• James E Fletcher and Christopher M B Heard.
• Department of Anesthesiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA. jfletcher@aims.unc.edu
• Paediatr Anaesth. 2004 Feb 1; 14 (2): 152-7.

BackgroundA synergistic effect has been described when rocuronium (Roc) and mivacurium (Miv) are combined in equal (i.e. 1:1) ED95 proportions at various total doses. We have investigated the effect of Roc or Miv alone and four different ratios (1:4, 2:3, 3:2 and 4:1) of Roc and Miv mixed to a total dose of 1.33 x ED95. The primary outcome is the ratio producing the maximum enhancement of duration of clinical effect.MethodsSixty-eight healthy children were anaesthetized with propofol, nitrous oxide and fentanyl. They then randomly received either Roc 0.4 (mg.kg(-1)), Miv 0.133 (mg.kg(-1)) or one of four Roc + Miv combinations (mg.kg(-1)): Roc 0.32 + Miv 0.027; Roc 0.24 + Miv 0.053; Roc 0.16 + Miv 0.08; and Roc 0.08 + Miv 0.106. The mechanical response of the adductor pollicis muscle to supramaximal stimulation of the ulnar nerve at the wrist was recorded.ResultsDuration of effect was greater in the combination groups than that predicted from the duration of Roc or Miv used alone. Duration was maximally increased around a 1:1 ratio (2:3 and 3:2) of Roc and Miv. The likelihood of achieving 100% block was greater in combination groups compared with Roc or Miv used alone.ConclusionsCombinations of Roc and Miv show a synergistic effect, which appears maximal as the mixture approaches a 1:1 ratio of their ED95s. This combination acted as if a larger effective dose of a single (new) drug had been given, but did not offer the advantage of both rapid onset and short duration of effect.

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