Nitrous oxide (N2O) is 35 times more soluble in blood than nitrogen. During emergence from N2O-based anaesthesia, it rapidly leaves blood into alveoli, diluting alveolar O2. This phenomenon is called:

• A Second gas effect
• B Concentration effect
• C Alveolar hypoventilation
• D Diffusion hypoxia (Fink phenomenon) ✓

Explanation

Diffusion hypoxia (Fink phenomenon) occurs during N2O washout: as N2O rapidly transfers from blood to alveoli, it dilutes alveolar O2 and CO2. The fall in alveolar PO2 can cause clinically significant hypoxia (SpO2 <90%) in patients breathing room air. Alveolar dilution of CO2 can also blunt the respiratory drive stimulus. Prevention: administer 100% O2 for 3–5 minutes at the end of N2O anaesthesia. The second gas effect and concentration effect are induction phenomena. Diffusion hypoxia is specifically an emergence phenomenon.

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

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Written and medically reviewed by the StethoPrep medical team.