Congenital heart disease comprises the most common form of congenital anomaly found in humans, affecting 8 of every 1000 live births and 10 to 12 out of every 1000 pregnancies. Over the past 20 years, fetal echocardiography has emerged as a reliable and accurate means to diagnose these problems associated with cardiac structure and function prior to birth. As imaging technologies continue to improve, a higher percentage of congenital heart defects and abnormalities of fetal cardiovascular function can be detected prior to birth, allowing practitioners to counsel parents about their unborn child's expected outcome and to implement prenatal and perinatal management strategies in order to maximize postnatal outcome.
In contrast to postnatal circulation, in which the pulmonary and systemic circulation are arranged in series, the fetal circulation is arranged in parallel because the placenta, rather than the lungs, serves as the site of oxygenation and ventilation. As a consequence, the right ventricle pumps approximately 55% to 60% of the combined cardiac output, whereas the left ventricle pumps approximately 40% to 45%.1 As shown in Figure 3-1, the fetal blood flow patterns are optimized to deliver oxygen and nutrition to vital organs, while shunting blood away from less important structures. Indeed, the placenta has extremely low resistance in order to promote blood flow to this site. Within the capillary bed of the placenta, oxygen is exchanged for carbon dioxide. A single umbilical vein then leaves the placenta carrying richly oxygenated blood back to the fetus through the umbilical cord. To bypass most of the liver, the umbilical vein inserts into the ductus venosus, which then connects with the inferior vena cava to enter the right atrium. The angle at which the ductus venosus inserts into the inferior vena cava–right atrium junction directs most of the richly oxygenated blood across the foramen ovale and into the left atrium and left ventricle. The left ventricle, in turn, perfuses the coronary arteries and the cerebral vasculature. These fetal physiologic adaptations ensure that the most richly oxygenated blood is delivered to the most vital structures in need of oxygen, namely the myocardial and cerebral circulations. Similarly, the most deoxygenated blood returning from the superior vena cava is directed to the tricuspid valve and into the right ventricle, which pumps blood into the main pulmonary artery and across the ductus arteriosus to return to the descending aorta and ultimately to the umbilical arteries. Because the fetal lungs are compressed and not responsible for oxygenation and ventilation prior to birth, the resistance within the pulmonary vasculature is quite high to ensure that deoxygenated blood crosses the ductus arteriosus rather than entering the fetal lungs, to return deoxygenated blood to the site of oxygenation within the fetus, namely the placenta.
The fetal circulation. The ductus venosus carries the most highly oxygenated blood across the foramen ovale (FO) to the left atrium (LA), the ...