Patients With Pulmonary Hypertension or Interstitial Lung Disease at Altitude - Effect of Oxygen on Breathing and Sleep

Inclusion Criteria: * Precapillary pulmonary hypertension, or interstitial lung disease. * New York Heart Association class 2-3. * Residence at low altitude (\<800m). Exclusion Criteria: * Unstable or exacerbated condition * Very severe pulmonary hypertension or interstitial lung disease, New York Heart Association class 4 * requirement for oxygen therapy at low altitude residence * hypoventilation * more than mild or unstable cardiovascular disease * use of drugs that affect respiratory center drive * internal, neurologic or psychiatric disease that interfere with protocol compliance including current heavy smoking (\>20 cigarettes per day), inability to perform 6 min walk test. * previous intolerance to moderate altitude (\<2600m). * Exposure to altitudes \>1500m for \>2 days within the last 4 weeks before the study. * Pregnant or nursing patients