Describe the factors that increase the risk of systemic toxicity of the amide local anaesthetics.
The amide group oflocal anaesthetics consist of lignocaine, prilocaine, ropivacaine and
bupivacaine. The systemic toxicity primarily relates to toxic plasma levels and the factors
that influence this. The main factors expected can be categorized under drug factors
(including kinetics), patient factors, site of injection and external factors. Many candidates
omitted important details such as pKA, lipid solubility and addition of vasoconstrictors. For
example, absorption is affected by drug pKA, (thecloser to physiological pH themore rapid
the absorption), use of vasoconstrictors and the drugs own vasoactive properties, site of
injection (intercostal>epidural>brachial plexus>subcutaneous infiltration). Distribution is
dependent on physicochemical properties of the amide. The rate of metabolism, mechanism
of action (bupivacaine, in comparison to lignocaine has stronger binding to inactivated
resting sodium channels and a slower rate of dissociation) and external factors (e.g.
systemic acidosis) are other factors that should have been mentioned, and expanded upon
with relevant detail.