The mechanism of local anaesthetic drugs is dependent on the degree of ionisation at any given pH (Fig. 6.2). Following administration, at physiological pH (7.4) the unionised lipophilic molecules cross the cell membrane by diffusion. The intracellular pH is lower than plasma pH causing the unionised molecules to become ionised and therefore trapped in the intracellular space. These molecules then bind to the intracellular surface of the cell membrane sodium channels, preventing the propagation of action potentials and stopping nerve transmission. Smaller nerve fibres which transmit pain are blocked more readily than larger nerve fibres for light touch.

Complications of local anaesthetic can either be localised or systemic. Localised reactions are rare, but mostly caused by an allergic reaction to the metabolites of the drug. This is more common with amino-esters than amino-amides.

Systemic toxicity is caused by high plasma levels of local anaesthetic agent. Typically toxicity presents with immediate and delayed systemic features mostly affecting the central nervous and cardiovascular systems, with some effects on the haematological and immune systems (Table 6.1).

Systemic toxicity is treated by administering an intravenous lipid infusion (lipid rescue) which draws out the lipophilic local anaesthetic agent from the blood stream and any affected tissues.

In epidural anaesthesia, local anaesthetic is injected into the potential space (extradural space) surrounding the Dura mater. This space extends from the craniocervical junction to the sacral hiatus and contains the segmental spinal nerves. A Tuohy needle may be inserted between vertebrae with the help of an air or fluid-filled syringe to administer the agent (Fig. 6.3). Indications for the use of an epidural include achieving surgical anaesthesia of a specific region of the body, (in the treatment of pain, both chronic and acute) treating acute or chronic pain e.g. in labour, or post-operatively. In addition to local anaesthetics, drugs including opioids, steroids and other medications may be given down the epidural as a one-off injection, or as a continuous infusion.

Examples include:

Spinal anaesthesia involves the injection of local anaesthetic directly into the cerebrospinal fluid (CSF) within the subarachnoid space. This space lies between the pia and arachnoid mater and in addition to CSF, contains blood vessels, spinal nerves and the spinal cord. In adults, the cord extends down to the level of L1/2, and therefore to avoid damage to the cord, the needle may only be inserted below the 2nd lumbar and above the 1st sacral vertebra. A single injection is used which acts for a limited amount of time, providing a denser block with a smaller volume of anaesthetic compared to an epidural (Table 6.2).