The patient was evaluated by a pulmonologist, and a polysomnogram was performed at an altitude of 2,500 m (8,200 feet). The results showed moderately severe sleep apnea (apnea-hypopnea index 43.8/h) that was mostly central in nature (central events 40/h) (Fig 1). The patient refused therapy with positive airway pressure and was placed on nocturnal oxygen. Given a history of persistent hypoxia, the patient pursued continuous daytime pulse oximetry testing and had his oxygen saturations measured at various altitudes. The results are as follows: 91% to 98% on 2 L/min nasal cannula at 2,500 m (8,200 feet), 72% to 92% on room air at 2,500 m (8,200 feet), 81% to 95% on room air at 1,676 m (5,500 feet), 82% to 97% on room air at 838 m (2,750 feet), and 96% to 98% on room air at sea level. Despite his low oxygen saturations, he remained asymptomatic except for periods of “forgetting to breathe during the day,” concurring with drops in his oxygen saturation on pulse oximetry.