Pulmonary Hypertension in Congenital Heart Disease.

The pulmonary circulation, which acts to maximize perfusion to aerated lung is a low-pressure system after birth. If there is a gap in the septum between the atria or ventricles, the blood will flow from the systemic side to the pulmonary side. The pulmonary arterial oxygen saturation will increase, but the oxygen tension in pulmonary veins and in systemic arteries will remain normal, unless the pressure forces the development of an intrapulmonary, arterio-venous, shunt.

As the pulmonary arterial pressure and perfusion increase, the pulmonary vessels dilate. There may develop pulmonary oedema. Eventually pulmonary resistance increases. The pulmonary mechanism evolved to match perfusion to ventilation and there exist mechanisms to change flow within the lungs, possibly involving nitric oxide. Prolonged high perfusion pressure to the lungs will generate a response, either to protect the lungs from this perfusion pressure or a consequence of damage. There is proliferation of smooth muscle cells in the wall of the pulmonary arterioles and fibrosis in the walls of small arteries.

When the pulmonary pressure increases to the level of the systemic circulation, flow across the defects will stop and the heart size will reduce, as a volume overload is converted to a pressure overload.

As the pressure in the right sided chambers increases, then blood will begin to flow to the systemic side. This will reduce the oxygen tension in the systemic circulating blood, which will be noted clinically.