Halogenated volatile anesthetics have a potentiating effect on nondepolarizing muscle relaxants. The mechanism appears to be incompletely understood, but is likely a combination of:
- effect on central motor neurons
- augmentation of the NMB’s affinity for the receptor site
- inhibition of post-synaptic nicotinic acetylcholine receptors
Clinically, the presence of inhaled anesthetics will decrease the ED50, as well as both prolong the duration of action and recovery from neuromuscular blockade. The degree of potentiation depends on the specific agent and concentration used. For example, desflurane > sevoflurane > isoflurane. With deeper planes of anesthesia, the degree of potentiation is enhanced due to a dose-dependent inhibition of current through the nicotinic receptor at the neuromuscular junction.
Studies attempting to quantify the magnitude of these effects have led to conflicting results due to the time factor, or duration of anesthesia. The older halogenated agents (halothane, enflurane, isoflurane) may take up to 2 hours to equilibrate with muscle, so in practice the potentiating effects of these vapors might not be immediately apparent. In contrast, newer agents such as sevoflurane and desflurane equilibrate more rapidly with muscle, and the effects may be seen after as little as 30 minutes.
Nitrous oxide has largely been considered as having no effect on neuromuscular blockade.