Platypnea-orthodeoxia syndrome in Fontan circulation: is it gravity, the height difference between the right and left pulmonary arteries, or both?

Summary A 22-year-old woman with an extracardiac Fontan circulation experienced hypoxemia while standing. Pulmonary ventilation perfusion scintigraphy showed reduction of right lung perfusion while sitting. Her pulmonary arteries were tilting to the left. We diagnosed her as platypnea-orthodeoxia syndrome, which was caused by gravity induced ventilation perfusion mismatch, and facilitated by a height difference between her right and left pulmonary arteries.