💊 Local Anaesthetics

Local anaesthetics are a class of drugs that reversibly block nerve conduction when applied to neural tissue, leading to temporary loss of sensation in a specific area of the body.

They work by reversibly blocking voltage-gated sodium channels in neuronal cell membranes. This prevents the influx of sodium ions, thereby inhibiting the generation and propagation of action potentials along nerve fibres. This blockade prevents pain signals from reaching the central nervous system.

Local anaesthetics are used for various procedures requiring localised pain control, such as minor surgical procedures, dental work, and obstetric analgesia (e.g., epidurals). They are also used for regional anaesthesia, nerve blocks, and topical anaesthesia for superficial pain or prior to venepuncture.

Contraindications include hypersensitivity to the specific local anaesthetic or to other amide/ester-type anaesthetics. Certain preparations (e.g., those with adrenaline) are contraindicated in areas with end-arteries (fingers, toes, penis, nose, earlobes) due to the risk of ischaemia. Severe heart block or uncontrolled epilepsy are also contraindications for some regional techniques.

Systemic toxicity can occur with inadvertent intravascular injection or excessive doses, leading to CNS effects (e.g., circumoral numbness, tinnitus, seizures, coma) and cardiovascular effects (e.g., bradycardia, hypotension, arrhythmias, cardiac arrest). Allergic reactions are rare. Local effects include bruising, swelling, and nerve damage (rare).

During and after administration, monitor for signs of systemic toxicity, including CNS symptoms and cardiovascular changes (heart rate, blood pressure, ECG). Resuscitation equipment and lipid emulsion (for severe toxicity) should be immediately available. Observe the anaesthetised area for effectiveness and any signs of complications.

Always check for allergies to local anaesthetics. Carefully calculate the maximum safe dose based on the patient's weight and the specific agent. Be aware of drug interactions, especially with other CNS depressants. Counsel the patient on the expected duration of numbness and to avoid injury to the anaesthetised area.

Understanding the principles of local anaesthetic toxicity and its management (LAST) is a high-yield topic. Knowledge of different types (amides vs. esters) and their metabolism is important. The role of adrenaline in prolonging action and its contraindications in specific anatomical sites is crucial.

SBAs often test the mechanism of action, particularly the sodium channel blockade. Questions may focus on the signs and management of local anaesthetic systemic toxicity (LAST), including the role of lipid emulsion. Differentiating between amide and ester types, and the implications of adding adrenaline, are also common themes.