Hypoplastic left heart syndrome occurs when parts of the left side of the heart (mitral valve, left ventricle, aortic valve, and aorta) do not develop completely. The condition is congenital (present at birth).

Hypoplastic left heart is a rare type of congenital heart disease. It is more common in males than in females.

As with most congenital heart defects, there is no known cause. Approximately 10% of patients with hypoplastic left heart syndrome also have other birth defects.

The problem develops before birth when there is not enough growth of the left ventricle and other structures, including the:

• Aorta -- the blood vessel that carries oxygen-rich blood from the left ventricle to the entire body
• Entrance and exit of the ventricle
• Mitral and aortic valves

This causes the left ventricle and aorta to be incompletely developed, or hypoplastic. In most cases, the left ventricle and aorta are much smaller than normal.

In patients with this condition, the left side of the heart is unable to send enough blood to the body. As a result, the right side of the heart must maintain the circulation for both the lungs and the body. The right ventricle can support the circulation to both the lungs and the body for a while, but this extra workload eventually causes the right side of the heart to fail.

The only possibility of survival is a connection between the right and the left side of the heart, or between the arteries and pulmonary arteries (the blood vesels that carry blood to the lungs). Babies are normally born with two of these connections:

• Foramen ovale (a hole between the right and left atrium)
• Ductus arteriosus (a small blood vesel that connects the aorta to the pulmonary artery)

Both of these connections normally close on their own a few days after birth.

In babies with hypoplastic left heart syndrome, blood from the right side of the heart travels through the ductus arteriosus. This is the only way for blood to get to the body. If the ductus arteriosus is allowed to close in a baby with hypoplastic left heart syndrome, the patient may quickly die because no blood will be pumped to the body. Babies with known hypoplastic left heart syndrome are usually started on a medicine to keep the ductus arteriosus open.

Because there is little or no flow out of the left heart, blood returning to the heart from the lungs needs to pass through the foramen ovale or an atrial septal defect (a hole connecting the collecting chambers on the left and right sides of the heart) back to the right side of the heart. If there is no foramen ovale, or if it is too small, the baby could die. Patients with this problem have the hole between their atria opened, either with surgery or using heart catheterization.