What is Tetralogy of Fallot?
Tetralogy of Fallot (TOF) is a complex congenital heart defect, congenital meaning present at birth. To understand a heart with TOF, you must understand a healthy heart first. The heart is made up of chambers, pulmonary arteries, pulmonary veins, an aorta and the vena cava, follow along with the diagram below to get a full understanding of the makeup of the heart.
The Chambers of the Heart
The four chambers of the heart are as follows: right atrium (1), left atrium (2), right ventricle (3), and left ventricle (4). The atria are smaller than the ventricles and make up the top of the heart while the ventricles make up the bottom. The atria receive blood coming in from the body, so they are connected to the pulmonary veins (5) which carry blood into the heart. The ventricles are the strongest part of the heart muscle, they send blood out of the heart, so they are connected to pulmonary arteries (6) which carry blood out to the the body.
The Valves of the Heart
The heart works by pumping blood to the lungs and other systems of the body. One-way valves are present within the heart to prevent blood from flowing back into it in the wrong direction. Two atrioventricular valves (AV) are located between the atria and ventricles, they ensure that blood only flows from the atria into the ventricles and not the opposite. The tricuspid valve (7) is on the right side of the heart while the mitral (bicuspid) valve (8) is on the left.
The semilunar valves (SV) are smaller than the AV valves and are located between the ventricles and arteries that take blood away from the heart and into the systems of the body. The pulmonary valve (9) is on the right and the aortic valve (10) is on the left. These valves prevent blood from flowing backwards into the heart.
The Aorta and Vena Cavae
The aorta (11) is the main artery in the human body, originating from the left ventricle of the heart and extending all the way down to the abdomen. It distributes oxygenated blood to all parts of the body. The vena cava (12) does just the opposite of the aorta. It is made up of the superior vena cava and inferior vena cava, both bring deoxygenated blood back to the heart and empty it into the right atrium.
How the Heart Pumps Blood
The right atrium (1) receives deoxygenated blood into the heart from the vena cava (12). The blood then goes through the tricuspid valve (7) into the right ventricle (3). From there, the blood is pumped upwards to the lungs through the pulmonary valve (9) and into the pulmonary arteries (6) to receive oxygen. Because the right ventricle isn’t strong enough to send the oxygenated blood throughout the body, it is pumped through the pulmonary veins (5) and into the left atrium (2). It then passes through the mitral valve (8) to reach the left ventricle (4). Finally, it goes through the aortic valve (10) and aorta (11) where it is then transferred throughout the entire body via veins and capillaries. Once it has made its way through the body, it will return to the heart by way of the vena cava to restart the cycle.
About Tetralogy of Fallot
Now that you know how a normal heart functions, you can better understand the complexities of TOF, which actually involves four heart defects in one:
- A large ventricular septal defect (VSD)
- Pulmonary Stenosis
- Right ventricular hypertrophy
- An overriding aorta
Ventricular Septal Defect
The heart has an inner wall that separates the two chambers on its left side (left atrium & left ventricle) from the two chambers on its right side (right atrium and right ventricle). This wall is called a septum and it prevents oxygenated blood from mixing with deoxygenated blood. A VSD is a hole in the septum between the heart’s left ventricle (4) and right ventricle (3). The hole allows oxygen-rich blood from the left ventricle to mix with oxygen-poor blood from the right ventricle, resulting in a lack of oxygen circulating throughout the body.
This defect involves the narrowing of the pulmonary valve (9), this creates an obstruction when blood tries to flow from the right ventricle (3) to the pulmonary artery (6) to receive oxygen from the lungs. Normally, oxygen-poor blood from the right ventricle flows through the pulmonary valve and into the pulmonary artery freely. From there, the blood travels to the lungs to pick up oxygen. With pulmonary stenosis, the pulmonary valve cannot fully open. Thus, the heart must work harder to pump blood through the valve. As a result, not enough blood reaches the lungs.
Right Ventricular Hypertrophy
With this defect, the muscle wall of the right ventricle (2) becomes enlarged due to underlying causes that put added stress on this part of the heart. In TOF, this occurs because the heart has to work harder than normal to move blood through the narrowed pulmonary valve.
This defect occurs in the aorta (11), the main artery that carries oxygen-rich blood from the heart to the body. In a healthy heart, the aorta is attached to the left ventricle, allowing only oxygen-rich blood to flow to the body. With TOF, the aorta is displaced between the ventricles and directly over the VSD. As a result, some oxygen-poor blood from the right ventricle (3) flows directly into the aorta and out to the body before it has the chance to be oxygenated by the lungs.
**Some babies born with TOF have additional heart defects, like pulmonary atresia. With this defect, the pulmonary valve doesn't form correctly. Instead of opening and closing to allow blood to travel from the heart to the lungs, a solid sheet of tissue forms. Because of this, blood can't travel by its normal route to pick up oxygen from the lungs. In some cases, small amounts of blood are able to travel to the lungs through other natural passages within the heart and its arteries.
TOF is a very rare congenital heart defect, occurring in about 1 out of every 2000 babies, affecting boys slightly more than girls.