Long-standing hypoxaemia in complex adult congenital heart disease: anatomical and functional interplay in an uncommon presentation.

We report a case of a 28-year-old man lost to follow-up with chronic hypoxaemia and a history of an uncorrected tricuspid hypoplasia, perimembranous ventricular septal defect (VSD) and pulmonary stenosis. Given this initial diagnosis, chronic hypoxaemia was deemed to result from right-to-left shunt through the VSD driven by elevated right ventricle pressures. However, the further investigation identified an inferior sinus venosus atrial septal defect, unveiling the true mechanism behind the clinical scenario. The patient was submitted to surgical correction with clinical improvement. This case illustrates the defiant evaluation of this type of atrial septal defect through echocardiography and underlines the importance of a multimodal evaluation to reach an accurate diagnosis and optimal management.