A ventricular septal defect (VSD) is a type of heart defect (congenital heart disease) in which there is a hole in the wall (septum) between the heart’s lower chambers (ventricles). The hole may be as small as a pinpoint, or as large as the space where the entire septum should be (i.e., the total absence of the septum). 

VSD is the most common congenital heart defect, occurring in as many as 1 percent of newborns. VSDs may occur in isolation or as part of a more complex series of congenital defects. The vast majority of VSDs are small and spontaneously close on their own during the first years of life, without any symptoms or harmful effects.

Although larger VSDs are more dangerous than smaller ones, most physicians agree that all VSDs that do not close on their own should be treated, no matter how small they are. This is because even small VSDs carry a risk of the formation of blood clots that may be carried to the brain and cause a stroke. VSDs can also increase the risk of a person developing infective endocarditis (infection within the heart). 

Larger VSDs, however, may result in a dangerous left-to-right shunt. This occurs when some of the oxygen-rich blood in the left ventricle is diverted (shunted) through the VSD and into the right ventricle, where it mixes with oxygen-poor blood. Left-to-right shunts, which also include atrial septal defect (ASD) and patent ductus arteriosus (PDA), account for over half of all heart diseases that are present at birth (congenital heart disease).

Because of the left-to-right shunt, the right ventricle of the heart receives blood from two different places: from the right atrium, as it should, and from the left ventricle (through the VSD). This results in too much blood flow to the right side of the heart and lungs. The more blood that is diverted to the right side of the heart, the harder the lungs and right ventricle must work to compensate for the problem.

Eventually, the stress on the overworked right ventricle may cause it to weaken and/or enlarge. Also, the lungs can become congested from constantly receiving more blood than is needed. Eventually, the congestion and overwork could lead to arrhythmias (abnormal heart rhythms) or heart failure.

There are a number of different types of VSDs, which include the following:

A VSD may be present by itself, or it may be one of a number of heart conditions, including:

• Tetralogy of Fallot. A complex congenital abnormality characterized by four anatomical defects.
  
  
• Double outlet right ventricle. This occurs when both the aorta and the pulmonary artery attach to the right ventricle, instead of the left and right ventricles, respectively.
  
  
• Transposition of the great arteries. In this congenital defect, the positions of the aorta and pulmonary artery are switched.
  
  
• Atrial septal defect (ASD). This defect refers to a hole in the septum between the upper chambers of the heart (atria).
  
  
• Patent ductus arteriosus. This occurs when the ductus arteriosus, a normal connection between the aorta and pulmonary artery that should seal within a few days of birth, remains open.
  
  
• Heart valve disorders. The position of the VSD may affect the tricuspid valve, mitral valve or aortic valve.
  
  
• Anomalous pulmonary venous connection. Instead of the pulmonary veins connecting with the left atrium, as they should, they connect to the right atrium, vena cavae or hepatic veins. The condition may be partial or complete. The associated ASD is usually at a high position in the atrial septum.

In the case of some complex congenital heart defects, the presence of a VSD may actually be beneficial. For example, if a patient is born with a defect that prevents blood from flowing normally through the lungs, into the left side of the heart and out to the body, a VSD may allow an alternative path for oxygen-rich blood to reach the body. In these cases, the VSD may actually be life-saving, and physicians will not attempt to correct the VSD until they have addressed the more serious heart defect.