A right-to-left shunt is a cardiac 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 cir ...

flow Flow may refer to: Science and technology * Fluid flow, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on a set * Flow (psyc ...

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

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 rhynchocephalia ...

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 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 a ...

is higher than left heart pressure and/or the shunt has a one-way

valvular A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

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 proper ...

(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:

(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 A tracheal tube is a catheter that is inserted into the trachea for the primary purpose of establishing and maintaining a patent airway and to ensure the adequate exchange of oxygen and carbon dioxide. Many different types of tracheal tubes ar ...

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 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, pa ...

. This is seen in Ventricular septal defect,

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 Tetralogy of Fallot, 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 (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 (thickening of the muscular walls of the right ventricle, this is a result of the increased amount of work the heart has to do) # Ventricular septal defect (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 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