Nitrous oxide (N2O) is avoided in patients with pneumothorax, bowel obstruction, and middle ear surgery because it:

• A Directly inhibits surfactant production
• B Causes vasoconstriction of feeding vessels to affected organs
• C Raises metabolic CO2 production in closed spaces
• D Has a blood:gas coefficient higher than nitrogen, allowing faster diffusion into closed air spaces ✓

Explanation

N2O has a blood:gas partition coefficient of 0.47, which is about 34 times more soluble in blood than nitrogen (blood:gas 0.014). N2O diffuses into air-filled closed cavities far faster than nitrogen can leave, dramatically expanding the volume (if compliant) or raising the pressure (if non-compliant). In pneumothorax this can cause tension pneumothorax; in bowel obstruction it distends bowel loops; in middle ear surgery it can cause barotrauma. Surfactant and vasoconstriction are not mechanisms.

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

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Written and medically reviewed by the StethoPrep medical team.