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 th e adductor pollicis response has completely recovered after the admini stration of a neuromuscular blocking agent, The response of the genioh yoid muscle to intubating doses of muscle relaxants is evaluated and c ompared to that of adductor pollicis. Methods: Sixteen patients underg oing 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) atr acurium, and the other eight received 0.1 mg . kg(-1) vecuronium, The evoked response (twitch height, TH) of the adductor pollicis was monit ored by measuring the integrated electromyographic response (AP EMG) o n one limb and the mechanical response, using a force transducer (AP f orce), on the other. The activity of geniohyoid muscle (GH EMG) was me asured using submental percutaneous electrodes. The following variable s were measured: maximal TH depression; onset time for neuromuscular b lockade 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 retur n of train-of-four ratio to return to 0.7. Results: The principal find ings were (1) OTmax was significantly (P < 0.01) shorter for geniohyoi d than for adductor pollicis after either atracurium or vecuronium (OT max was 216, 256, and 175 s for AP force, AP EMG, and GH EMG, with atr acurium 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 vecuron ium (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 vecuro nium, respectively). Conclusions: Once the adductor pollicis response has returned to normal values after a single intubating dose of atracu rium or vecuronium, the risk of residual depression of the TH of the g eniohyoid muscle, one of the principal muscles contributing to airway patency, appears unlikely.