Conotruncal defects occur about 5-6 weeks after conception, when the heart is essentially an S-shaped tube with upper and lower bulges. A dividing wall bisects the lower heart area—called the conotruncus—resulting in 2 ventricles (pumping chambers) and 2 major blood vessels, the pulmonary artery (leading to the lungs) and the aorta (leading to the rest of the body).

All conotruncal defects cause improper circulation of oxygenated and depleted blood. Affected infants require complex open heart surgery; many die from their heart defects or other birth defects. Among chromosomally normal infants, conotruncal heart defects occur in 7.3 per 10,000 livebirths and fetal deaths. They are more frequent in males than females.

The most common types are:

	Double outlet right ventricle (0.4 per 10,000): pulmonary artery and aorta both arise from the right side of the heart.
	Pulmonary atresia (0.3 per 10,000): an absent valve at the entrance to the pulmonary artery blocks blood flow to the lungs.
	Tetralogy of Fallot (2.9 per 10,000 births): its 4 components are underdeveloped right heart, hole between the ventricles, overriding aorta and underdeveloped pulmonary artery.
	Transposition of the great vessels (2.6 per 10,000 births): reversal of the main blood vessels leading from the heart.
	Truncus arteriosus (0.9 per 10,000 births): a single blood vessel replaces the pulmonary artery and aorta.

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