Describe the factors that influence the speed of ONSET of neuromuscular blockade.

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College Answer

The better candidates had an organised approach to their answer. For example factors could
be broadly classified as pharmacokinetic and pharmacodynamic factors or patient and drug
factors. For example, pharmacokinetic factors could include dose, the use of a priming dose,
patient’s volume status, cardiac output, muscle group and skeletal muscle blood flow.
Pharmacodynamic factors expected were those of mechanism of blockade, receptor affinity,
agent potency, neuromuscular disorders, age, drug interactions and electrolyte disorders.
Many answers listed factors which affect the duration of neuromuscular blockade instead of
the onset. Marks were not allocated for information provided by candidates that did not
address the question asked. It is important that candidates provide an answer specific to the
question asked. Many candidates applied Fick’s Law inappropriately. Lipid solubility and
pKa are not relevant as these drugs do not cross the nerve membrane.
Syllabus - H2a, 2c
Reference: Pharmacology and Physiology in clinical practice, Stoelting Pg 186, Foundations
of Anaesthesia: Basic clinical Science, Hemmings and Hopkins ( 2nd ed)