SummaryRecovery from potent non-depolarising muscle relaxants is slower than from the less potent agents. However, recovery from mivacurium, which is more potent than atracurium, is faster than from atracuriurn following systemic administration. In an attempt to conjrm this discrepancy we compared recovery times following simultaneous administration of equipotent doses of atracurium and mivacurium into the isolated forearms of human volunteers (n = 10). This method enabled us to study the interaction of muscle relaxants with receptors at the neuromuscular junction separated from the effects of plasma drug concentration. In these experiments, the recovery times from maximum block to 50% recovery of control twitch height were significantly longer with rnivacurium than with atracurium (mean 25.2(SD 4.7) versus 22.6(3.1) min, p < 0.01). We found that the evidence that mivacurium has a slower recovery than the less potent atracurium may be true using the bilateral isolated forearm technique and that the discrepancy might be due to a difference in the pharmacokinetic variables of the two drugs. Key wordsNeuromuscular relaxants; atracurium, mivacurium. Pharmacokinetics; recovery.Previous studies have shown that speed of recovery from neuromuscular blockade is fast with low potency nondepolarising muscle relaxants and slow with those with a high potency [ 11. However, following systemic bolus administration of equipotent doses the recovery time of mivacurium is faster than that of atracurium in spite of being three times more potent [2]. This discrepancy has been attributed to the difference in their metabolism. Mivacurium undergoes rapid hydrolysis by plasma cholinesterase whereas atracurium is metabolised at a slower rate by the Hofmann reaction and ester hydrolysis [2, 31. The comparison of recovery times is influenced by their different plasma concentration. This study was carried out in the isolated forearm in order to ascertain what correlation exists between the recovery time and potency of the two drugs following simultaneously administered equipotent doses. The isolated forearm technique allows separation of the effect at the neuromuscular junction from changes secondary to the plasma concentration of drug [4]. MethodsApproval from the ethics committee of Soon Chun Hyang University Hospital was obtained for this study. We investigated the spontaneous recovery of a mivacurium-and atracurium-induced neuromuscular block following simultaneous administration in the isolated forearm of ten volunteers. All subjects were conscious healthy adults who were not receiving any medication which might have influenced neuromuscular transmission. A vein on the dorsum of each hand was cannulated and forearm pneumatic tourniquets were placed on each forearm. The 3M surface electrodes (Red Dot, Ag/AgCl) were applied over the ulnar nerve at the wrists and supramaximal transcutaneous stimulation from a Grass S88 two-channel nerve stimulator using a 0.2 ms square wave pattern at 0.2 Hz were applied.The twitch response of the thumb a...
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