Which effect of nitrous oxide on closed gas spaces makes it contraindicated in middle ear surgery?

• A N2O directly causes vasoconstriction of middle ear vessels
• B N2O reacts with metal prostheses in the ossicular chain
• C N2O produces excessive secretions that obstruct the Eustachian tube
• D N2O is 34 times more soluble in blood than nitrogen, causing rapid expansion of nitrogen-containing spaces ✓

Explanation

Nitrous oxide (N2O) has a blood-gas partition coefficient (0.47) much higher than nitrogen (0.014), making it approximately 34 times more soluble in blood than nitrogen. When N2O diffuses into a closed nitrogen-containing space (middle ear, pneumothorax, bowel) faster than nitrogen can exit, the space expands. In the middle ear this can displace tympanic membrane grafts or cause barotrauma; in the eye after vitreoretinal surgery with SF6 or C3F8 it can raise intraocular pressure dangerously.

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

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Written and medically reviewed by the StethoPrep medical team.