Ketamine has an established effect on analgesia but the effects of ketamine on ventilation have not been clearly defined. The lack of validated and sensitive instruments to evaluate the effects of ketamine on ventilation is an important reason for the conflicting results.The investigators have demonstrated that the transcutaneous carbon dioxide monitor is accurate in detecting hypoventilation in patients undergoing deep sedation. Animal data suggest that when added to propofol in a sedation regimen, ketamine decreased hypoventilation when compared to propofol alone. It is unknown if ketamine added to a commonly used sedative agent (propofol) can decrease the incidence and severity of hypoventilation in patients undergoing deep sedation. It is also unknown if the effect of ketamine on ventilation are different in patients with and without obstructive sleep apnea.