Nitrous oxide Is contraindicated In patients with pneu-mothorax, pneumopericardium or Intestinal obstruction because it:

Nitrous oxide displaces nitrogen in the air filled cavities and while doing so , it enters cavities faster that is even before nitrogen escapes. This results in expansion of such cavities leading to pnuemothorax and air embolus. Hence nitrous oxide should be avoided in such patients. From padmaja 4th edition Page no 191