Acetazolamide to Prevent Impending Altitude-illness in Patients With COPD

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, forced expiratory volume in one second 40-80% predicted, pulse oximetry ≥92%, PaCO2 \<6 kilopascal, breathing ambient air at 760 m. * One of the following early signs and/or symptoms of impending altitude-illness 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/light-headedness 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 altitude-illness by self-monitoring (as defined above) at 3'100m will not be included.