A 'box-shaped' heart is a radiographic description given to the cardiac silhouette in some cases of Ebstein anomaly. The appearance of this finding is caused by the combination of the following features: huge right atrium that may fill the entire right hemithorax shelved appearance of the left cardiac contour due to the dilated right ventricular outflow tract or elevated residual right ventricle small aorta and main pulmonary artery. http://radiopaedia.org/articles/box-shaped-heart
Ebstein's anomaly is a rare heart defect in which parts of the tricuspid valve are abnormal. The tricuspid valve is normally made of three parts, called leaflets or flaps. In patients with Ebstein's anomaly, the leaflets are larger and unusually deep in the right ventricle. The defect usually causes the valve to work poorly, and blood may go the wrong way back into the right atrium. The backup of blood flow can lead to heart swelling and fluid buildup in the lungs or liver.