Local anaesthetic requirements and activity vary considerably. Selection of an appropriate agent in a specific situation requires knowledge of the clinical needs and pharmacological properties of the various anaesthetic drugs currently available ( table 2).

 
Topical Anaesthesia

Local anaesthetics may be applied to the skin, the eye, the ear, the nose and the mouth as well as other mucous membranes. In general, cocaine, amethocaine, lignocaine and prilocaine are the most useful and effective local anaesthetics for this purpose. When used to produce topical anaesthesia, they usually have a rapid onset of action (5-10mins) and a moderate duration of action (30-60 mins). Cocaine is a potent vasoconstrictor and is useful in the reduction of bleeding as well as topical anaesthesia. Other local anaesthetic agents may be absorbed in significant amounts particularly after topical application to the more vascular areas, and fatalities have occurred after application of these agents to mucosal surfaces.

Absorption of local anaesthetics through intact skin is usually slow and unreliable and high concentrations (e.g. 20% benzocaine or 40% lignocaine) are required.

 
Infiltration Anaesthesia

Infiltration techniques are used to provide anaesthesia for minor surgical procedures. Amide anaesthetics with a moderate duration of action are commonly used (lignocaine, prilocaine and mepivacaine). The site of action is at unmyelinated nerve endings and onset is almost immediate. The duration of local anaesthesia is variable. Procaine has a short duration of action (15-30 min), while lignocaine, mepivacaine and prilocaine have a moderate duration of action (70-140 min). Bupivacaine has the longest duration of action (approximately 200 min). The addition of adrenaline (1 in 200,000) will increase the quality and prolong the duration of anaesthesia.

 
Conduction Anaesthesia

Conduction anaesthesia can be divided into minor nerve blockade (e.g. ulnar, radial or intercostal), and major blockade of deeper nerves or trunks with a wide dermatomal distribution (e.g. brachial plexus blockade). For each individual agent the duration of anaesthesia will be determined more by the total dose of the drug rather than the volume or concentration of drug used.

When amide local anaesthetics are used to produce minor nerve blockade, they have a relatively rapid onset of action (5-10min). Lignocaine, mepivacaine and prilocaine have a moderate duration of action (1-2 hr), while bupivacaine and etidocaine produce local anaesthesia for 2-6 hrs.

In major nerve blockade the onset is more variable, mainly due to anatomical factors which can delay or restrict the access of the local anaesthetic to its site of action. In general lignocaine, mepivacaine and prilocaine have a faster onset of action (10-15 min) than bupivacaine (15-30 min). Analgesia persists for 3- 4 hr with lignocaine, prilocaine and mepivacaine, but up to 10 hrs with bupivacaine.

 
Extradural Anaesthesia

Local anaesthetic solutions are deposited in the epidural space between the dura mater and the periosteum lining the vertebral canal. The epidural space contains adipose tissue, lymphatics and blood vessels. The injected local anaesthetic solution produces analgesia by blocking conduction at the intradural spinal nerve roots.

The quality and extent of the blockade produced by each agent is determined by the volume as well as the total dose of the drug. The spread of local anaesthetic solutions may be more extensive ipregnant women as the volume of the potential space is reduced by venous engorgement in the epidural space. Enhanced effects may also be seen in the elderly and in patients with arteriosclerosis due to impairment of vascular absorption from the epidural space.

Bupivacaine (0.5%) or lignocaine (1.5-2.0%) are usually used to produce extradural anaesthesia. Repeated administration of lignocaine or mepivacaine into the epidural space may result in a diminished response with each subsequent dose (tachyphylaxis). This may be due to local changes in pH due to the relative acidity of these solutions. The reduction in pH may reduce the amount of free base available for diffusion across the neuronal membrane.

 
Spinal Anaesthesia

The introduction of local anaesthetic solutions directly into the cerebrospinal fluid (CSF) produces spinal anaesthesia. The local anaesthetics do not have to cross tissue or diffusion barriers and also the central attachments of the ventral and dorsal nerve roots are unmyelinated, which allows rapid uptake of the free base. There is a faster onset of action and a smaller dose is required. Spinal anaesthesia produces a similar clinical effect with a dose approximately ten times smaller than that needed for extradural anaesthesia.

Solutions of amethocaine (0.2%), lignocaine (5%), prilocaine (5%) bupivacaine (0.5%) and mepivacaine (4%) are commonly used to produce spinal anaesthesia. Prilocaine and mepivacaine have a slightly longer duration of action than lignocaine; bupivacaine has the longest duration of action.

In pregnancy, compression of the inferior vena cava by the pregnant uterus leads to distension of the vertebral venous plexus and reduces the volume of the subarachnoid space. Consequently the degree of blockade is enhanced and reduced doses are required.

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