Rational Use of Local Anaesthetics

Summary

The judicious use of local anaesthetic agents requires knowledge of the pharmacological properties of the various drugs, technical skill in the performance of the different nerve blocks, and a thorough evaluation of the patient’s clinical status.

Regional anaesthesia may be classified anatomically as follows: (a) infiltration anaesthesia (extravascular or intravascular); (b) peripheral nerve blockade (minor or major nerve block); and (c) central neural blockade (epidural or subarachnoid block). Anaesthetic potency, onset time and duration of action are the most important clinical properties of any local anaesthetic agent. In general, onset of anaesthesia occurs most rapidly during infiltration techniques and subarachnoid administration, followed in order of increasing onset time by minor nerve blockade, epidural block and major nerve blocks. Duration of anaesthesia is most prolonged when major nerve blockade is performed, followed in order of decreasing duration by epidural and infiltration procedures, minor nerve and subarachnoid blocks.

The local anaesthetic agents commonly employed for regional anaesthesia may be classified according to their relative potency and duration of activity into: (1) agents of low potency and short duration, e.g. procaine and chloroprocaine; (2) agents of moderate potency and duration, e.g. lignocaine (lidocaine), mepivacaine and prilocaine; and (3) agents of high potency and long duration, e.g. amethocaine (tetracaine), bupivacaine and etidocaine.

In general, the onset, duration and quality of regional anaesthesia are enhanced by an increase in dose achieved by either an increase in concentration or in the volume of anaesthetic solution, and by the concomitant use of a vasoconstrictor drug, adrenaline (epinephrine). However, the local anaesthetic properties of the intrinsically more potent and longer acting agents are influenced less by the addition of adrenaline, particularly when such agents are employed for central neural blockade of the epidural type.