Congenital Heart Defects

Congenital heart defects, also sometimes called congenital heart disease, are a group of conditions involving structural abnormalities of the heart, great vessels, or both. They can be cyanotic or acyanotic based on the presence or absence of cyanosis, which is a blue discoloration of the skin and mucous membranes.

As a quick review, during gestation, the fetal lungs are fluid-filled and have high vascular resistance, so they don’t participate in gas exchange. Instead, blood is shunted away from the lungs, and the fetus receives oxygenated blood from the placenta, where gas exchange occurs and metabolic wastes are removed.

Now, the high vascular resistance in the lungs also causes the pressure in the right side of the heart to be much higher than the left side, so, as oxygenated blood enters the right atrium, most of the blood is shunted into the left atrium through the foramen ovale, an opening between the right and left atria. Blood then moves into the left ventricle where it’s pumped through the aortic valve, into the aorta, and out to the fetal body. Now, some blood from the right atrium passes into the right ventricle, through the pulmonary valve and into the pulmonary artery, heading for the lungs. But before it can get to the lungs, the blood is shunted through the ductus arteriosus, a vessel connecting the pulmonary artery to the aorta. The aorta then sends the blood out to the fetal body.

Following birth, the foramen ovale and ductus arteriosus close. The pressure on the left side of the heart increases and pressure on the right side of the heart decreases as pulmonary vascular resistance lessens, and the lungs begin to participate in gas exchange. Deoxygenated blood now moves from the right side of the heart to the lungs for oxygenation and returns to the left side of the heart before being pumped to the rest of the body.

Congenital heart defects don’t have a single cause, but there are several risk factors that can interfere with development of the fetal heart and blood vessels. These include advanced maternal age; maternal exposure to infections, like rubella; or the presence of underlying metabolic conditions, such as diabetes mellitus or phenylketonuria. Additionally, alcohol and certain medications, like lithium or phenytoin, are teratogens, meaning they are known to cause fetal abnormalities if taken during pregnancy. Prematurity and fetal chromosomal abnormalities, such as trisomy 13, 18, and 21 also increase the risk of congenital heart defects.

Okay so, defects that cause a right-to-left shunt, where blood flows from the right side of the heart to the left side of the heart without being oxygenated in the lungs are called cyanotic heart defects because they lead to decreased oxygen saturation and cyanosis. They include tetralogy of Fallot, or TOF, and transposition of the great arteries, or TGA.

Tetralogy of Fallot consists of four heart abnormalities including pulmonary stenosis, or narrowing of the pulmonary valve; right ventricular hypertrophy, or enlargement of the right ventricle; ventricular septal defect, or VSD, which is an opening between the ventricles; and overriding aorta, where the aorta is abnormally positioned above the ventricular septal defect and between the right and left ventricles. First, pulmonary stenosis makes it more difficult for deoxygenated blood to reach the pulmonary circulation for oxygenation. As a result, the right ventricle hypertrophies as it works harder to push blood against the obstruction. Next, the ventricular septal defect allows deoxygenated blood in the right ventricle to mix with oxygenated blood in the left ventricle and move into systemic circulation. And finally, the overriding aorta makes it easier for deoxygenated blood to be pumped into the systemic circulation.

Now, transposition of the great arteries is a condition where the great arteries, the aorta and pulmonary artery, are flipped, or transposed. This causes deoxygenated blood in the right ventricle to flow into the aorta and out to systemic circulation, while oxygenated blood in the left ventricle flows into the pulmonary artery and recirculates in the lungs. Often, associated defects like ventricular septal defect are also present, allowing mixing of deoxygenated and oxygenated blood, which can help maintain some tissue oxygenation.

On the other hand, defects that cause a left-to-right shunt, where oxygenated blood moves from the left side of the heart to the right and back into the lungs, are called acyanotic heart defects, because blood entering the systemic circulation is oxygenated, so cyanosis doesn’t occur. Acyanotic defects include atrial septal defect, or ASD; ventricular septal defect, or VSD; patent ductus arteriosus, or PDA; and coarctation of the aorta, or COA.

An atrial septal defect occurs when the septum between the left and right atria doesn't develop properly, creating an opening between the two chambers. Likewise, a ventricular septal defect occurs when the septum between the right and left ventricles doesn’t develop properly, creating an opening between them. Now, in these defects, the pressure is higher on the left side of the heart, so blood is forced from the left side, through the defect to the right side. Next, a patent ductus arteriosus is when the ductus arteriosus fails to close, so there’s a persistent connection between the pulmonary artery and aorta. This allows some deoxygenated blood from the right ventricle to mix with oxygenated blood and enter the systemic circulation. Finally, coarctation of the aorta occurs when there’s narrowing of the aorta which obstructs oxygenated blood from exiting the left ventricle, which hypertrophies as it works harder to pump blood out to the systemic circulation.