Acute Mountain Sickness (AMS) is a common condition affecting individuals traveling to elevations greater than 2500 meters (8200 feet). While more gradual ascent profiles, as well as the use of acetazolamide, have been shown to decrease the incidence of AMS, it remains a common condition that can affect anyone who travels to altitude. Many pharmacologic options for treatment of AMS exist, however these are not always effective or able to be taken by all patients. Continuous positive airway pressure (CPAP) has been shown in some small studies and reports to be a potential non-pharmacologic method of both preventing and treating AMS. No large trials to validate the efficacy of CPAP in altitude related illness have been done. This study aims to evaluate the degree to which CPAP can improve symptoms of AMS, as well as improve oxygen saturation among individuals traveling in a high altitude location.

AMS is commonly experienced among travelers in all high altitude locations, and can affect anyone who travels to one of these places. This illness can be prevented, and after its symptoms have been started, can be treated with medication, as well as by descending to a lower elevation. The medications for AMS symptoms, including acetazolamide, dexamethasone, and ibuprofen are not always effective, and even when they are, often take considerable time to show their effect. Additionally, not all patients can take these medications. When these medications are not effective, or there is a contraindication to their use, descent to a lower elevation is necessary, however this is not always possible due to weather, debilitating symptoms, or coinciding diagnoses of high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE). These situations require a difficult and complicated evacuation, requiring multiple rescuers and heavy equipment, or increased risk of clinical deterioration.

CPAP has been shown to be a potential non-pharmacologic method of both preventing and treating AMS. These limited studies quantified symptoms using the Lake Louise score {LLS}, a clinical tool used to assess the severity of AMS. They showed that LLS among subjects receiving CPAP, LLS lower when using it prophylactically or after AMS symptoms had started, however these were not compared against placebo. Additionally, over the last five years, CPAP devices have become quite portable and battery powered, making them much more suitable for travel to wilderness and high altitude locations. They are also available to consumers at low cost, are designed for travel and weigh less than 1 kg. Taking advantage of these devices' portability, a potential new treatment modality for AMS and other altitude related illnesses exists. Additionally, these devices can offer new solutions for mountain rescue and evacuation of patients suffering from altitude related illness, as these devices have been shown in a yet-to-be-published study to improve oxygen saturation and work of breathing while ambulating and self-carrying the device. Improvements in these assessments has been rapid (less than 2 minutes), potentially offering a more rapid treatment of AMS than existing pharmacologic agents as well. This study will be done in Pheriche, Nepal, as it is a location that has a uniquely elevated prevalence of altitude-related illness.

Inclusion Criteria:

• present in Pheriche, Nepal;
• Lake Louise score > 1 (with or without formal AMS diagnosis)

Exclusion Criteria:

• History of chronic respiratory conditions (uncontrolled asthma, COPD, ILD)
• obstructive sleep apnea (if currently using nighttime CPAP)
• congestive heart failure
• coronary artery disease
• history of myocardial infarction
• neurologic disorder
• cognitive disorder
• altered mental status
• pregnancy
• current use of supplemental oxygen
• excessive facial hair,
• claustrophobia

Each participant will not be aware of which arm to which they have been assigned. The on-site investigator will be responsible for randomization, so they will not be able to be blinded.