The concentration effect and second gas effect are phenomena relevant to inhalational anaesthesia induction. Which statement BEST describes the second gas effect?

• A Adding N2O reduces the MAC requirement of volatile agents by a fixed percentage that equals N2O MAC contribution
• B The second gas effect refers to the augmented ventilation caused by N2O stimulating respiratory centres
• C High N2O concentrations reduce the FRC, increasing functional residual capacity for the second gas
• D The rapid uptake of large volumes of N2O concentrates co-administered volatile agents in the alveoli, accelerating their rise in alveolar partial pressure ✓

Explanation

The second gas effect occurs because N2O is administered in large concentrations (50–70%) and is rapidly absorbed from alveoli. This absorption concentrates any co-administered volatile (the 'second gas') in the remaining alveolar gas, raising its alveolar partial pressure faster than would occur without N2O. Additionally, the reduction in alveolar volume from N2O absorption draws more gas in from the airways (augmenting alveolar ventilation for all gases). The concentration effect itself refers specifically to the faster-than-expected rise in alveolar partial pressure of N2O at high inspired concentrations.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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