Multicenter, double-blinded, randomized control trial. Enrollment is based on presentation to one of the treating facilities with clinical signs and symptoms of high-altitude pulmonary edema with a recent arrival at high altitude, an oxygen saturation below 85%, and radiographic evidence of noncardiogenic pulmonary edema. A total of 264 patients will be randomized 1:1 across 2 study arms: a CPAP mask set to 15cm H2O and an altered CPAP mask in which the pressure valve has been modified to deliver no positive pressure. All patients will be treated with 30mg nifedipine XR. Except for the application of positive pressure, all other interventions are performed in the context of everyday clinical practice, and thus will be equivalent for participants in both arms. Patients will be evaluated every 30 minutes for their ability to maintain an oxygen saturation of 92% with a normal respiratory rate and no signs of increased work of breathing on 2 liters of nasal cannula oxygen for a total of up to a maximum treatment length of 2 hours. The primary efficacy endpoint will be assessed every 30 minutes for 2 hours.
