Which of the following correctly describes the second gas effect and its clinical relevance when nitrous oxide is used alongside volatile agents?

• A N2O increases MAC of volatile agents, requiring higher concentrations
• B N2O slows induction by increasing FRC and diluting volatile agent partial pressure
• C N2O competitively inhibits volatile agent binding at GABA-A receptors, requiring higher doses
• D Rapid uptake of high volumes of N2O concentrates the simultaneously administered volatile agent in alveoli, accelerating induction ✓

Explanation

The second gas effect: when large volumes of N2O (high concentration, typically 60–70%) are administered, its rapid uptake from alveoli decreases alveolar volume, concentrating the concomitantly administered volatile agent and thereby increasing its alveolar partial pressure faster than if given alone. This accelerates induction. The first gas effect (concentrating effect) explains why 70% N2O achieves near-70% alveolar concentration faster than predicted by its blood:gas coefficient. N2O has a MAC-reducing (additive) effect — adding 60–70% N2O reduces the requirement for volatile agent by approximately 0.6 MAC equivalents.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.