Heart defects that are present at birth (congenital) arise from problems early in the heart's development, but there's often no clear cause. Genetics and environmental factors probably play a role.

A ventricular septal defect occurs when the septum, the muscular wall separating the heart into left and right sides, fails to form fully between the lower chambers of the heart (ventricles) during fetal development. This leaves an opening that allows mixing of oxygenated blood and deoxygenated blood, meaning the heart has to work harder to provide enough oxygen to your body's tissues.

If a ventricular septal defect is large, blood overfills the lungs and overworks the heart. If left untreated, the blood pressure in the lungs goes up (pulmonary hypertension) and the ventricles enlarge and no longer work efficiently. Ultimately, this can lead to irreversible damage to the lung arteries and to heart failure. In contrast, small ventricular septal defects don't usually cause any problems, except for a loud heart murmur that doctors may note during physical exams.

Oct. 26, 2011