MAC Additivity Calculation — FRCA Primary MCQ

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HardPharmacology - Inhalational AgentsMAC Additivity CalculationFRCA Primary

Sevoflurane has a MAC of 2.0% in oxygen. When 60% N2O is added the required sevoflurane concentration for 1 MAC total is approximately:

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Correct answer: B — 0.8%

N2O MAC is 104%. At 60% N2O the MAC fraction from N2O = 60/104 ≈ 0.58 MAC. Remaining MAC needed from sevoflurane = 1.0 - 0.58 = 0.42 MAC. Sevoflurane concentration = 0.42 × 2.0% = 0.84% ≈ 0.8%. MAC values are additive for inhaled agents.

Reference: Peck Hill Williams 2014 Pharmacology for Anaesthesia and Intensive Care