All regional techniques may lead to loss of consciousness or loss of airway, and so require the same facilities, expertise, and precautions as for general anaesthesia (eg full resuscitation facilities and patient fasted).
Local anaesthesia is used either alone or to supplement general anaesthesia, the aim is to prevent or reduce nerve conduction of painful impulses to higher centres (via the thalamus), where the perception of pain occurs. (Action is by a membrane-stabilizing effect, impairing membrane permeability to sodium, so blocking impulse propagation.)
Types of local anaesthesia (LA) Amides:
- Lidocaine (=lignocaine) t 2h; max dose in typical healthy adult=3mg/kg up to 200mg (500mg with epinephrine, but use with caution).
- Prilocaine t 2h. Moderate onset. The dose is 3-5mg/kg. (400mg is the maximum dose in adults; 300mg if used with felypressin.) Very low toxicity, so it is the drug of choice for Bier's block (IV regional anaesthesia).
- Bupivacaine (levobupivocaine1) t 3h. Slow onset and prolonged duration. Contraindicated in intravenous regional anaesthesia (Bier's block). (NB: levobupivacaine is a newer less cardiotoxic local analgesic, otherwise similar in action to bupivacaine.) For dose see BNF.
- Ropivacaine t 1.8h. Dose: see BNF. Less cardiotoxic than bupivacaine. Less motor block when used epidurally. Contraindicated for regional anaesthesia and paracervical block in obstetrics.
Note: 0.5% solution=5mg/mL. 1% solution=10mg/mL. So for a 70kg man, the max dose of lidocaine is 20mL of 1% or 10mL of 2% solution. NB: lidocaine injections are less painful if they are warm, or at lower concentrations.
Certain commercially available preparations contain adrenaline (=epinephrine); these should be used with extreme caution, as systemic effects from the adrenaline may arise and prove hazardous, especially in CVS disease orBP.
Adrenaline (epinephrine) is ABSOLUTELY contraindicated in digital or penile blocks, and around the nose or ears. (Ischaemia produced may cause gangrene.)
Esters
(Infrequently used now.)
- Cocaine. Very high toxicity. Short duration of action. Used as a paste preparation for anaesthesia and vasoconstriction prior to nasal intubation or nasal surgery.
- Tetracaine t 1h. Slow onset. High toxicity. Drops for topical anaesthesia to eye (the eye must be covered with a patch following use), and now topically as an alternative to EMLA.
Toxicity
From excess dose, too rapid absorption, or direct IV injection.
Features
Perioral tingling; numb tongue; anxiety; lightheadedness; tinnitus; seizures; apnoea; collapse; direct myocardial depression; coma.
Treatment
Ensure oxygenation.
- Hypotension: try IV fluids first, then consider ephedrine 5mg IV as a slow bolus (may be repeated every 3-4min to a total of 30mg).
- Convulsions: thiopental (=thiopentone) sodium, or benzodiazepines, eg IV Diazemuls 2.5-5mg, or midazolam 5mg boluses. Intubation/IPPV will probably also be needed.
- CVS collapse: full resuscitation, ie intubation, adrenaline (=epinephrine), atropine, and cardiac massage as required.
Anaphylaxis
Occurs more commonly with the esters, but can occur with amides. A 2mL test dose can detect possible allergic reactions, but not anaphylaxis (and can warn of inadvertent intravascular injection).
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