The purpose of the current study is to evaluate the right ventricular function during graded cycling on an ergometer by transthoracic echocardiography in Kyrgyz highlanders with high altitude pulmonary hypertension (HAPH) by performing a cross-sectional case-control study.

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.