• patients with significant respiratory compromise.
• pneumothorax, pulmonary blebs, air embolism, bowel obstruction, and those undergoing surgery of the middle ear.
  • The blood:gas partition coefficient of nitrous oxide is 34 times greater than that of nitrogen. This differential solubility means that nitrous oxide can leave the bloodstream and enter air-filled cavities 34 times faster than nitrogen. As a result, nitrous is contraindicated in patients in whom expansion of these air-filled cavities could compromise patient safety.

• Caution must be exercised in individuals with a history of stroke, hypotension, and known cardiac conditions.
• Nitrous oxide is known to interfere with vitamin B12 and folate metabolism. In patients with these preexisting conditions, nitrous oxide should only be used with full precautionary consideration and close monitoring. Particular precautions should be taken in pediatric patients with underlying vitamin B12 deficiency (genetic or environmental) and conditions related to vitamin B12 deficiency.
• Nitrous oxide is relatively contraindicated in pregnancy. It is known to have potential teratogenic and fetal toxic effects, particularly with chronic use.

http://emedicine.medscape.com/article/1413427-overview#a05