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Section IICoredrugs in anaesthetic practiceThe former is further hydrolyzed while the latter is hydroxylated to 4-hydroxy–2,
6-xylidine forming the main metabolite, which is excreted in the urine. Some of the
metabolic products of lidocaine have anti-arrhythmic properties while others may
potentiate lidocaine-induced seizures.
Clearance is reduced in the presence of hepatic or cardiac failure.Eutectic mixture of local anaesthetic (EMLA)
When two compounds are mixed to produce a substance that behaves with a sin-
gle set of physical characteristics, it is said to be eutectic. Eutectic mixture of local
anaesthetic (5%) contains a mixture of crystalline bases of 2.5% lidocaine and 2.5%
prilocaine in a white oil:water emulsion. The mixture has a lower melting point, being
an oil at room temperature, while the individual components would be crystalline
solids.Presentation and uses
Eutectic mixture of local anaesthetic is presented as an emulsion in tubes containing
5gor 30 g. It is used to anaesthetize skin before vascular cannulation or harvesting
for skin grafts. It should be applied to intact skin under an occlusive dressing for at
least 60 minutes to ensure adequate anaesthesia.Cautions
EMLA cream should be avoided in patients with congenital or idiopathic
methaemoglobinaemia, or in infants less than 12 months of age who are receiv-
ing treatment with methaemoglobin-inducing drugs. Patients taking drugs associ-
ated with methaemoglobinaemia (e.g. sulphonamides or phenytoin) are at greater
risk of developing methaemoglobinaemia if concurrently treated with EMLA cream.
Methaemoglobinaemia is caused by o-toluidine, a metabolite of prilocaine.
EMLA should not be used on mucous membranes due to rapid systemic absorp-
tion. EMLA should be used with caution in patients receiving class I anti-arrhythmic
drugs (e.g. tocainide, mexiletine) because the toxic effects are additive and poten-
tially synergistic.Bupivacaine
Presentation and uses
Bupivacaine is prepared as a 0.25% and 0.5% (with or without 1:200 000 adrenaline)
solution. A 0.5% preparation containing 80 mg.ml−^1 glucose (specific gravity 1.026)
is available for subarachnoid block.
Itremains the mainstay of epidural infusions in labour and post-operatively
despite concerns regarding its potential cardiac toxicity and the availability of newer
drugs (ropivacaine and levobupivacaine). The maximum dose is said to be 2 mg.kg–1.