The purpose of this randomized, placebo-controlled double-blind trial is to evaluate the effect of acetazolamide on right heart function at rest in lowlanders with chronic obstructive pulmonary disease (COPD) traveling to high altitude (HA) and developing early signs of altitude-illness.

This randomized placebo-controlled, double-blind, parallel-design trial will evaluate effect of acetazolamide on right heart function in lowlanders with chronic obstructive pulmonary disease (COPD) travelling to high altitude and developping early symptoms and/or signs of impending altitude-illness. Qualifying participants will be randomized 1:1 to acetazolamide or placebo treatment during their further stay of 2 days at 3'100 m.

Inclusion Criteria:

• Men and women, age 35-75 y, living at low altitude (<800 m).

• COPD diagnosed according to the Global Initiative for Obstructive Lung Disease (GOLD) guidelines, FEV1 40-80% predicted, pulse oximetry

  ≥92%, PaCO2 <6 kPa, breathing ambient air at 760 m.

• One of the following early signs and/or symptoms of impending altitudeillness identified by self-monitoring during ascent to or stay at 3100 m:

• Pulse oximetry SpO2≤84%

• Headache or nausea/vomiting or fatigue/weakness or dizziness/lightheadedness of at least moderate intensity

Exclusion Criteria:

• COPD exacerbation, very severe COPD with hypoxemia or hypercapnia at 760 m (see above).
• Other lung disease, relevant comorbidities (such as uncontrolled cardiovascular disease, i.e., unstable arterial hypertension, coronary artery disease; previous stroke; obesity (body mass index >35 kg/m2); internal, neurologic, rheumatologic or psychiatric disease; current heavy smoking (>20 cigarettes per day).
• Renal failure and/or allergy to sulfonamides.
• Patients who do not have early signs and/or signs of impending altitudeillness by self-monitoring (as defined above) at 3'100m will not be included.

Quadruple (Participant, Care Provider, Investigator, Outcomes Assessor) An independent pharmacist will prepare identically looking active and placebo capsules labelled with secret codes. Codes will be concealed to investigators and patients until completion of data analysis.