Cisatracurium in the orbicularis oculi muscle

Objectives: Muscle relaxants have different pharmacodynamic profiles in various muscles. Therefore, results obtained for one muscle cannot be extrapolated to other muscles. In the adductor pollicis muscle cisatracurium exerts a pharmacodynamic profile comparable to atracurium, despite the known difference in onset time. However, studies evaluating the neuromuscular effect of cisatracurium in different muscles are lacking. Accordingly, this study compares the pharmacodynamic profile of cisatracurium and atracurium in the orbicularis oculi muscle (00)-which shows a neuromuscular course similar to the diaphragm and the laryngeal muscles - and the adductor pollicis muscle (AP). Methods: Forty-five patients (ASA I-II), scheduled for elective spinal surgery were anaesthetized with propofol and fentanyl. Endotracheal intubation was performed without using a muscle relaxant. Neuromuscular transmission was monitored using acceleromyography in both muscles. Patients received 0.1 mg/kg (2x ED95) or 0.15 mg/kg (3x ED95) cisatracurium, or 0.5 mg/kg atracurium (2x ED95) at random. Onset and recovery times were measured according to the recommendation of the Copenhagen Consensus Conference. Results: Onset time was significantly shorter in the 00 than in the AP following 0.15 mg/kg cisatracurium and 0.5 mg/kg atracurium (P<0.05). No differences in onset time between the two muscles were found after 0.1 mg/kg cisatracurium. The recovery of T-1 to 10% of its control was completed sooner in the 00 than in the AP in all three groups (P<0.05). Conclusions: Cisatracurium shows a dose-dependent shorter onset time in the 00 than in the AP. This is consistent with the current view that the onset of non-depolarizing neuromuscular blockers is more rapid in the 00 than in the AP. However; at least a dose of 3x ED95 of cisatracurium was necessary to show a difference in onset time between both muscles. ln contrast, atracurium is reported to lead to a significantly shorter onset of neuromuscular block in the 00 following 2x the ED95. The more rapid recovery of T-1 to 10% of its control in all three groups in the 00 is due to the relative resistance of this muscle to muscle relaxants.