As noted above, propofol is the most widely used agent in OBA. The drug has a rapid onset, with induction of anesthesia occurring in one arm to brain circulation time. It has a large volume of distribution and a high metabolic clearance. Recovery, primarily by redistribution, is also rapid. Propofol is not associated with nausea and vomiting, and in addition to its role as an induction agent, it is often used as an infusion for maintenance of both monitored anesthesia care and general anesthesia.

Ketamine is a dissociative anesthetic; its mechanism of action is primarily as an antagonist of the N-methyl-D-aspartate (NMDA) receptor. Its early use was plagued with post procedure delirium and nightmares. The judicious use of benzodiazepines and propofol have minimized this issue.

An evidence-based approach to use of ketamine/benzodiazepine is provided by Deng et al. [23]; patients received a bolus of 0.05 mg/kg midazolam. Two minutes before the infiltration of local anesthetic solution, a bolus of ketamine 0.3 mg/kg IV was administered, followed by a stepwise infusion of ketamine: 16.67 mcg/kg/min for 30 min, 13.3 mcg/kg/min for 90 min, and subsequently 10 mcg/kg/min. This approach was statistically superior to other combinations.

One note of caution is appropriate: there have been several scattered reports of ketamine causing damage to the bladder and the ureters when used on a repetitive basis [24]. In brief reports of three palliative care patients who were given ketamine as an analgesic, debilitating urological symptoms developed in one patient with resolution of symptoms following cessation of ketamine, but in the other two, some symptoms persisted until their death. The mechanism is unclear as to whether the underlying disease or ketamine was causative of their demise.

In a head-to-head trial of propofol/ketamine versus propofol/fentanyl, Fabbri et al. studied whether propofol in combination with fentanyl or ketamine provided good quality of anesthesia and recovery time in urological endoscopic outpatient surgery [25]. Sixty patients (ASA I and II) were assigned randomly to receive either 2.5 mcg/kg fentanyl or 1 mg/kg ketamine. In both groups, anesthesia was induced with propofol 1.5 mg/kg and maintained with 7 mg/kg/h. Patients breathed nitrous oxide and oxygen 3:2 spontaneously. Cardiovascular parameters were more stable after ketamine. The most important side effect was the presence of apnea lasting longer than 60s in 14 patients receiving fentanyl. The time to establish alertness was shorter in the ketamine group, who also had a better (P < 0.05) as well as postanesthetic recovery room score.