| Local anaesthetics: precautions |
Figures for maximum safe dose are imprecise, due to the influence of multiple factors. Use the lowest dose and concentration required to produce effective anaesthesia. Following careful aspiration to avoid inadvertent intravascular bolus or unwanted intrathecal injection, local anaesthetic solutions should be administered incrementally. Local anaesthetics rapidly cross the placenta and can expose the fetus to dangerous levels of the drug in some procedures, depending on the dose and technique of administration. Solutions containing preservative should not be used for intrathecal use.
Aminoamide local anaesthetics are metabolised in the liver and should be used with caution in patients with significant hepatic impairment. Lignocaine, bupivacaine and ropivacaine have metabolites that can contribute to activity and toxicity. Many metabolites are formed by the cytochrome P450 isoenzymes 3A4, 2C9 and 1A2. Prilocaine is metabolised to ortho-toluidine (o-toluidine), which can oxidise haemoglobin to methaemoglobin.
Atypical plasma cholinesterase (pseudocholinesterase deficiency) is a hereditary condition (affecting 1:3000 people) that results in molecular differences in the pseudocholinesterases, resulting in the poor metabolism of the ester molecules and an increased risk of adverse outcomes with ester-based local anaesthetics.
It is important to read the manufacturer’s guidelines as the concentrations of the local anaesthetics and associated vasoconstrictors change as new products come on to the market.
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