Neuromuscular blocking drugs: onset and intubation.

For more than 40 years, succinylcholine has been the traditional choice of muscle relaxant to facilitate tracheal intubation, particularly for anesthesia in the emergency patient with a full stomach. This presentation reviews factors that determine the onset of neuromuscular blockade, particularly with regard to tracheal intubation. Measurement of neuromuscular block, both clinical and via nerve stimulators, is described and compared, and correlations with intubating conditions are attempted. The onset of action of different muscle groups in humans is examined in an attempt to explain timing differences leading to the more rapid onset of block seen in the laryngeal and ventilatory muscles than in the muscles of the hand, which are the usual site of neuromuscular monitoring. The onset of different relaxants is compared, and attempts are made to relate the differences to neuromuscular pharmacology so as to understand the reasons for the rapid onset of succinylcholine (rapid metabolism) and rocuronium (poor potency). None of the currently available drugs, or those undergoing clinical investigation, possesses the rapid onset and prompt recovery of succinylcholine. Despite the formidable side effect profile of succinylcholine, it has not been replaced by a nondepolarizing agent for use in emergency conditions. However, the alternatives, particularly rocuronium and mivacurium, are drugs with a greater safety profile that, in many circumstances, can substitute for succinylcholine.