Local anaesthetic agents reversibly block nerve conduction, predominantly by inactivating sodium channels, and prevent electrical depolarisation. Smaller nerve fibres are more sensitive so that increasing doses are required to block fibres transmitting sympathetic pain, temperature, propriocep-tion and motor impulses. This phenomenon, known as differential conduction blockade, explains why patients may feel pressure but no pain and why a nerve block should be tested by checking for sharp pain or temperature. Furthermore, a limb which cannot be moved is quite likely to be painless.
Local anaesthetics are divided into those with an amide (-N-H-C-) or an ester (-C-O-) bond. The most commonly used drugs are amides. Termination of action is by redistribution into the intravascular space and by metabolism. Redistribution inversely affects duration and efficacy and directly increases toxicity. Vasoconstrictors, commonly adrenaline, reduce redistribution and toxicity and increase duration and efficacy.
Toxicity of local anaesthetics occurs by two mechanisms:
1. Toxicity of the drug: This is due to the result of absolute overdose or accidental intravascular injection; it causes excessive impairment of nerve conduction. Central nervous system toxicity is manifested, with increasing blood concentrations, by perioral numbness and paraesthesia, vertigo, tinnitus and muscle twitching and culminates in seizures. Cardiovascular toxicity occurs at relatively higher doses. The fall in vascular resistance and capacitance, poor myocardial contractility and a variety of arrhythmias may cause various degrees of cardiovascular collapse.
2. Toxicity of the technique: This includes sympathetic blockade and excessive cephalad spread of local anaesthetic which can affect the brain stem and cause apnoea, unconsciousness, convulsions and loss of vasomotor tone.
Allergy to local anaesthetics is mostly due to anaphylaxis to para-amino benzoic acid, a metabolite of most ester drugs. Genuine allergy to amide local anaesthetics is very rare. Local anaesthetic drugs probably do not trigger malignant hyperpyrexia.
Some of the more salient aspects of the common drugs are discussed below. Toxic doses are often quoted, but it is important to realise these vary greatly depending on the medical condition of the patient, site and rate of injection and use of a vasoconstrictor. The doses in parentheses are intended as a guide in standard patients without vasoconstrictor; the use of vasoconstrictors reduces toxicity.
Lignocaine (3 mg/kg)
This is the standard against which others are measured; it is also an antiarrhythmic.
Bupivacaine (2 mg/kg)
This is about four times more potent than lignocaine, which is of slower onset and longer duration. It has selective cardiac toxicity, particularly in pregnancy, often causing life-threatening ventricular arrhythmias which may be difficult to treat. Therefore, it should not be used for i.v. regional anaesthesia.
This is the newest amide local anaesthetic with similar potency and duration of action to bupivacaine. It appears less toxic to the cardiovascular and central nervous system than bupivacaine and is associated with less motor block.
(5 mg/kg): this is equipotent with lignocaine and has a similar time of onset but a shorter duration. It has a wider margin of safety if the tourniquet becomes deflated during i.v. regional anaesthesia, for which it is the drug of choice. It can cause methaemoglobinaemia.
(7 mg/kg): one of the few ester local anaesthetics in use today, cocaine is also a vasoconstrictor. This makes it the drug of choice in ENT surgery. It is a drug of abuse worldwide; its vasoconstrictor effects can cause myocardial ischaemia, tachycardia and cerebral vascular accident.
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