A concealed atriopleural fistula resulting from a cardiac stab wound

A young male presented with a right parasternal stab wound. The chest radiography was normal and transthoracic echocardiography ruled out pericardial tamponade. He remained hemodynamically stable until three hours later when signs of progressive anemia were observed. Chest computed tomography showed massive right-sided hemothorax. The patient underwent surgery, which revealed an active bleeding atriopleural fistula connecting the right atrium and draining into the right pleura resulting from the negative pressure generated during respiration. This mechanism prevented cardiac tamponade and maintained initial hemodynamically stability.