In transposition of the great vessels, the major blood vessels attached to the heart—the aorta and the pulmonary artery—are reversed. This reversal results in the blood going to the wrong places. This leads to low oxygen levels in the body.

The aorta, which normally carries oxygen-rich blood from the left side of the heart to the body, instead receives oxygen-poor blood from the right side of the heart. The pulmonary artery, which normally carries oxygen-poor blood from the right side of the heart to the lungs, instead receives oxygen-rich blood from the left side of the heart.

In transposition of the great vessels, the right lower chamber of the heart (rather than the left lower chamber) pumps blood to the body. But the right side of the heart normally is not strong enough to pump blood effectively to the whole body. This increased workload on the right side of the heart can lead to a weakened heart.

There are several types of transposition of the great vessels. Each has slightly different placement of the vessels and openings that result in mixing of blood between the two sides of the heart. The most common form of transposition of the great vessels results in oxygen-poor blood being pumped to the body.

Certain other heart defects must be present to allow a child with transposition of the great vessels to live. Other defects ultimately compensate for the transposition of the great vessels by allowing oxygen-rich blood to mix with oxygen-poor blood so that some oxygen can get to the tissues of the body. Surgery is usually needed for long-term survival.