A right-to-left shunt is a cardiac shunt which allows, or is designed to cause, blood to flow from the right heart to the left heart. This occurs when:

1. there is an opening or passage between the atria, ventricles, and/or great vessels; and,
2. right heart pressure is higher than left heart pressure and/or the shunt has a one-way valvular opening.

Commonest example of "Right-to-left shunt" is a congenital anomaly named "Tetralogy of Fallot" which is a cardiac anomaly that is a combination of four heart defects that commonly occur together. The four defects include:

(1):Pulmonary stenosis (narrowing of the pulmonary valve and outflow tract , that creates an obstruction (blockage) of blood flow from the right ventricle to the pulmonary artery (2):Ventricular septal defect (VSD) (3):Overriding aorta (the aortic valve is enlarged and appears to arise from both the left and right ventricles instead of the left ventricle as occurs in normal hearts) (4):Right ventricular hypertrophy (thickening of the muscular walls of the right ventricle)