A right-to-left shunt is a cardiac shunt which allows blood to flow from the right heart to the left heart. This terminology is used both for the abnormal state in humans and for normal physiological shunts in reptiles. Clinical Significance A right-to-left shunt occurs when: there is an opening or passage between the atria, ventricles, and/or great vessels; and, right heart pressure is higher than left heart pressure and/or the shunt has a one-way valvular opening. Small physiological, or "normal", shunts are seen due to the return of bronchial artery blood and coronary blood through the Thebesian veins, which are deoxygenated, to the left side of the heart. Causes Congenital defects can lead to right-to-left shunting immediately after birth: Persistent truncus arteriosus (minimal cyanosis) Transposition of great vessels Tricuspid atresia Tetralogy of Fallot Total anomalous pulmonary venous return A mnemonic to remember the conditions associated with right-to-left shunting involves the numbers 1-5, as follows: 1 Combination Vessel: Persistent truncus arteriosus (minimal cyanosis) 2 Vessels involved: Transposition of great vessels 3 Leaflets: Tricuspid atresia 4 Tetra- prefix: Tetralogy of Fallot 5 Words: Total anomalous pulmonary venous return A mainstem intubation with an endotracheal tube can lead to right-to-left shunting. This occurs when the tip of the endotracheal tube is placed beyond the carina. In this way only one lung is oxygenated and oxygen-poor blood from the non-ventilated lung dilutes the oxygen level of blood returning from the lungs in the left ventricle. Eisenmenger syndrome An uncorrected left-to-right shunt can progress to a right-to-left shunt; this process is termed Eisenmenger syndrome. This is seen in Ventricular septal defect, Atrial septal defect, and patent ductus arteriosus, and can manifest as late as adult life. This switch in blood flow direction is precipitated by pulmonary hypertension due to increased pulmonary blood flow in a left-to-right shunt. The right ventricle hypertrophies to compensate for this pulmonary hypertension, so the right ventricular pressure becomes higher than the pressure in the left ventricle. Because of this switch in the pressure gradient, blood starts flowing right to left, forming a right-to-left shunt. As with any right-to-left shunt, there is decreased blood flow to the lungs, resulting in decreased oxygenation of blood and cyanosis. Tetralogy of Fallot The most common cause of right-to-left shunt is the Tetralogy of Fallot, a congenital cardiac anomaly characterized by four co-existing heart defects. Pulmonary stenosis (narrowing of the pulmonary valve and outflow tract,