Second Gas Effect — FRCA Primary MCQ

Correct answer: D — Rapid nitrous oxide uptake contracts alveolar gas volume and promotes augmented inflow, accelerating sevoflurane transfer; the arterial effect may exceed the end-tidal effect.

The correct answer is D. Rapid uptake of high-concentration nitrous oxide reduces alveolar gas volume. This concentrates the remaining gases and promotes additional inflow of the inspired mixture, accelerating the rise and transfer of the concurrently administered volatile agent. Human studies demonstrate increased arterial as well as end-tidal sevoflurane partial pressure, with the arterial effect potentially larger because of ventilation-perfusion heterogeneity. The concentration effect is related but describes acceleration of nitrous oxide's own alveolar rise, whereas the second gas effect concerns another gas. It is not merely a monitoring artefact and is generally more prominent for poorly soluble companion agents. Events after stopping nitrous oxide describe reverse diffusion and possible diffusion hypoxia, not the second gas effect.

Reference: Peyton PJ et al. Magnitude of the second gas effect on arterial sevoflurane partial pressure. Anesthesiology. 2008;108:381-387. https://pubmed.ncbi.nlm.nih.gov/18292675/