In this trial, the investigators will evaluate the effect of acetazolamide (375 mg per day) vs. placebo on exercise performance at altitude in patients with COPD.

This is a randomized, placebo-controlled, double-blind parallel trial evaluating the effect of acetazolamide (375 mg per day) vs. placebo on exercise performance in lowlanders with chronic obstructive pulmonary disease travelling to altitude. Participants living in the Bishkek area, Kyrgyzstan (760m), will be transferred by car within 4h to the Tuja Ashu high altitude clinic (3200 m), and stay there for 2 days. Acetazolamide 375mg/day (or placebo), will be administered 24 hours before departure at 760 m and during the stay at altitude. Outcomes will be assessed during the stay at 3200 m.

Exercise performance will be evaluated by maximal cycling spiroergometry and constant load ergometry.

Inclusion Criteria:

• Male and female patients, age 18-75 yrs.
• COPD diagnosed according to GOLD, FEV1 40-80% predicted, SpO2 ≥92% at 750 m.
• Born, raised and currently living at low altitude (<800m).
• Written informed consent.

Exclusion Criteria:

• COPD exacerbation, very severe COPD with hypoxemia at low altitude (FEV1/FVC <0.7, FEV1 <40% predicted, oxygen saturation on room air <92% at 750 m).
• Comorbidities such as uncontrolled cardiovascular disease, i.e., unstable systemic arterial hypertension, coronary artery disease; previous stroke; OSA; pneumothorax in the last 2 months.
• Internal, neurologic, rheumatologic or psychiatric disease including current heavy smoking (>20 cigarettes per day)
• Known renal failure or allergy to acetazolamide and other sulfonamides