Write a short note on Maximum Safe Doses of Local Anaesthetics.

Concept

Every local anaesthetic has a maximum safe dose — the largest amount that can be given without an unacceptable risk of systemic toxicity — expressed in mg/kg of body weight. Because toxicity depends on the plasma level, the dose must always be calculated by weight before injection.

Calculating the Dose

It helps to know that a 1% solution contains 10 mg/mL (so 2% = 20 mg/mL, 0.25% = 2.5 mg/mL). For example, the maximum plain lignocaine for a 70 kg adult is 3 × 70 = 210 mg = 21 mL of 1%. Adrenaline raises the lignocaine ceiling to 7 mg/kg by slowing absorption; it makes little difference to bupivacaine.

WHY the Limit Matters

The maximum safe dose is not an abstract figure but the practical safeguard against systemic toxicity, and it is easy to exceed inadvertently when several syringes are used, when a more concentrated solution is chosen, or in a small or elderly patient, which is why the dose is worked out in milligrams per kilogram and checked against the actual volume drawn up. It is also worth remembering that the ‘safe’ figure assumes correct placement in the tissues; an accidental intravascular injection can cause toxicity at a dose well below the stated maximum, so staying within the limit is necessary but not by itself sufficient for safety.

Note

A quick worked example fixes the method: for a 60 kg patient the maximum plain lignocaine is 180 mg, which is just 18 mL of 1% or 9 mL of 2% — smaller than many people expect.

Always calculate the maximum dose before injecting, not after.

• Max safe dose in mg/kg — calculate by weight before injecting.
• Lignocaine 3 (7 with adrenaline); bupivacaine 2; prilocaine 6 mg/kg.
• 1% solution = 10 mg/mL (2% = 20 mg/mL, 0.25% = 2.5 mg/mL).
• Adrenaline raises the lignocaine ceiling (little effect for bupivacaine).