monitoring the effect of muscle relaxants on laryngeal muscles using surface laryngeal electromyography.

I n recent years, attempts have been made to find a nondepolarizing muscle relaxant with a short enough onset to replace succinylcholine as the muscle relaxant of choice for rapid sequence induction. The onset time of new muscle relaxants has been measured mainly by mechanomyographically, electromyographically or, more recently, by acceleromyographically obtained responses of muscular sites, e.g., the adductor pollicis muscle. They are easily accessible, but not the site of interest for sufficient muscle relaxation at intubation: the larynx. Furthermore, the pharmacodynamic data obtained by using different forms of neuromuscular monitoring can vary quite substantially (1,2). Thus, when comparing the onset time or the clinical duration of a specific neuromuscular blockade at various sites, care must be taken to use similar forms of neuromuscular monitoring, e.g., electromyography (EMG) at all sites. EMG has long been a tool to measure responses of laryngeal muscles, especially in surgery of the thyroid gland, where damage to the recurrent laryngeal nerve is a potential hazard. Various electrodes have been used to obtain evoked electromyographic responses, such as hooked-wire electrodes endoscopically introduced into the adducting laryngeal muscles (cricothyroid posterior) (3) or, more recently, a less invasive, easily attachable laryngeal electrode (4). This form of monitoring proved reliable in obtaining reproducable electromyographic signals of the larynx. The purpose of this study was to develop a method that uses the attachable laryngeal surface electrode to obtain electromyographic responses of the adducting laryngeal muscles, evoked by transcutaneous stimulation of the recurrent laryngeal nerve, and measure the onset of the neuromuscular blockade of rocuronium and mivacurium; this was compared with the onset at the adductor pollicis muscle using EMG via surface skin electrodes.