N2 Washout FRC — FRCA Primary MCQ
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Correct answer: D — Functional residual capacity
The nitrogen washout technique measures FRC. The patient breathes 100% O2 and exhaled N2 is collected until the concentration reaches near zero. Total N2 washed out divided by the initial alveolar N2 concentration gives the volume at which washout started (FRC). Helium dilution is an alternative closed-circuit method.
Reference: Power Kam 2018 Principles of Physiology for the Anaesthetist