contraindicated in patients who have otitis media It is also contraindicated in pregnant patients, and pregnant staff members should not administer N2 O Commercially available N2 O devices provide between a 50–50 mixture and a 70–30 mixture of N2 O–oxygen, and contain a fail-safe system that shuts off the flow of N2 O when oxygen flow stops Some delivery units require the patient to create a negative pressure of to cm H2 O to open the mask’s “demand valve” that delivers the gas This type of delivery system is best for children years of age and older A continuous-flow nasal mask can also be used to deliver N2 O and is more acceptable to younger children; some practitioners have developed a continuous-flow face mask that is potentially more reliable in children who typically breathe through their mouths Each unit is fitted with a scavenger system that is connected to the vacuum system in the patient’s room The equipment is portable, but can cost several thousand dollars and must be inspected daily to ensure safety In addition, the patient who receives N2 O requires the attention of a staff member who is skilled and experienced in the administration of the agent Ketamine Ketamine hydrochloride is a phencyclidine derivative that is commonly used for PSA in children Ketamine is an NMDA receptor antagonist and is both water and lipid soluble Ketamine’s effect on blood pressure is particularly notable as it causes hypertension due to catecholamine release, as opposed to the hypotension frequently seen with other sedatives Other advantages include preservation of airway reflexes and respiratory rate, and bronchodilation Ketamine also causes an increase in cerebral metabolism and blood flow Of note, even ketamine can cause cardiovascular collapse in patients who are catecholamine depleted Ketamine’s effects are dose responsive: At doses less than mg/kg IV, the primary effect is analgesia and disorientation; at doses above mg/kg IV, the primary effect is dissociation in addition to amnesia and analgesia Reaching this dissociative state is the aim when using ketamine for PSA Witnessing this dissociative state can be unnerving to families who have not been prepared or expect that sedation means patients will be “asleep” and will not move With ketamine sedation, patients may moan, move slightly, maintain