The investigators aim to include 30 patients with a Fontan circulation and 30 age and gender-matched healthy adults. Baseline testing in Bern (540m over sea level)includes: a symptom limited cardiopulmonary exercise stress test on a cycle ergometer with a ramp protocol (CPX); a pulmonary function test (spirometry); a symptom limited, stepwise increasing workload test with non-invasive inert gas rebreathing method for pulmonary blood flow measurement during exercise (Innocor®); analysis of heart rate variability (HRV); Holter-ECG monitoring for detection of arrhythmias.