At birth, a number of interrelated circulatory and pulmonary changes must occur in order for there to be a smooth transition from the fetal to transitional neonatal circulation. If any of these changes do not occur, the result will be a neonate who is hypoxic and may not have adequate pulmonary and systemic perfusion.
Fetal circulation functions as a parallel circuit, where both the right and left sides of the heart provide systemic blood flow.(Figure 2-1) Thus, cardiac output (450 mL/kg/min) in the fetus is the sum of both the right and left ventricular outputs. In utero, the right ventricle contributes approximately 67% and the left ventricle approximately 33% of the total cardiac output. This is in contrast to the extrauterine (adult) circulation that functions in series, where the right side of the heart provides pulmonary blood flow and the left side of the heart provides systemic blood flow. Total cardiac output in adult circulation is determined by each ventricle, and, at birth, approximately 350 mL/kg/min is needed to meet the high metabolic demands of the newborn.
Fetal circulation. From Greeley WJ, Berkowitz DH, Nathan AT. Anesthesia for pediatric cardiac surgery. In: Miller RD, Eriksson LI, Fleisher LA, et al. Miller’s Anesthesia. 7th ed. Philadelphia, PA: Elsevier; 2011.
Blood leaving the placenta enters the fetus via the umbilical vein. Approximately 40% to 60% of this blood bypasses the liver via the ductus venosus.1 On entering the right atrium, one-third of this blood will preferentially flow across the foramen ovale (FO) to the left atrium because of deflection by the crista dividens. This blood will then pass through the left ventricle and exit to the ascending aorta supplying the coronary and cerebral circulation and the upper body of the fetus. Blood returning from the cerebral circulation via the superior vena cava enters the right atrium and is preferentially directed toward the right ventricle. Approximately 90% of the blood exiting the right ventricle will be shunted across the ductus arteriosus (DA), while the remaining 10% of the blood will enter the pulmonary circulation providing nutrients for growth. Shunting across the DA occurs because the pulmonary vascular resistance (PVR) is high secondary to pulmonary vasoconstriction and the systemic vascular resistance (SVR) is low secondary to the low-resistance placenta and large-caliber DA.2 Blood shunted across the DA enters the aorta just before the left subclavian artery and provides nutrients to the lower body of the fetus. From the descending aorta, blood flows to the internal iliac arteries and then umbilical arteries to the placenta.
In the fetus, gas exchange occurs in the placenta. The partial pressure of oxygen (pO2) of maternal blood in the placenta is 30 to 35 mm Hg (oxygen saturation 65%), much like ...