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 (Grown-up congenital heart disease GUCH). Among adults with congenital heart disease, patients with a Fontan circulation typically show the lowest values of peak oxygen consumption with peak VO2 values between 15 and 29 ml/kg/min, corresponding to 43-64 % of the predicted values in healthy controls. For these patients, strict exclusion from activities at high altitude may have an impact on quality of life. Unfortunately, the most recent recommendations for physical activity, recreation sport, and exercise training in paediatric patients with congenital heart disease do not provide specific recommendations. The investigators study therefore aims 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. These results are necessary to counsel GUCH patients with respect to their leasure activities in Switzerland.