Single ventricle is one of the most complex and rare types of heart defects present at birth (congenital heart defect). It occurs when the fetal heart does not develop normally early in the first trimester of pregnancy.

The fetal heart begins its development as a long tube. During the first trimester, the lower part of this tube normally divides into two pumping chambers: the left ventricle and the right ventricle. However, there are rare cases in which this normal separation does not occur, and the baby is born with one functional ventricle instead of two.

As a result of this defect, oxygen-rich blood and oxygen-poor blood mix together in the single ventricle instead of remaining separate. Some oxygen-rich blood needlessly travels back to the lungs, and some oxygen-poor blood uselessly travels to the rest of the oxygen-demanding body. Unless corrected surgically, single ventricle usually results in heart failure and death.

However, surgical correction is available. Most patients will undergo at least two surgeries to correct single ventricle: the bi-directional Glenn (or the hemi-Fontan) and the Fontan. Together, these operations redirect oxygen–poor blood from the body straight to the lungs, bypassing the heart. In turn, the single ventricle is responsible for pumping freshly oxygenated blood to the body. Although these procedures may correct the defect, no procedure may cure it.