Diagnosis of systemic venous drainage is mandatory for patients with congenital heart disease planned for cavopulmonary anastomosis or any Fontan-type palliation. Preexisting venous channels as the persistent left superior vena cava are common in cardiac anomalies and may lead to postoperative deterioration and cyanosis. We describe a 6 month old boy with a complex imbalanced atrioventricular septal defect who developed cyanosis in the very early postoperative period, following bidirectional cavopulmonary anastomosis. It was caused by ineffective lung perfusion due to a reopened persistent left superior vena cava with drainage to the coronary sinus.He underwent coil embolization of the persistent left superior vena cava with retrievable coils and cyanosis improved. Coil embolization is an effective alternative to secondary surgery, especially for hemodynamically compromised patients in the postoperative period.