Normal Fetal and Pediatric Cardiac Anatomy and Physiology
Fetal circulation and the transition to extrauterine life
Fetal circulation depends on three shunts: the ductus venosus, the foramen ovale, and the ductus arteriosus. Oxygenated umbilical venous blood travels through the ductus venosus into the inferior vena cava and is preferentially streamed across the foramen ovale into the left atrium, left ventricle, and ascending aorta, supplying the brain. Superior vena caval blood enters the right ventricle and pulmonary artery, with most flow bypassing the high-resistance pulmonary bed through the right-to-left ductus arteriosus. The right ventricle is the dominant ventricle in fetal life and handles the majority of combined cardiac output. At birth, lung expansion and a fall in pulmonary vascular resistance increase pulmonary blood flow, left atrial pressure rises above right atrial pressure, and the foramen ovale closes functionally. The ductus arteriosus constricts as oxygen tension rises and prostaglandin levels fall.
Pediatric anatomy and physiology
Normal segmental anatomy includes situs solitus, levocardia, atrioventricular concordance, and ventriculoarterial concordance. Coronary arteries arise from the aortic sinuses. Fetal heart rate is typically 110–160 beats/min, and a small pericardial fluid collection can be normal.
Traps
Do not assume left ventricular dominance in the fetus; a slightly larger right ventricle is normal. Do not call a normal shunting vessel abnormal. Do not expect adult oxygen saturations or adult ventricular dominance in a fetus.
Rule of thumb
In the fetus, the right ventricle is dominant and the ductus arteriosus is right-to-left. If the left ventricle is larger than the right at mid-gestation, look for a cause such as coarctation, aortic stenosis, or a restrictive foramen ovale.