Outline the factors which affect the onset, duration of action and toxicity of local anaesthetic agents.

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College Answer

Marks were equally divided between all three parts. Structure to the answer using a table and
list of facts gained credit. Factors affecting onset would be well described by stating Ficks
law of diffusion and followed with an explanation of the equation. Factors affecting duration
such as protein binding, regional blood flow, metabolism and use of vasoconstrictors scored
marks.
Regarding toxicity, an explanation of the CC/CNS ratio was required (the ratio of plasma
levels at which CVSCollapse vs. Convulsions occur). Other factors included structure of
agents, accumulation e.g. due to liver disease. A mention of features of particular agents’
toxicity such as prilocaine and methaemoglobinaemia was expected.
Syllabus G2b 2a-c
References: Peck, Hill and Williams 2nd edition p163-174
Stoelting and Hillier 4th edition p179-203
Evers and Maze p507-533