Nitrous oxide is avoided in patients with air-containing closed body spaces (bowel obstruction, pneumothorax, middle ear surgery) because:

• A N2O reacts with haemoglobin to form methaemoglobin, reducing oxygen delivery
• B N2O inhibits surfactant synthesis, causing alveolar collapse around air pockets
• C N2O displaces oxygen from alveoli, creating diffusion hypoxia within the spaces
• D N2O is more soluble in blood than nitrogen, and diffuses into air-filled spaces faster than nitrogen can exit ✓

Explanation

Nitrous oxide has a blood-gas partition coefficient (0.47) approximately 34 times more soluble in blood than nitrogen (0.014). When N2O is administered, it diffuses into nitrogen-containing closed body spaces far more rapidly than nitrogen can diffuse out, because of this solubility differential. The result is expansion of the space (pneumothorax, bowel loops) or a rise in pressure within a non-distensible cavity (middle ear, sinus). Methaemoglobin formation is a concern with prilocaine and benzocaine, not nitrous oxide. Diffusion hypoxia occurs in the alveoli at emergence when N2O rapidly exits the blood, diluting alveolar oxygen.

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

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Written and medically reviewed by the StethoPrep medical team.