Which of the following train-of-four (TOF) patterns shown on the right is MOST consistent with partial recovery from a phase II block following neuromuscular blockade using a succinylcholine infusion?
Succinylcholine is among the oldest anesthesia medications still in common use. An intubating dose of succinylcholine produces complete neuromuscular blockade – and optimal intubating conditions – within seconds of administration. Recovery of neuromuscular function is similarly rapid, with return of full motor function typically within 10 to 15 minutes. Succinylcholine is indicated for use in rapid sequence induction and intubation when the goal is to achieve a protected (intubated) airway as quickly as possible after loss of protective reflexes. Succinylcholine is also useful in elective cases when rapid return of neuromuscular function is desired, either because the case is likely to be short or because monitoring of motor nerve function is part of the operation.
Succinylcholine is a depolarizing neuromuscular blocking drug that works by binding to (and triggering) postjunctional nicotinic acetylcholine receptors on skeletal muscle. However, succinylcholine is metabolized more slowly than the natural neurotransmitter, acetylcholine. This means that it blocks the receptor for a longer time, during which further muscle stimulation cannot occur. The result is short-term paralysis, with the duration determined by the level of plasma pseudocholinesterase available to metabolize succinylcholine.
TOF monitoring of neuromuscular blockade induced by a single dose of succinylcholine demonstrates a characteristic pattern that is different from those seen with nondepolarizing agents (Figure). Twitch height decreases and returns symmetrically, rather than the characteristic fade seen with nondepolarizing agents.
Larger or repeated doses of succinylcholine can lead to a phenomenon known as phase II block. In phase II block, recovery of TOF response follows the more common pattern of fade typical of nondepolarizing blockade (Table).