In cardiac anomalies causing severe obstruction in the left side of the heart, such as aortic atresia, mitral atresia, or occasionally severe aortic stenosis, maintenance of circulation depends upon shunting of pulmonary venous blood into the right atrium. The usual pathway by which the shunt is achieved is across the atrial septum through the foramen ovale. When this route is closed or severely narrowed, alternate but less common pathways may exist. These involve either anomalous connections of pulmonary veins to systemic veins or communications with the coronary venous system. In the latter, as commonly occurs in aortic atresia, left ventricular myocardial sinusoids carry pulmonary venous blood from the left ventricular cavity and into the cardiac veins. In other instances of severe left sided obstruction, a direct communication may exist between the left atrium and the coronary sinus.