Comparison of the neuromuscular blocking effect of cisatracurium and atracurium on the larynx and the adductor pollicis

Background: Cisatracurium unlike atracurium is devoid of histamine-induced cardiovascular effects and this alone would be the greatest advantage in replacing atracurium for the facilitation of tracheal intubation. On the other hand, 2 ED95 doses of cisatracurium (100 mug/kg) do not yield satisfactory intubating conditions such as those seen with equipotent doses of atracurium and therefore the recommended intubating dose of cisatracurium is 3 ED95. To understand this discrepancy better, we evaluated the potency and onset of atracurium and cisatracurium directly at the larynx adductors in humans. Methods: The study was conducted in 54 patients (ASA class I or II) undergoing peripheral surgery requiring general anesthesia. Cisatracurium 25-150 mug/kg or atracurium 120-500 mug/kg intravenous (i.v.) boluses doses were administered during anesthesia with propofol, nitrous oxide, oxygen and fentanyl. Neuromuscular block was measured by electromyography (single twitch stimulation every 10 s) at the larynx and the adductor pollicis. The dose-response effect measured at both muscles included maximum neuromuscular blockade achieved (Emax), the time to maximum depression of twitch height (onset) and time to spontaneous recovery of the twitch height to 25%, 75% and 90% (T25, T75, T90) of control value. Result: The onset at the larynx was of 196 +/- 28 s after the 100 mug/kg cisatracurium dose compared with 140 +/- 14 s after the 500 mug/kg atracurium dose (P < 0.05). Emax at the larynx was 92 +/- 1% and 98 +/- 1% after 100 mug/kg cisatracurium and 500 mug/kg atracurium, respectively (P < 0.05). The time to onset of maximum suppression Emax = 100 +/- 0% after a 150 mug/kg cisatracurium dose was 148 +/- 29 s. At the larynx, the ED50 was 25 mug/kg for cisatracurium and 180 mug/kg for atracurium and the ED95 was 87 mug/kg for cisatracurium compared with 400 mug/kg for atracurium. Conclusion: The slow onset time at the laryngeal muscles after cisatracurium can be explained by the higher potency as compared with atracurium.