Ecmo

SYMPOSIUM: CARDIOVASCULAR MEDICINE

Cardiovascular system adaptation at birth
Joanna Berhrsin Alan Gibson

The fetal cardiovascular system
The cardiovascular system is the first to function in the embryo, with blood beginning to circulate by the end of the third week when heart contractions begin. The myocardium continues to grow by progressive cell division until birth and cardiaccontractility steadily improves over the second half of pregnancy. An effective functional circulation is maintained by the presence of three shunts in the fetal cardiovascular system; the ductus venosus, ductus arteriosus and foramen ovale (Figure 1). In addition the presence of fetal haemoglobin means that the organs in the fetus are able to extract oxygen at low saturations. Relatively oxygenated bloodleaves the placenta through the umbilical vein and passes through the ductus venosus into the inferior vena cava. It then enters the right side of the heart and traverses the foramen ovale in the inter-atrial septum to reach the left ventricle from which it is pumped through the aorta to feed the coronary circulation and the major body organs. Deoxygenated blood passes through the caval veins intothe right side of the heart where there is some mixing in the right atrium before blood either passes through the foramen ovale to the left atrium or through the tricuspid valve to the right ventricle. High pulmonary vascular resistance results in the majority of the blood that leaves the right ventricle being preferentially shunted through the ductus arteriosus into the aorta. The amount ofblood entering the pulmonary circulation varies during pregnancy, increasing from around 13% of the combined cardiac output at midgestation to around 25% after 30 weeks. The ductus arteriosus bypasses the pulmonary circulation and the different impedance between the pulmonary vasculature and the ductal circulation results in the majority of blood remaining in the systemic circulation. This is underthe control of prostacyclin (PGI2). Oxygen saturations in different parts of the fetal circulation are shown in Table 1. It is of interest to note that the difference in oxygen saturations between the blood leaving the left and right ventricles is only 10%, increasing to 12% during hypoxaemia. This is a reflection of the large volume of blood that is shunted through the foramen ovale. In the maturecirculation, where no intracardiac shunting takes place, the stroke volume of the right ventricle should equal that of the left ventricle. In the fetus the two ventricles pump in parallel and the right ventricle is responsible for the majority of the cardiac output, pumping blood into the pulmonary artery with the majority then passing through the ductus arteriosus to enter the descending aorta andsupply the lower body and placenta. Blood enters the left ventricle through the foramen ovale and left atrium and is pumped into the ascending aorta to supply the cerebral and coronary circulations. This blood is more oxygenated than that in the descending aorta, hence facilitating the delivery of oxygen to the tissues that require it most. In contrast to the mature heart, where the leftventricle is thicker than the right, the comparative thickness of the fetal ventricles is similar. This results in a minimal pressure difference between left and right ventricles in contrast to the significant gradient observed in postnatal life. The ability of the fetal heart to alter stroke volume according to preload is impaired when compared to the mature heart. The fetal

Abstract
Due to thepresence of fetal haemoglobin and the patency of anatomical shunts the fetal circulation is perfectly adapted to intrauterine life, utilizing the placenta as the organ of gas exchange. Immediate adaptation must occur at birth as the lungs take over this role. Initially the pulmonary vascular resistance is high but pulmonary blood flow progressively increases in the newborn due to vasodilatation as a...