COMPARISON OF THE NEUROMUSCULAR BLOCKING EFFECT OF ATRACURIUM AND VECURONIUM ON THE ADDUCTOR POLLICIS AND THE GENIOHYOID MUSCLE IN HUMANS

Background: Residual paralysis of suprahyoid muscles may occur when the adductor pollicis response has completely recovered after the administration of a neuromuscular blocking agent, The response of the geniohyoid muscle to intubating doses of muscle relaxants is evaluated and compared to that of adductor pollicis. Methods: Sixteen patients undergoing elective surgery under general anesthesia were given 5-7 mg . kg(-1) thiopental and 2 mu g kg(-1) fentanyl intravenously for induction of anesthesia. Eight (half) patients then received 0.5 mg . kg(-1) atracurium, and the other eight received 0.1 mg . kg(-1) vecuronium, The evoked response (twitch height, TH) of the adductor pollicis was monitored by measuring the integrated electromyographic response (AP EMG) on one limb and the mechanical response, using a force transducer (AP force), on the other. The activity of geniohyoid muscle (GH EMG) was measured using submental percutaneous electrodes. The following variables were measured: maximal TH depression; onset time for neuromuscular blockade to 50%, 90%, and maximal TH depression (OT50, OT90, and OTmax); times between administration of neuromuscular blocking agent and TH recovery to 10%, 25%, 50%, 75%, and 90% of control; and time for return of train-of-four ratio to return to 0.7. Results: The principal findings were (1) OTmax was significantly (P < 0.01) shorter for geniohyoid than for adductor pollicis after either atracurium or vecuronium (OTmax was 216, 256, and 175 s for AP force, AP EMG, and GH EMG, with atracurium and 181, 199, and 144 s with vecuronium, respectively), and (2) the evoked EMG of geniohyoid recovered at the same speed as the EMG of adductor pollicis after an intubating dose of atracurium or vecuronium (recovery of TH to 75% of control at 50, 48, 42 min with AP force, AP EMG, and GB EMG with atracurium and 46, 45, and 42 min with vecuronium, respectively). Conclusions: Once the adductor pollicis response has returned to normal values after a single intubating dose of atracurium or vecuronium, the risk of residual depression of the TH of the geniohyoid muscle, one of the principal muscles contributing to airway patency, appears unlikely.