pediatrics
Perinatal Physiology
Neonatal Cardiovascular Function
Fetal circulation is marked by right-to-left shunting of blood around the unventilated lungs through a Patent Ductus Arteriosus (PDA) (connecting the pulmonary artery to the aorta) and foramen ovale (connecting the right and left atria). Shunting is encouraged by high pulmonary arteriolar resistance and relatively low resistance to blood flow in the systemic (including placental) circulation. About 90 to 95% of the right heart output bypasses the lungs and goes directly to the systemic circulation. The fetal ductus arteriosus is kept open by low fetal systemic PaO2 (about 25 mm Hg) along with locally produced prostaglandins. The foramen ovale is kept open by differences in atrial pressures: left atrial pressure is relatively low because little blood is returned from the lungs, but right atrial pressure is relatively high because large volumes of blood return from the placenta. In the fetus, blood entering the right side of the heart has already been oxygenated via the placenta. Because the lungs are not ventilated, only a small amount of blood needs to go through the pulmonary artery. Most of the blood from the right side of the heart bypasses the lungs through the Foramen ovale Ductus arteriosus Normally, these two structures close shortly after birth. Profound changes to this system occur after the first few breaths, resulting in increased pulmonary blood flow and functional closure of the foramen ovale. Pulmonary arteriolar resistance drops acutely as a result of vasodilation caused by lung expansion, increased PaO2, and reduced PaCO2. The elastic forces of the ribs and chest wall decrease pulmonary interstitial pressure, further enhancing blood flow through pulmonary capillaries. Increased venous return from the lungs raises left atrial pressure, thus reducing the pressure differential between left and right atria; this effect contributes to the functional closure of the foramen ovale. As pulmonary blood flow is established, venous return from the lungs increases, raising left atrial pressure. Air breathing increases the PaO2, which constricts the umbilical arteries. Placental blood flow is reduced or stops, reducing blood return to the right atrium. Thus, right atrial pressure decreases while left atrial pressure increases; as a result, the two fetal components of the interatrial septum (septum primum and septum secundum) are pushed together, stopping flow through the foramen ovale. In most people, the two septa eventually fuse and the foramen ovale ceases to exist. Soon after birth, systemic resistance becomes higher than pulmonary resistance, a reversal from the fetal state. Therefore, the direction of blood flow through the patent ductus arteriosus reverses, creating left-to-right shunting of blood (called transitional circulation). This state lasts from moments after birth (when the pulmonary blood flow increases and functional closure of the foramen ovale occurs) until about 24 to 72 hours of age, when the ductus arteriosus constricts. Blood entering the ductus and its vasa vasorum from the aorta has a high PO2, which, along with alterations in prostaglandin metabolism, leads to constriction and closure of the ductus arteriosus. Once the ductus arteriosus closes, an adult-type circulation exists. The 2 ventricles now pump in series, and there are no major shunts between the pulmonary and systemic circulations. Prenatal stress, postnatal stress, and anatomical differences that result in the persistence of elevated pulmonary vascular resistance after birth may result in Persistent Pulmonary Hypertension of the Newborn , previously known as persistent fetal circulation. Hypoxemia and acidosis cause the pulmonary arterioles to constrict and the ductus arteriosus to dilate, reversing the processes described previously and resulting in right-to-left shunting through the ductus arteriosus, foramen ovale, or both. The goal of treatment is to reverse the conditions that caused pulmonary vasoconstriction and treat the underlying conditions. 