the passage of blood from the right side of the heart into the left (as through a septal defect), or from the pulmonary artery into the aorta (as through a patent ductus arteriosus); such a shunt can occur only when the pressure on the right side exceeds that in the left, as in advanced pulmonic stenosis, or when the pulmonary artery pressure exceeds aortic pressure, as in one form of Eisenmenger syndrome or in tricuspid atresia.