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. 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:

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
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.

Outside of heart-related conditions, right-to-left shunts of the heart can be seen with Pulmonary Arteriovenous Malformations.

Symptoms

Early cyanosis is a symptom of a right-to-left shunt. A right-to-left shunt results in decreased blood flow through the pulmonary system, leading to decreased blood oxygen levels. Hypoxemia manifests as cyanosis, causing "blue babies."

Diagnosis

Differentiation between a right-to-left shunt and pulmonary disease is often aided clinically by the results of a hyperoxia test. Using high levels of inspired oxygen should have little effect on the dissolved O2 in the blood because highly oxygenated blood is diluted by shunted blood.

Shunt equation

Q_p/Q_s is the shunt fraction
CcO₂ is the end-capillary oxygen content
CaO₂ is the arterial oxygen content
CvO₂ is the mixed venous oxygen content.

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.