Local Anaesthetics
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Direct answer
Lignocaine blocks sodium channels from their inner face, binding when the channel is open or inactivated — which is why rapidly firing pain fibres are blocked first, producing the clinical sequence of pain, then cold, warmth, touch and pressure loss with motor power preserved. The chemistry splits the family in two: esters (procaine, tetracaine, cocaine, benzocaine) are hydrolysed by plasma cholinesterase and cross-react with sulphonamide allergy through their PABA product, while the amides (lignocaine, bupivacaine, ropivacaine, prilocaine) survive to be hepatically metabolised. Dosing arithmetic is where marks are won: 1 per cent solution means 10 mg/mL, lignocaine's ceiling is about 4–5 mg/kg plain and 7 mg/kg with adrenaline, and bupivacaine's cardiotoxicity — collapse that resists resuscitation until a 20 per cent lipid emulsion is infused — is the chapter's most feared complication.
What you must remember
- Mechanism detail: use-dependent (state-dependent) block of voltage-gated sodium channels; smaller unmyelinated pain fibres fall before larger motor fibres — the basis of differential block in spinal and epidural anaesthesia.
- Ester versus amide rule: amides carry two letter i's before "-caine" in the international spelling (lidocaine, bupivacaine, prilocaine); esters such as procaine and benzocaine are PABA-releasing and sulfa-cross-reactive.
- Maximum doses worth memorising: lignocaine about 4–5 mg/kg (roughly 300 mg) plain and 7 mg/kg (roughly 500 mg) with adrenaline; bupivacaine about 2.5–3 mg/kg; always calculate against the concentration — 1 per cent is 10 mg/mL, 2 per cent is 20 mg/mL.
- Adrenaline addition: vasoconstriction prolongs duration, reduces bleeding and slows systemic absorption (raising the safe ceiling); traditional exam answer — avoid in end-arterial territories (digits, penis, nose, ears), despite modern evidence permitting low concentrations; contraindicated with tricyclic antidepressants and uncontrolled hyperthyroidism.
- Systemic toxicity sequence: perioral numbness, tinnitus, metallic taste, slurred speech, then seizures, then cardiovascular collapse; aspirate before injecting; treatment is airway, oxygen, seizure control, lipid emulsion 20 per cent at 1.5 mL/kg bolus repeated, and prolonged resuscitation for bupivacaine arrests.
- Bupivacaine is the most cardiotoxic (tight channel binding); levobupivacaine and ropivacaine are the safer single-enantiomer choices; bupivacaine is never used for intravenous regional (Bier) block — prilocaine or lignocaine is.
- Prilocaine in doses above about 600 mg causes methaemoglobinaemia — chocolate-brown blood, pulse oximeter stuck near 85 per cent, treated with methylene blue 1–2 mg/kg.
- EMLA is the eutectic 2.5 per cent lignocaine with 2.5 per cent prilocaine cream, applied under occlusion for about 60 minutes before venepuncture in children.
- Local anaesthetics work poorly in infected, acidic tissue — the inflamed environment ionises the weak base and abscess fields anaesthetise incompletely; this is also why they fail around inflamed tooth pulps.
- Spinal anaesthesia: hyperbaric bupivacaine 0.5 per cent is standard; lignocaine spinal carries a transient neurological symptoms risk; add clonidine or fentanyl to spare total dose.
- Benzocaine topical gels cause methaemoglobinaemia in children — the FDA warning behind avoiding over-the-counter teething gels.
Worked example: suturing a child's laceration
A 15 kg, three-year-old needs a forehead laceration sutured. The arithmetic first: lignocaine plain 2 per cent at 4 mg/kg gives 60 mg, which is 3 mL — the entire safe budget for this child; at 2 per cent (20 mg/mL), that is exactly 3 mL, and a second surgeon topping up "a little more" crosses into toxicity. Adrenaline-containing solution would raise the ceiling to about 7 mg/kg (105 mg, roughly 5 mL of 2 per cent with adrenaline), and on a vascular face it also improves the field — the end-artery dogma does not apply to the face in current practice, but the exam stem set on a finger or toe still expects the traditional "avoid adrenaline" answer.
Infiltration technique: buffer helps the sting, inject slowly through wound edges, aspirate, and watch the child for the early words she cannot yet say — perioral tingling, tinnitus, drowsiness — because in a crying toddler the prodrome announces itself as sudden quiet. If the safe volume will not cover the field, the correct moves are a regional block, topical application with time, or sedation — never exceeding the calculated ceiling. Lipid emulsion availability is the resuscitation plan, and methaemoglobinaemia from prilocaine joins the differential if the pulse oximeter sits near 85 per cent with chocolate blood.
The arithmetic that fails candidates
Percentage-to-mg/mL conversion is the single commonest examination failure in this chapter: 0.5 per cent bupivacaine is 5 mg/mL, so 20 mL delivers 100 mg — already near the adult ceiling of 2.5 mg/kg for a 40 kg patient. Second, the "top-up" in a long procedure: lignocaine's peak effect on plasma is additive, so the clock and the cumulative total, not the injection site, decide safety. Third, ester allergy is claimed for all local anaesthetics, but a true amide allergy is vanishingly rare — the correct switch is to an amide from an ester (or vice versa in the rare amide reactor). Fourth, the Bier block question hides bupivacaine as the wrong answer. Fifth, tachyphylaxis — repeated doses of the same agent working less — is attributed to "bad batch" rather than recognised pharmacology.
Frequently asked questions
What does 1 per cent lignocaine contain per millilitre?
10 mg per mL (percentage times 10); 2 per cent contains 20 mg/mL — every maximum-dose calculation depends on this conversion.
What are the maximum safe doses of lignocaine?
About 4–5 mg/kg plain (roughly 300 mg in an adult) and 7 mg/kg (roughly 500 mg) with adrenaline, calculated on actual body weight and the volume-concentration product.
How is local anaesthetic systemic toxicity treated?
Stop injecting, give oxygen and airway support, control seizures with a benzodiazepine, and infuse 20 per cent lipid emulsion 1.5 mL/kg bolus with repeat doses and an infusion; resuscitation is prolonged in bupivacaine arrests.
Why does adrenaline extend local anaesthetic action?
Vasoconstriction retains the drug at the nerve, prolonging duration and reducing systemic absorption — it also permits a higher total dose and a drier surgical field.
Which local anaesthetic causes methaemoglobinaemia?
Prilocaine above about 600 mg (and topical benzocaine), treated with methylene blue 1–2 mg/kg — chocolate-brown blood with a pulse oximeter reading near 85 per cent.
Why do local anaesthetics fail in infected tissue?
Inflammation acidifies the tissue, ionising the weakly basic anaesthetic so it cannot cross the nerve membrane — an argument for regional block beyond the infected field.