Lidocaine Cartridge Calculation — MFDS Part 1 MCQ

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EasyPharmacologyLidocaine Cartridge CalculationMFDS Part 1

A healthy 70 kg adult is to receive 2% lidocaine with adrenaline 1:80,000 from 2.2 mL dental cartridges. For this calculation, use a maximum lidocaine dose of 4.4 mg/kg. What is the maximum number of complete cartridges?

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Correct answer: B — 7 cartridges

Explanation lettering: C = shown as A · A = shown as B · D = shown as C · B = shown as D

The correct answer is A: 7 cartridges. Using the stated convention, the maximum lidocaine dose is 70 × 4.4 = 308 mg. A 2% solution contains 20 mg/mL, so a 2.2 mL UK dental cartridge contains 44 mg lidocaine. Therefore 308 ÷ 44 = 7 cartridges. Options E (2) and B (3) are below the calculated ceiling and so are not the maximum requested; they may attract candidates who confuse a safe working dose with the calculated limit. Option C (10 cartridges, 440 mg) and option D (14 cartridges, 616 mg) exceed the weight-based limit; D reflects the error of using 1.1 mL or a 1:1 mL-to-cartridge conversion. Clinically, the smallest effective dose should always be used: this figure is a ceiling, not a target.

Reference: NHS Business Services Authority, dm+d browser. Lignospan Special 20 mg/ml / 12.5 micrograms/ml solution for injection 2.2 ml cartridges (lidocaine 44 mg/2.2 ml, 2%; adrenaline 1:80,000), accessed 2025. https://dmd-browser.nhsbsa.nhs.uk/ampp/view/95558