Important advances in cardiac surgery, intensive care, and diagnostic modalities over the last decades have led to a steady growth in the number of adults with congenital heart defects. Among adults with congenital heart disease, patients with a Fontan circulation typically show the lowest values of peak oxygen consumption. For these patients, strict exclusion from activities at high altitude may have an impact on quality of life. The investigators aim to elucidate the short term effects of a stay at high altitude on hemodynamic adaptation, exercise capacity and clinical well being in adult Fontan patients. 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 and therefore tolerate the journey to the Jungfraujoch well.