[Rocuronium: onset time or speed of action?]

Area of Science:

• Anesthesiology
• Pharmacology
• Neuromuscular Blockade

Background:

• Rocuronium is an established neuromuscular blocking agent.
• Previous studies suggested a rapid initial onset followed by a slower progression to maximum effect.
• The precise velocity of rocuronium's onset has been debated due to measurement methods.

Purpose of the Study:

• To confirm the biphasic onset characteristics of rocuronium.
• To quantify the onset velocity of rocuronium using precise calculations.
• To elucidate the speed of neuromuscular blockade progression.

Main Methods:

• Electromyography was used to measure neuromuscular blockade in 25 adult surgical patients.
• Patients received a 400 microg Kg(-1) bolus dose of rocuronium.
• Velocity was calculated as percentage of block per second (%/s) and time per percentage of block (s/%) for different phases of onset.

Main Results:

• The velocity of blockade was significantly higher during the initial phase (0.55%/s) compared to the later phase (0.109%/s).
• The overall average onset velocity was 0.33%/s.
• Rocuronium achieved initial blockade faster (1.95 s/%) than final blockade (10.8 s/%), with an early to late speed ratio of approximately 5.8.

Conclusions:

• Rocuronium demonstrates a definitively biphasic onset of neuromuscular blockade.
• The initial rate of action is approximately 5.8 times faster than the subsequent rate.
• Precise velocity calculations confirm rocuronium's rapid initial neuromuscular blockade.