Onset of rocuronium-induced neuromuscular block evaluated subjectively and accerelomyographically at the masseter muscle

The main aim of this study was to compare the onset times of rocuronium evaluated subjectively and by acceleromyography at the masseter muscle (MM). Forty female patients were sequentially enrolled in this study. In the first 20 patients, neuromuscular block was evaluated subjectively. After induction of anesthesia with fentanyl and propofol, both the left masseter and ulnar nerves were stimulated in 2-Hz train-of-four (TOF) mode using peripheral nerve stimulators. Contractions of the MM were felt with an anesthesiologist's left hand holding an anesthesia facemask; those of the adductor pollicis (APM) were visually observed. All the patients received a bolus of rocuronium, 0.6 mg/kg. Onset times after rocuronium were defined as the duration until the contractions became impalpable at the MM or invisible at the APM. At the time contraction of the MM had not been felt, intubating conditions were assessed. In the next 20 patients, contractions of the MM and the APM were concurrently quantified using acceleromyography after induction of anesthesia and laryngeal mask insertion. Following 0.6 mg/kg rocuronium, onset of the action was recorded. Onset of the action of rocuronium at the MM evaluated subjectively [mean (SD), 70.3 (17.7) s] was similar to that monitored acceleromyographically [73.3 (27.6) s, P > 0.05], and significantly shorter than that at the APM acceleromyographically [111.0 (34.8) s, P = 0.016]. Intubating conditions of 20 patients were graded either excellent or good. Subjective evaluation of contractions of the MM by an anesthesiologist's hand may be reliable to determine faster timing for safe tracheal intubation.