Monitoring masseter muscle evoked responses enables faster tracheal intubation

Purpose The aim of this study was to investigate whether monitoring neuromuscular block at the masseter muscle (MM) would allow faster tracheal intubation when compared with that at the adductor pollicis muscle (APM). Methods Twenty female patients undergoing gynecological surgery were enrolled into this study. Immediately after inducing anesthesia with fentanyl and propofol, both the left masseter and ulnar nerves were stimulated in a 2 Hz train-of-four (TOF) mode using peripheral nerve stimulators. Contractions of the MM were felt with the anesthesiologist’s left hand lifting the patient’s jaw and holding an anesthesia facemask, while those of the APM were visually observed. Immediately after the contracting responses of the muscles were confirmed, all of the patients received an iv bolus of vecuronium 0.1 mg kg −1 . Onset times after vecuronium were defined as the duration until the contractions became impalpable at the MM or invisible at the APM. When the contraction of the MM could no longer be felt, the conditions for laryngoscopy and tracheal intubation were assessed. Results Onset time evaluated tactually at the MM (mean ± SD, 108.4 ± 27.7 s) was significantly shorter than that evaluated visually at the APM (181.2 ± 32.1 s, P