To the Editor:—

In their article on the pharmacodynamics of rapacuronium, Wright et al.  1stated that after a 1.5-mg/kg dose, the drug's time course of action (at the adductor pollicis) is similar to what they observed in a previous study after 1.0 mg/kg of succinylcholine. 2I think this statement is somewhat misleading. Although the onset profiles of both drugs do indeed seem equivalent, the same is not true of offset.

In their study, they reported a bolus to 25% recovery interval of 13.4 ± 3.2 min after rapacuronium. This is not clinically comparable to their data for succinylcholine (8.0 ± 2 min). Certainly, the recovery index noted for rapacuronium (8.8 ± 1.6 min) is far longer than the value usually cited for succinylcholine, which is at most 2–3 min. It should also be noted that 1.5 mg/kg rapacuronium represents not more than 2 times the ED95, whereas 1.0 mg/kg succinylcholine represents 3–4 times the ED95.

Based on currently available information, rapacuronium should be viewed as a rapid-onset blocking agent of short rather than ultrashort duration.

Wright PMC, Brown R, Lau M, Fisher DM: A pharmacodynamic explanation for the rapid onset/offset of rapacuronium bromide. ANESTHESIOLOGY 1999; 90:16–23
Wright PMC, Caldwell JE, Miller RD: Onset and duration of rocuronium and succinylcholine at the adductor pollicis and laryngeal adductor muscles in anesthetized humans. ANESTHESIOLOGY 1994; 81:1110–5