Contraindications Absolute contraindications Urgent need for treatment (eg, hemodynamic instability) that cannot await sedation Hypersensitivity to one of the drugs or to the delivery vehicle * Particular to ketamine: Age < 3 months (increased risk of airway compromise) Particular to nitrous oxide: Pneumothorax, pneumomediastinum, bowel obstruction, or intraocular gas bubble (post-vitreoretinal surgery), which are sites of air accumulation into which nitrous oxide can expand * Allergy to eggs or soy are no longer a contraindication to propofol (the egg / soy allergens are different from the egg / soy moieties in the propofol formulation).