A right-to-left shunt is a

cardiac The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

shunt which allows

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...

to flow from the

right heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to ...

to the

left heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...

. This terminology is used both for the abnormal state in humans and for normal physiological shunts in

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

Clinical Significance

A right-to-left shunt occurs when: #there is an opening or passage between the atria, ventricles, and/or

great vessels Great vessels are the large vessels that bring blood to and from the heart. These are: *Superior vena cava *Inferior vena cava *Pulmonary arteries * Pulmonary veins *Aorta Transposition of the great vessels is a group of congenital heart defec ...

; ''and'', #right heart

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

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 Persistent truncus arteriosus (PTA), often referred to simply as truncus arteriosus, is a rare form of congenital heart disease that presents at birth. In this condition, the embryological structure known as the truncus arteriosus fails to prop ...

(minimal cyanosis) * Transposition of great vessels *

Tricuspid atresia Tricuspid atresia is a form of congenital heart disease whereby there is a complete absence of the tricuspid valve. Therefore, there is an absence of right atrioventricular connection. This leads to a hypoplastic (undersized) or absent right ventri ...

Tetralogy of Fallot Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are: *pulmonary stenosis, which is narrowing of the exit from the r ...

* 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:

(minimal cyanosis) *2 Vessels involved: Transposition of great vessels *3 Leaflets:

*4 Tetra- prefix:

*5 Words: Total anomalous pulmonary venous return A mainstem intubation with an

endotracheal tube A tracheal tube is a catheter that is inserted into the Vertebrate trachea, trachea for the primary purpose of establishing and maintaining a patent airway and to ensure the adequate Gas exchange, exchange of oxygen and carbon dioxide. Many diffe ...

can lead to right-to-left shunting. This occurs when the tip of the endotracheal tube is placed beyond the

carina Carina may refer to: Places Australia * Carina, Queensland, a suburb in Brisbane * Carina Heights, Queensland, a suburb in Brisbane * Carina, Victoria, a locality in Mildura Serbia * Carina, Osečina, a village in the Kolubara District ...

. 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 Eisenmenger syndrome or Eisenmenger's syndrome is defined as the process in which a long-standing left-to-right cardiac shunt caused by a congenital heart defect (typically by a ventricular septal defect, atrial septal defect, or less commonly, pat ...

. This is seen in

Ventricular septal defect A ventricular septal defect (VSD) is a defect in the ventricular septum, the wall dividing the left and right ventricles of the heart. The extent of the opening may vary from pin size to complete absence of the ventricular septum, creating one ...

Atrial septal defect Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale; however, when this d ...

, and

patent ductus arteriosus ''Patent ductus arteriosus'' (PDA) is a medical condition in which the ''ductus arteriosus'' fails to close after birth: this allows a portion of oxygenated blood from the left heart to flow back to the lungs by flowing from the aorta, which has ...

, 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 Cyanosis is the change of body tissue color to a bluish-purple hue as a result of having decreased amounts of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Body tissues that show cyanosis are usually in locations ...

Tetralogy of Fallot

The most common cause of right-to-left shunt is the

, a congenital cardiac anomaly characterized by four co-existing heart defects. #

Pulmonary stenosis Pulmonic stenosis, is a dynamic or fixed obstruction of flow from the right ventricle of the heart to the pulmonary artery. It is usually first diagnosed in childhood. Signs and symptoms Cause Pulmonic stenosis is usually due to isolated valvula ...

(narrowing of the pulmonary valve and outflow tract, obstructing blood flow from the right ventricle to the pulmonary artery) #

Overriding aorta An overriding aorta is a congenital heart defect where the aorta is positioned directly over a ventricular septal defect (VSD), instead of over the left ventricle. The result is that the aorta receives some blood from the right ventricle, causing ...

(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) #Right

ventricular hypertrophy Ventricular hypertrophy (VH) is thickening of the walls of a ventricle (lower chamber) of the heart. Although left ventricular hypertrophy (LVH) is more common, right ventricular hypertrophy (RVH), as well as concurrent hypertrophy of both ventr ...

(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 hole exists in the septum that divides the left and right ventricles) Outside of heart-related conditions, right-to-left shunts of the heart can be seen with Pulmonary Arteriovenous Malformations (PAVMs).

Symptoms

Early

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 Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the bod ...

). 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 (low oxygenation) blood.

Shunt equation

Q_s/Q_t = (CcO_2  - CaO_2)/(CcO_2  - CvO_2)

* Qs/Qt 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.

References

{{Congenital malformations and deformations of circulatory system Medical terminology