Background: Pulmonary hypertension (PH) causes significant morbidity and mortality in children at altitude.

Materials and Methods: Fifty-two children living at 2,640 m were included. During hyperoxia test (O₂Test), patients received high oxygen concentrations (FiO₂ >80, through Mask, using Venturi or nonrebreathing mask); echocardiography was used to evaluate pulmonary vasculature reactivity. A decrease >20% from the basal pulmonary artery systolic pressure was considered a positive response.

Results: Most of the patients had severe PH. The median age at diagnosis was 4.5 years; 34 were female (65.4%). Idiopathic PH was present in 44 patients (84.6%). Six developed severe PH after ductus closure. They were classified in responders (n = 25), and nonresponders (n = 26). Responders were younger (3 years vs. 7 years, p = 0.02), and 22 (88%), had better functional class (FC) 1–2, than nonresponders: 18 (69.23%) of them had worse FC: 3–4 (p = 0.000). In responders, 10/12 who went to live at low altitude became asymptomatic, compared with 7/13 who remained at high altitude. FC 1–2 was achieved by 70% of the patients with idiopathic PH who went to a low altitude, compared with 30% who continued at high altitude (p = 0.03). In nonresponders, 10/26 patients moved to a low altitude: four improved, one worsened, and five died; of the 16/26 patients living at high altitude, four are stable, eight worsened, and four died. Four patients (30.76%) in responder group and nine (69.24%) in the nonresponder group died (p = 0.03). There were differences between both groups in systolic (88 mm Hg vs. 110 mm Hg; p = 0.037), diastolic (37 mm Hg vs. 56 mm Hg; p = 0.035), and mean pulmonary artery pressures (57 mm Hg vs. 88 mm Hg; p = 0.038).

Conclusions: This specific hyperoxia test applied until 24 hours (not published before) helps to predict survival and prognosis of children with PH. Children with PH at a high altitude improve at low altitude.