Background:
Aortopulmonary window (APW) is a rare congenital cardiac anomaly characterized by abnormal communication between the ascending aorta and the main pulmonary artery. Without early surgical correction, this condition has a poor prognosis. However, few case reports have described adult survival of patients with untreated APW.

Case presentation:
We report the case of a 15-year-old girl who developed irreversible pulmonary hypertension due to untreated APW. Initially, we suspected the presence of an extracardiac shunt using a simple calculation of the echocardiography-derived pulmonary-to-systemic flow (Qp/Qs) ratio during a routine echocardiography study, which provided us with a clue to proceed with further multimodal diagnostic evaluation. In this report, we describe a comprehensive diagnostic workup, including right heart catheterization and computed tomography imaging, which confirmed the diagnosis of severe irreversible pulmonary hypertension secondary to a large untreated APW.

Conclusion:
This case report highlights the clinical utility of the echocardiography-derived Qp/Qs ratio as a valuable, noninvasive tool for diagnosing APW, which can lead to severe irreversible pulmonary hypertension. The multidisciplinary approach demonstrated in this case serves as a valuable example for clinicians evaluating similar cases. Therefore, APW should be considered in the differential diagnosis of severe pulmonary hypertension, even in adult patients.