The second-gas effect of nitrous oxide accelerates induction with a co-administered volatile agent. The underlying mechanism is:
- A N2O inhibits cytochrome P450 enzymes, slowing volatile metabolism and raising alveolar concentration
- B N2O lowers blood-gas partition coefficient of the volatile agent through diffusion competition
- C Rapid uptake of a large volume of N2O concentrates the co-administered gas in residual alveolar volume ✓
- D N2O increases cardiac output, accelerating alveolar-to-blood transfer of the co-administered agent
Correct answer: C. Rapid uptake of a large volume of N2O concentrates the co-administered gas in residual alveolar volume
Explanation
The second-gas effect occurs because N2O is taken up in enormous volumes (litres per minute) from alveoli during early induction. This rapid uptake shrinks alveolar volume, concentrating any co-administered agent (e.g., sevoflurane) and raising its partial pressure faster than it would rise alone. Simultaneously, the negative volume effect augments tidal gas flow into alveoli (augmented inflow effect), further increasing delivery of the second gas. N2O does not alter the blood-gas partition coefficient of other agents.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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