Patients with a univentricular heart live with chronic hypoxia (75-85%) in their first years of life, which could affect adaptation to altitude or other hypoxic insults later in life. To test this hypothesis, we exposed 18 patients with Fontan circulation (age: 24.5 [16.3-38.8] years
f/m 9/9) to simulated altitude using normobaric hypoxia (15.2% oxygen, equivalent to 2500 masl) for 24 h. In blood samples obtained in normoxia (T1, 21% oxygen) and after 24 h hypoxia after a submaximal stress test, we measured hypoxia-regulated molecules involved in angiogenesis and tissue homeostasis. A significant increase was displayed for IL-10 (