The aims of the study are to measure a)the hemodynamic adaptation (pulmonary blood flow) at rest and during exercise; b)the response of cardiopulmonary parameters to exercise (ventilation, O2 uptake) and c) heart rate variability and occurrence of arrhythmias during the journey and while performing study measures in patients with a Fontan circulation. The measurements will be performed at high altitude and at lowland and compared with an age- and gender-matched control group of healthy subjects. The objective is to test the clinical tolerance of high altitude exposure after a rapid ascent with public transport in patients with a Fontan circulation. The investigators hypothesize that patients with a Fontan circulation in NYHA functional class I-II are able to increase their cardiac output during exercise at high altitude in order to meet the required metabolic demands despite hypoxia induced pulmonary vasoconstriction and therefore tolerate the journey to the Jungfraujoch well.