Visual estimation of onset time at the orbicularis oculi after five muscle relaxants: Application to clinical monitoring of tracheal intubation

The onset time of neuromuscular blockade at the adductor pollicis (AP) is different among neuromuscular blocking drugs, but these discrepancies had never been studied at the orbicularis oculi (OO). The purpose of this study was to verify if the differences in onset time observed at the AP still existed at the OO and to score the intubating conditions using monitoring at the OO after five muscle relaxants. The study included 172 adults aged 18-75 yr. Anesthesia was induced with fentanyl and propofol, Atracurium (0.5 mg / kg), mivacurium (0.20 mg / kg), rocuronium (0.6 mg/kg), succinylcholine (1.0 mg/kg), or vecuronium (0.08 mg/kg) was injected by random allocation. Time to complete disappearance of the response at the OO was assessed visually after train-of-four stimulation of the facial nerve. Laryngoscopy was then performed, and intubating conditions were determined on a scale of 1-4. Results were based on 150 patients. Onset time at the OO was (mean +/- sD): succinylcholine (57 +/- 17 s) < mivacurium (99 +/- 19 s) = rocuronium (99 +/- 47 s) ( atracurium (129 +/- 33 s) = vecuronium (135 +/- 38 s) (P < 0.05). Overall intubating conditions were excellent (84%), good (14%), poor (1.3%), impossible (0.7%), and were similar among the five groups. We conclude that differences in onset time of muscle relaxants observed at the AP were also found at the OO. Visual estimation of the response at the OO correctly predicted good-to-excellent intubating conditions in more than 90% of cases for all the currently available muscle relaxants. Implications: Onset time of neuromuscular blockade, as estimated visually at the orbicularis oculi, depends on the muscle relaxants given. Regardless of the relaxant used, intubating conditions at loss of orbicularis oculi are acceptable.