A 60-year-old man is taken to the post-anesthesia care unit less than an hour after an elective laparoscopic cholecystectomy. The surgery was uneventful and the patient was extubated in the recovery room. Medical history is significant for hypertension, hyperlipidemia, chronic kidney disease, and obstructive sleep apnea. The patient has no history of alcohol, tobacco, or recreational drug use. Temperature is 100 F (37.8 C), blood pressure is 125/65 mm Hg, pulse is 75/min, and respirations are 7/min. Oxygen saturation is 100% on 4 L of oxygen via nasal cannula. When the nurse attempts to wean the patient from the oxygen, pulse oximetry drops to 87%. The patient is somnolent but arousable when his name is called. Auscultation reveals shallow respirations with no rales, wheezing, or rhonchi. Abdominal examination shows a surgical wound with mild distension and tympany. There is no lower extremity edema. Chest x-ray reveals mild hypoinflation. Arterial blood gas analysis on 36% FiO₂ is as follows: Which of the following is the most likely cause of this patient's requirement of supplemental oxygen?

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Explanation: 11ec2b69_c1b8_3ce9_8f63_6dbd3432d9f6_MD0005_00 This patient has postoperative hypercapnic and hypoxic respiratory failure, most likely due to residual anesthesia effect. The effects of sedation, opioid analgesia, and anesthesia place patients at risk for respiratory failure due to decreased central respiratory drive and a depressed state of arousal. Patients with underlying obstructive sleep apnea are at further risk due to sedation and neuromuscular blockers causing decreased pharyngeal muscle dilator tone and a higher propensity for obstructive apneic or hypopneic events. These factors can lead to severe hypoventilation and respiratory failure immediately following or shortly after surgery. Patients with respiratory failure due to residual anesthesia effect are typically somnolent with decreased respiratory rate and tidal volume (suggested by hypoinflation on chest x-ray). Arterial blood gas demonstrates respiratory acidosis with a normal alveolar-arterial (A-a) gradient. Because the hypoxemia is due to hypoventilation and A-a gas exchange is intact, the hypoxemia typically corrects with supplemental oxygen. alveolar-arterial (A-a) gradient (Choice A) Atelectasis is a common cause of postoperative hypoxemia that can occur 2-5 days following surgery; however, it is uncommon immediately following surgery. In addition, because atelectasis causes hypoxemia due to intrapulmonary shunting, the hypoxemia fails to correct with supplemental oxygen and the A-a gradient is typically elevated. (Choice B) Bronchospasm is a potential cause of early postoperative hypoxemia; however, the absence of wheezing on physical examination makes this diagnosis unlikely. (Choice C) Postoperative patients are at increased risk of pulmonary embolism (PE); however, PE in the immediate postoperative period is rare. In addition, acute PE usually presents with respiratory alkalosis due to hyperventilation, and an elevated A-a gradient is expected due to ventilation/perfusion mismatch. (Choice E) Upper airway (laryngeal) edema is a potential complication of endotracheal intubation that presents in the early postoperative period. Patients usually have stridor with tachypnea and increased work of breathing rather than somnolence and bradypnea. Educational objective: The residual effects of anesthetics, opioids, sedatives, and neuromuscular blockers can cause severe hypoventilation leading to hypercapnic and hypoxic respiratory failure in the early postoperative period. In such patients, the alveolar-arterial gradient is normal and hypoxemia typically corrects with supplemental oxygen. References: Postoperative pulmonary complications, early mortality, and hospital stay following noncardiothoracic surgery: a multicenter study by the perioperative research network investigators. (http://www.ncbi.nlm.nih.gov/pubmed/27829093)

A 60-Year-Old Man Is Taken to the Post-Anesthesia Care Unit