A 65-year-old, 80-kg woman has undergone a total knee replacement under general anesthesia with fentanyl (250 µg), isoflurane and 70 percent nitrous oxide. At the end of the two-hour procedure, the patient is somnolent with a respiratory rate of 20 breaths/min and a spontaneous tidal volume of 300 mL. She is transported to the postanesthesia care unit and the tracheal tube is connected to a T-piece with an FiO2 of .4 at a flow rate of 3 L/min. Over the ensuing five minutes, her SpO2 decreases from 98 percent to 84 percent. Which of the following is the most likely etiology of this decrease in SpO2?

With a tidal volume of 300 mL and respiratory rate of 20 breaths/min, the patient’s minute ventilation is 6 L/min. Assuming a normal inspiratory:expiratory (I:E) ratio...

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