The purpose of this study is to investigate the effect of travelling to moderate altitude and of nocturnal oxygen therapy during a stay at moderate altitude on breathing and sleep of patients with pulmonary hypertension or with interstitial lung disease.

Patients with pulmonary hypertension or with interstitial lung disease living below 800 m will be invited to participate in a randomized, cross-over field trial evaluating the hypotheses that: a), breathing and sleep during a 2 day sojourn at moderate altitude are impaired in comparison to low altitude; b), breathing and sleep during a 2 day sojourn at moderate altitude are improved by nocturnal oxygen therapy compared to room air (sham oxygen) administered during nights at altitude. Outcomes will be assessed at low altitude (Zurich, 490 m, baseline), and during 2 study periods of 2 days each spent at moderate altitude (St. Moritz Salastrains, 2048 m), separated by a wash-out period of at least 2 weeks spent at low altitude (<800 m). The order of stays at the different altitudes and of the treatments will be randomized.

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