Question 9

Discuss the factors that may potentially influence the speed of onset of neuromuscular blockade.

[Click here to toggle visibility of the answers]

College Answer

Speed of onset is related to how quickly an effective dose reaches the neuromuscular junction, the 
type of interaction with the receptor and the margin of safety of the receptors. Examples of 
parameters that increase speed of onset include a high drug rate of delivery (high CO, high muscle 
group blood flow, and fast injection rate), high drug concentration (higher dose, low potency, higher ED 95, lower protein binding) or a depolarising block. A good answer would include a list of these factors, with a brief explanation. Mention of other factors such as electrolyte disturbances 
gained additional marks. It was expected that the direction of an effect would be clearly indicated 
(e.g. “potency” would not score a mark unless the candidate wrote – “low potency increases 
speed of onset”, etc.). These drugs are charged molecules which do not cross cell membranes 
and have a low volume of distribution. Absorption from GIT, Lipid solubility, pKa, metabolism 
and clearance have minimal relevance to speed of onset.