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Potency of Nondepolarizing Neuromuscular Blockers. The potency of neuromuscular blockers can be expressed as the dose of drug required to produce an effect—for example, 50% or 95% depression of twitch height (ED50 and ED95, respectively) (see Table 12-2). It has been suggested that the ED50 value should be employed rather than the ED95 value when comparing the potency of neuromuscular blockers.

  1. Factors that Increase the Potency of Nondepolarizing Neuromuscular Blockers

    1. Inhalational anesthetics potentiate the neuromuscular blocking effect of nondepolarizing neuromuscular blockers (results mainly in a decrease in the required dosage and prolongation of both the duration of action).

      1. The magnitude of this potentiation depends on several factors, including the duration of inhalational anesthesia, the specific inhalational anesthetic used, and the concentration of inhalational agent used.

      2. The rank order of potentiation is desflurane > sevoflurane > isoflurane > halothane > nitrous oxide-barbiturate-opioid or propofol anesthesia.

    2. Some antibiotics potentiate neuromuscular blockade.

      1. The aminoglycoside antibiotics, the polymyxins, and lincomycin and clindamycin primarily inhibit the prejunctional release of acetylcholine and also depress postjunctional nicotinic acetylcholine receptor sensitivity to acetylcholine.

      2. Tetracyclines exhibit postjunctional activity only.

    3. Hypothermia or magnesium sulfate potentiates the neuromuscular blockade induced by nondepolarizing neuromuscular blockers.

    4. Some local anesthetics when given in large doses potentiate neuromuscular block (no clinically significant potentiation occurs with smaller doses).

    5. Antidysrhythmic drugs (quinidine) potentiate neuromuscular block.

  2. Factors that Decrease the Potency of Nondepolarizing Neuromuscular Blockers. Resistance to nondepolarizing muscle blockers has been demonstrated in patients receiving chronic anticonvulsant therapy (attributed to increased clearance, increased binding of the neuromuscular blockers to 1-acid glycoproteins, and/or upregulation of neuromuscular acetylcholine receptors).

  3. Effect of Drug Potency on Speed of Onset

    1. The speed of onset of action is inversely proportional to the potency of nondepolarizing neuromuscular blockers (low potency is predictive of rapid onset and high potency is predictive of slow onset).

    2. The influence of potency on speed of onset is explained by the fact that, for an equipotent dose (a dose that results in 50% receptor occupancy), a low-potency drug (such as rocuronium) will have a higher number of molecules than a high-potency drug.

    3. The concept of “buffered diffusion” explains the slow recovery of long-acting neuromuscular blockers (diffusion of a neuromuscular blocker from the neuromuscular junction is impeded because it binds to extremely high-density receptors within a restricted space represented by the neuromuscular junction).