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Pulmonary Hypertension of the Newborn
Background
Persistent pulmonary hypertension of the newborn (PPHN) is a rare disorder of the lungs which occurs at birth or shortly thereafter. The symptoms include cyanosis (blue cast of the skin due to deficient oxygenation of the blood), respiratory distress and tachypnea (increased rate of respiration) during the first day of life.
The circulation of the unborn baby differs from that of an infant outside the womb. The unborn baby does not use its lungs to oxygenate its blood supply, but instead receives oxygen and gets rid of carbon dioxide via the placenta. All the many thousands of blood vessels in the lung are thick walled and have a small lumen (cavity of the blood vessel) so that the resistance to flow through the lungs is high. Also, the structure of the heart in the unborn functions slightly differently from newborns and adults because the fetus does not use the lung to breathe and obtain oxygen from the air (oxygen comes from the placenta) and therefore blood is diverted away from the lungs through two channels. One channel is a small hole between the two chambers in the upper part of the heart, called the foramen ovale. The other channel is a connection between the two main blood vessels leaving the heart (aorta and pulmonary artery) and this communication is called the ductus arteriosus. Both channels normally close soon after birth.
Birth acts as a signal for changes to take place so that blood can be pumped to and from the lungs and the baby can take its first breath. As the baby breathes in, the lungs inflate for the first time, the blood vessels in the lungs widen (dilate), and blood rushes into the lungs from the heart. As the lung vessels dilate the pressure within them falls and so does the pressure within the right side of the heart which connects with them. The pressure on the left side of the heart is now greater than that in the right and the patent foramen ovale closes like a trap door. The ductus arteriosus constricts and eventually closes up after a few days.
Although the pattern of blood flow in the newborn baby is now the same as that of the adult, the structure of the blood vessels within the lung are still very different. It takes several weeks for the walls of the blood vessel to become thin and for the resistance to flow to decrease. Sometimes this fails to happen. The resistance to the flow of blood through the lungs remains high and the pulmonary arterial pressure remains high after birth. This is called persistent pulmonary hypertension of the newborn.
