The Effect of Muscle Relaxants on the Cricothyroid Muscle

Abstract

The speed of onset and recovery rate from muscle relaxants depends on the potency of the drug, the dose of the drug, and the muscle group that is monitored. The diaphragm and the laryngeal muscles are more resistant to the effects of muscle relaxants than the more frequently monitored muscles of the hand, such as the adductor pollicis, which is innervated by the ulnar nerve. Several studies have been performed using electromyographic (EMG) responses of the diaphragm and transgastric pressure changes after stimulation of the phrenic nerve (1-7). Other studies have measured the response of the laryngeal muscles to neuromuscular blocking drugs by measuring tracheal tube cuff pressures after stimulation of the recurrent laryngeal nerve (8-10). The conclusion from all of these studies has been that the diaphragm and laryngeal muscles are more resistant to the effects of muscle relaxants, demonstrate the effects of the relaxant more rapidly, and have faster recovery than muscles of the hand. Despite these aforementioned studies, direct EMG measurement of the effects of muscle relaxants on the function of the cricothyroid muscle has not been previously reported. This article reports the electromyographic responses of the cricothyroid muscle to succinylcholine in one patient and to mivacurium in two patients undergoing modified bilateral radical neck dissection for malignant head and neck tumors. None of the patients received muscle relaxants after tracheal intubation until the motor nerves were identified prior to surgical resection. Body temperature was maintained between 35.5 and 37.0°C with the aid of forced air warming blankets in all three patients.