COMPARISON OF THE EFFECTS OF MIVACURIUM ON THE DIAPHRAGM AND GENIOHYOID MUSCLES

Although subjects often report difficulty with swallowing shortly afte r receiving neuromuscular blocking agents, difficulty with swallowing during recovery from neuromuscular blocking agents appears to be infre quent. We have used electromyography to compare onset and recovery at the diaphragm and geniohyoid airway muscles after an intubating dose o f mivacurium (0.2 mg kg(-1)) to determine if the geniohyoid muscles we re particularly sensitive to neuromuscular blocking agents. Twelve adu lts undergoing elective surgery were anaesthetized with propofol and f entanyl and the trachea intubated without neuromuscular blocking agent s. The left hypoglossal and right phrenic nerves were stimulated with percutaneous needle electrodes and the electromyogram recorded with su rface electrodes. EMG responses were measured after a bolus dose of mi vacurium 0.2 mg kg(-1). Recordings were also made of the mechanical re sponse of the adductor pollicis to supramaximal ulnar nerve stimulatio n. There was no difference in the rate of onset of block for geniohyoi d muscles and the diaphragm, but recovery to 25% and 90% of the contro l response was shorter at the diaphragm (median 14.5 (95% confidence l imits 12.9-15.3) min and 23.8 (21.7-26) min) than at the geniohyoid mu scle (19.4 (15.6-20.1) min and 29.2 (26.3-31.4) min), respectively (P < 0.05). When the train-of-four ratio of the mechanical response of th e thumb reached 70%, the diaphragm and geniohyoid muscles had recovere d completely in all patients.