Timing of Opioid Administration and Postoperative Respiratory Depression.

The goal of this study is to determine if a relationship can be detected between the administration of an opioid and quantitative respiratory depression in spontaneously breathing non-intubated patients who have undergone general anesthesia and have received perioperative opioids. The primary aim is to determine the temporal effect of opioid administration on respiratory depression as assessed by minute ventilation with a reduction of at least 20% in the percent of predicted minute ventilation. A secondary aim is to determine a dose response of opioid administration and its effect on minute ventilation. The investigators aim to determine the influence of opioid administration on minute ventilation on spontaneously breathing patients during post-anesthesia care unit (PACU) stay.

Inclusion Criteria: * Written informed consent * Adults ≥18 years of age and ≤85 years of age * Patients planned for general anesthesia with an expected anesthetic time of at least 2 hours * Patients expected to receive postoperative opioid analgesia Exclusion Criteria: * Non-operative procedure (imaging) * Patients not admitted to the PACU postoperatively * Use of patient-controlled analgesia systems * Female patients who are pregnant or breastfeeding * Patients with asymmetric lung disease * American Society of Anesthesiologists (ASA) classification V * Emergency surgery (ASA E Modifier)