In short
Organize nitrous contraindications by mechanism, not as a memorized list: closed gas spaces (nitrous is roughly 34 times more soluble in blood than nitrogen and expands any trapped gas pocket — middle ear, sinus, bowel, a recent intraocular gas bubble); inactivation of vitamin B12-dependent methionine synthase with chronic or repeated exposure; nasal obstruction, which is the single most common reason nitrous simply does not work, not a contraindication in the strict sense; and pregnancy, which splits into the patient (occasional analgesic use appears low-risk) versus staff (occupational exposure without scavenging is the actual concern). The blood-pressure drop some patients get is the anxiety leaving, not vasodilation — my own earlier shorthand for that had been sloppy. Two things cut the other way: a randomized tympanoplasty trial found no significant difference in graft failure with nitrous, softening the classic middle-ear caution, and B12 levels are typically normal even in documented nitrous neurotoxicity, so a normal blood test is not a clearance.
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