Pharmacology for Anaesthesia and Intensive Care

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10 Local anaesthetics

Prilocaine

Presentation and uses

Prilocaine is presented as a 0.5–2.0% solution. It is also available as a 3% solution with

felypressin (0.03 unit.ml−^1 )for dental use. It has similar indications to lidocaine but

is most frequently used for intravenous regional anaesthesia. The maximum dose is

6 mg.kg–1or 8 mg.kg–1when administered with felypressin.

Kinetics

Prilocaine is the most rapidly metabolized amide local anaesthetic, metabolism

occurring not only in the liver, but also the kidney and lung. When given in large doses

one of its metabolites, o-toluidine, may precipitate methaemoglobinaemia. This may

require treatment with ascorbic acid or methylene blue, which act as reducing agents.

The neonate is at special risk as its red blood cells are deficient in methaemoglobin

reductase. EMLA cream may precipitate the same reaction.

Cocaine

Presentation and uses

Cocaine is an ester local anaesthetic derived from the leaves ofErythroxylon coca,

aplant indigenous to Bolivia and Peru. It is used for topical anaesthesia and local

vasoconstriction. Moffatt’s solution (2 ml 8% cocaine, 2 ml 1% sodium bicarbonate,

1ml1:1000 adrenaline) has been used in the nasal cavities, although its potential for

side effects has rendered it less popular. Cocaine is also formulated as a paste ranging

from 1% to 4%. A maximum dose of 1.5 mg.kg–1or 100 mg is currently recommended.

Mechanism of action

Cocaine blocks uptake 1 and MAO while also stimulating the central nervous system.

These combined effects increase the likelihood of precipitating hypertension and

arrhythmias. Its use also provokes hyperthermia.

Side effects

When taken or administered in high doses it can cause confusion, hallucinations,

seizures, arrhythmias and cardiac rupture.

Kinetics

Cocaine is absorbed well from mucous membranes and is highly protein bound

(about 95%). Unlike other esters it undergoes significant hepatic hydrolysis to inac-

tive products, which are excreted in the urine.

Amethocaine

Amethocaine is an ester local anaesthetic used for topical anaesthesia. It is presented

as 0.5% and 1% drops for topical use before local anaesthetic block or as a sole agent