Normally, the left heart system and pressure is high and the right heart system is low. As mentioned earlier, the blood from the left side is easily shunted to the right side. So why would the right heart shunt to the left heart system? There must be another mechanism that is. In general, right-to-left shunts are associated with complex congenital heart disease. The most common ones are tetralogy of Fallot and tetralogy of Fallot. This is mainly due to the presence of narrowing of the right heart outflow tract in these diseases and obstruction in the pathway for right heart ejection, causing higher pressure in the right heart than in the left heart, resulting in a right-to-left shunt.