Right-to-left shunt

A right-to-left shunt is a cardiac shunt which allows blood to flow from the right heart to the left heart.^[1] This terminology is used both for the abnormal state in humans and for normal physiological shunts in reptiles. Commonly found in turtles.

Human medical

A right-to-left shunt 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.

Small physiological, or "normal", shunts are seen due to the return of bronchial artery blood and coronary blood through the Thebesian veins, which is deoxygenated, to the left side of the heart.

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. The four defects include:

1. Pulmonary stenosis (narrowing of the pulmonary valve and outflow tract, obstructing blood flow from the right ventricle to the pulmonary artery)
2. Overriding aorta (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)
3. Ventricular septal defect (an abnormal hole between the ventricles)
4. Right ventricular hypertrophy (thickening of the muscular walls of the right ventricle, this is a result of the increased amount of work the heart has to do)

A right to left shunt frequently causes hypoxemia and is characterised by frequent chest infection.

Differentiation between a right to left shunt and pulmonary disease is often aided clinically by the results of a hyperoxia test.

Reptiles

Because most reptiles have a single ventricle and all reptiles have both a right aortic arch and a left aortic arch, all reptiles have the capacity for right-to-left shunt.^{[citation needed]}

References