The purpose of the current study is to evaluate the cerebral tissue oxygenation of Kyrgyz highlanders with high altitude pulmonary hypertension (HAPH) by performing a longitudinal cohort study. To this end, the investigators will invite the same highlanders who participated in the study in 2012 to undergo follow-up examinations in 2017, in order to allow comparisons of current results with baseline data from 2012.

Inclusion Criteria:

• high altitude pulmonary hypertension confirmed by clinical presentation and mean pulmonary artery pressure >30 mmHg measured by echocardiography at altitude of residence.
• healthy subjects (high altitude controls)
• Both genders
• Age >16 y
• Kyrgyz ethnicity
• born, raised and currently living at >2500 m
• healthy subjects currently living at <1000 m (low altitude controls)

Exclusion Criteria:

• Pulmonary hypertension from other causes, in particular from left ventricular failure as judged clinically and by echocardiography
• Excessive erythrocytosis
• Other coexistent disorders that may interfere with the cardio-respiratory system and sleep
• Regular use of medication that affects control of breathing
• Heavy smoking

Highlanders with high altitude pumonary hypertension living at an altitude of 2500-4000 m; healthy highlanders living at an altitude of 2500-4000 m; healthy lowlanders living at an altitude of <1000 m.