Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Cardiac shunts refer to the abnormal flow of blood or mixing of blood between anatomic structures that are ordinarily separated. An example would be mixing of blood between the right atrium and the left atrium in a patient with an atrial septal defect. The direction of flow of the blood may be described as right-to-left, left-to-right or bidirectional, or as systemic-to-pulmonary or pulmonary-to-systemic. The direction may be controlled by left and/or right heart pressure, a biological or artificial valve, or both.
The most common congenital heart defects (CHDs) which cause shunting are the atrial septal defect (ASD), ventricular septal defect (VSD), patent ductus arteriosus (PDA), and patent foramen ovale (PFO). In isolation, these defects may be asymptomatic, or they may produce symptoms which can range from mild to severe, and which can either be acute or have delayed onset. However, these shunts are often present in combination with other defects; in these cases, they may still be asymptomatic, mild or severe, acute or delayed, but they may also work to counteract the negative symptoms caused by another defect (as with d-Transposition of the great arteries).
Some acquired shunts are modifications of congenital ones: a balloon septostomy can enlarge a foramen ovale (if performed on a newborn), PFO or ASD; or prostaglandin can be administered to a newborn to prevent the ductus arteriosus from closing. Biological tissues may also be used to construct artificial passages.
Mechanical shunts are used in some cases of CHD to control blood flow or blood pressure. One example is the modern version of the Blalock-Taussig shunt.