Pulmonary atresia

In the condition known as pulmonary atresia, no pulmonary valve exists. Blood is therefore unable to flow from the right ventricle into the pulmonary artery and onward to the lungs. As a further result, the right ventricle usually remains hypoplastic. In addition, the tricuspid valve or right atrioventricular valve is often poorly developed.

With pulmonary atresia, the only means of getting blood flow to the lungs is through the ductus arteriosus, which connects the pulmonary artery and the aorta. The infant is usually cyanotic.

Blood returning to the right atrium and right ventricle therefore needs to shunt across to the left side of the circulation, through either an ASD or a patent foramen ovale. The foramen ovale is a natural septal opening in the fetal heart that allows blood to flow between the atria. In most newborns this opening closes at birth. Failure of the foramen ovale to close results in a type of atrial septal defect, known as a patent foramen ovale (PFO). Where there is an ASD or a PFO, blood leaves the right atrium and mixes with the oxygen-rich blood in the left atrium. This blood is then pumped by the left ventricle into the aorta and out to the body.

A further problem arises when the ductus arteriosus closes, causing severe cyanosis. The infant is then usually given prostaglandin to keep the ductus arteriosus open. As a palliative measure, a surgeon may create a shunt between the pulmonary artery and the aorta to increase blood flow to the lungs. If the right ventricle is too small to be effective as a pump, a Fontan procedure is performed later, connecting the right atrium directly to the pulmonary artery. The ASD or PFO is also closed to relieve the cyanosis.

Diagram 2.14 Pulmonary atresia 1 - atrial septal defect 2 - patent ductus arteriosus 3 - absent pulmonary valve 4 - hypoplastic right ventricle Abnormal blood flow (as indicated by the shaded blue arrow) is from the right atrium and right ventricle through an atrial septal defect to the left side of the heart. Blood can reach the pulmonary arteries only through a patent ductus arteriosus.