[Rocuronium: onset time or speed of action?]

Rocuronium has been said to have an initially rapid onset of action followed by a slower final approach to maximum effect, the supporting data showing only that time to 75% of blockade is more rapid. Expressions of velocity are derived from basic measures such as length and time. The aim of our study was to confirm the onset characteristics of rocuronium by using appropriate expressions of velocity.

After consenting, 25 adults scheduled for elective surgery were anesthetized, using a bolus dose of rocuronium of 400 microg Kg(-1) for relaxation. Electromyography was used to determine maximal effect, time to reach 80% of a full neuromuscular block, onset time, the time interval between these two points, and the differential blockade produced during each period. Velocity for each period was calculated as the ratio between the percentage of block produced per amount of time elapsed (%/s) during each respective period. The ratio between time and blockade (s/%) was also calculated.

More blockade was produced per second (velocity) during the initial period of activity (0.55%/s) than during the final period (0.109%/s). The average velocity during the onset period overall (0.33%/s) lay between those two figures. Rocuronium requires less time (acting fast) to achieve an initial effect (1.95 s/%) than it does to reach the final effect (10.8 s/%). The ratio of early to late speed of onset was 5.8 +/- 3.5.

Using appropriate expressions and calculations, we can unequivocally conclude that rocuronium has a biphasic onset that is initially rapid and then slower. The rate of action is nearly 6 times faster at the beginning.