Left-to-Right Shunt Lesions - Acyanotic Congenital Heart Disease

Introduction:

A shunt is an irregular pattern of blood flow through the heart. It is a passage through which blood moves from one area to another. A cardiac shunt is a congenital heart disease. Congenital heart diseases (CHD) are a group of diseases with malformations of the heart and the heart's blood vessels. It occurs in one percent of live births. In premature babies and stillborns, the incidence is more. There are three categories of CHD. They are left to right shunt, right to left shunt, and obstructive heart disease.

What Are the Basic Structure and Blood Supply of the Heart?

The heart's structure is explained below:

• Four chambers are present in the heart: two upper chambers and two lower chambers.

• The upper chambers are the right and left atrium, and the lower chambers are the right and left ventricles.

• The septum divides the left and right atria and the left and right ventricles.

• The aorta is a large vessel that delivers oxygen-rich blood to the body.

• The pulmonary vessels supply blood from the heart to the lungs. They carry oxygen-poor (deoxygenated) blood. The main pulmonary artery (pulmonary trunk) leaves the right ventricle at the pulmonary valve.

• The pulmonary artery carries blood to the lungs.

The heart pumps blood into the lungs. The lungs oxygenate the blood and remove carbon dioxide from it through the process of breathing. The oxygenated blood flows to the left side of the heart from the lungs. The blood is pumped into the aorta. The aorta supplies oxygenated blood to the rest of the body.

What Is Cardiac Shunt?

Cardiac shunts are congenital heart diseases. In cardiac shunts, the blood mixes pathologically from the right to the left ventricle or from the right atrium to the left and vice-versa. As explained previously, the blood does not flow normally in cardiac shunts. The oxygenated blood can leak back into the lungs, or the oxygenated blood may not reach the lungs.

What Is Left to Right Shunt?

In the left-to-right shunt, oxygenated blood mixes with deoxygenated blood. The blood in this shunt moves from the left ventricle to the right ventricle, or the blood moves from the left to the right atrium. The oxygenated blood leaks back to the lungs. The types of left-to-right shunts are:

• Atrial septal defect (ASD).

• Ventricular septal defect (VSD).

• Patent ductus arteriosus (PDA).

What Are Atrial Septal Defects?

This defect has a hole in the septum. The septum separates the right and left atriums. This hole allows an abnormal passage of the blood flow. This defect causes the backward flow of blood from the left atrium to the left ventricle. It also causes the blood to flow from the left atrium into the right atrium. So the right atrium will have extra blood that goes to the lungs. The associated genetic disorders are Down syndrome and Holt-Oram syndrome.

What Are Ventricular Septal Defects?

In this condition, the hole is present in the septum dividing the two chambers of the heart. It is the most common congenital heart disease. Under normal conditions, the ventricle pumps the deoxygenated blood to the lungs to oxygenate the blood. After the blood gets oxygenated in the lungs, it enters the left ventricle and is supplied to the entire body. In this shunt, the oxygen-rich blood mixes with deoxygenated blood and is directed to the lungs. This causes extra load in the lungs, which can be dangerous.

What Is Patent Ductus Arteriosus?

• Ductus arteriosus is a channel between the pulmonary trunk and the aorta.

• It affects how blood flows through a baby's lungs. The classic example of left to right shunt is patent ductus arteriosus. The major hemodynamic problem is the left-to-right shunt.

• During intrafetal life, the lungs do not work. The highly oxygenated atrial side of the blood is mixed with the less oxygenated venous side of the blood. At the bifurcation of the pulmonary artery, ductus arteriosus is located.

• When the baby is in the uterus, the baby will not breathe. So the lungs do not work because there is no oxygen going to the lungs. Since oxygen is the dilator for the blood vessels, the blood arteries become spastic (closed). The blood flows through the ductus arteriosus to reach the aorta and does not flow through the pulmonary artery. The pulmonary artery supplies the lungs.

• Hypoxia is a contributing factor for pulmonary arterioles. When the baby is in the uterus, the pulmonary artery is constricted because there is no oxygen. During intrauterine life, there is no oxygen going through the lungs. The blood from the vena cava moves into the right atrium and to the right ventricle, which then moves through the pulmonary artery; there is very high resistance in the pulmonary artery. During intrafetal life, nature does not want the blood to flow through the pulmonary artery because the placenta oxygenates this blood.

• During intrafetal life, the blood is not oxygenated by the lungs. The placenta oxygenates it. The pulmonary arteries are constricted, and blood flows through the pulmonary artery and into the aorta, which is proximal to the origin of the subclavian artery.

• There is always a physiological hole between the left and right atrium. After the baby is born, the pulmonary vessels dilate, and the resistance falls into the pulmonary vasculature, so blood will start flowing through pulmonary arteries and eventually to the lungs.

• So, the blood flow through ductus arteriosus will reduce after birth. Secondly, prostaglandins (PG E2) play an important role in prenatal life because they open the ductus arteriosus.

• After PG E2 levels go down, the ductus arteriosus closes.

• Patent ductus arteriosus (PDA) is a heart defect that may develop soon after birth.

• If someone has patent ductus arteriosus, there is persistent blood flow through the aorta. The blood pressure in the aorta becomes very high. This is a classic example of well-oxygenated blood mixing with deoxygenated blood. This is another example of a functional left-to-right shunt.

• Patent ductus arteriosus is a vessel connecting the pulmonary artery to the aorta at its bifurcation.

• Whenever the right ventricle pumps blood into the pulmonary circulation, as lungs are not functional in babies, the blood goes to the right heart, the pulmonary artery, the ductus arteriosus, and the aorta. The Ductus arteriosus opens before birth.

• Ideally, after birth, the ductus arteriosus has to close. But if it does not close, it causes significant large cardiovascular problems. After it closes, fibrosis takes place. This fibrotic piece is called the ligamentous arteries in adults.

What Are the Symptoms of a Cardiac Shunt?

The symptoms are:

• Shortness of breath.

• Fatigue.

• Pulmonary hypertension (high blood pressure in the lung arteries).

What Are the Diagnostic Tests for Left to Right Shunt?

• Imaging Test - The CT (computed tomography) scan or chest X-ray shows the physical changes in the heart.

• Echocardiogram - It defines the heart's structure and functions.

• Electrocardiogram - It determines the signs of pulmonary hypertension.

• Cardiac Catheterization - It assesses the pressure in the heart chamber.

Conclusion:

A left-to-right shunt is an abnormal flow of blood from the left side of the heart to the right side. The defect can be congenital or acquired, allowing oxygen-rich blood from the left atrium to mix with oxygen-poor blood from the right atrium. This leads to reduced oxygenation of tissues. Left to right shunts are irregular patterns of blood flow in the heart. The symptoms can be life-threatening. It is better to seek medical help to recover from the condition.