5.10.1 Haemodynamic effects
A PDA allows blood to pass from the high pressure aorta to the relatively low pressure PA, i.e. in the opposite direction to blood flow in the fetus. This leads to overcirculation of the lungs. If the ductus is large it will lead to volume overload of the left side of the heart; a small PDA will be less significant. Frequently the ductus remains patent because other defects have resulted in abnormal pressures and oxygen concentrations within the circulatory system. For example, a PDA is essential for life in animals with severe pulmonary hypoplasia.