[1] This terminology is used both for the abnormal state in humans and for normal physiological shunts in reptiles.
A right-to-left shunt occurs when: 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.
[citation needed] This occurs when the tip of the endotracheal tube is placed beyond the carina.
This switch in blood flow direction is precipitated by pulmonary hypertension due to increased pulmonary blood flow in a left-to-right shunt.
Because of this switch in the pressure gradient, blood starts flowing right to left, forming a right-to-left shunt.
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.
Outside of heart-related conditions, right-to-left shunts of the heart can be seen with Pulmonary Arteriovenous Malformations (PAVMs).
Differentiation between a right-to-left shunt and pulmonary disease is often aided clinically by the results of a hyperoxia test.