Neonatal cardiac physiology varies considerably from that of an adult and is uniquely situated to meet the needs of the neonate at a time of significant change from the environment in utero. A full understanding of these differences first requires knowledge of the fetal circulation as well as the changes that occur at birth, known as the transitional circulation.
The fetal circulation is comprised of several structures that differentiate it from later circulation (Figure 131-1). Most importantly, the placenta allows for appropriate oxygenation of the fetus as well as nutritional and metabolic exchange to occur with the mother. A single umbilical vein leaves the placenta and carries oxygenated blood to the fetus. When blood enters the fetal liver, approximately 60% bypasses the liver’s circulation via the ductus venosus and enters the inferior vena cava (IVC) directly. About one-third of this IVC blood, which is more highly oxygenated, is then directed by the Eustachian valve to the right atrium (RA) where it crosses the foramen ovale and enters the left atrium (LA). From there, it enters the left ventricle (LV) and then the ascending aorta, ultimately perfusing the coronary and cerebral circulation. This blood, termed the preductal blood, has the highest oxygenation and can be measured after birth with pulse oximetry on the right upper extremity.
Diagram of the fetal circulation. The numbers within the circles are percentages of the combined ventricular output.
The remaining two-thirds of the IVC blood joins with superior vena cava (SVC) blood in the RA and enters the right ventricle (RV) followed by the pulmonary artery. Due to the significant hypoxia in the lungs and their collapsed state, the pulmonary vascular resistance (PVR) is very high. Therefore, approximately 90% of the pulmonary artery blood flow bypasses the lungs, instead flowing across the ductus arteriosus into the descending aorta and the systemic circulation. This blood returns to the placenta via two umbilical arteries.
As opposed to the adult circulation, which is in series, the fetal circulation is in parallel. With the shunting described above, the RV ejects approximately two-thirds of the combined ventricular output compared to one-third ejected by the LV. This increased workload, in addition to the high PVR, which it is pumping against, results in the RV having a greater muscle mass relative to the LV.
At birth, several changes occur which allow for the transition to postnatal cardiac physiology. As the baby is born, he/she takes the first breaths of life, opens the lungs, and fills them with air, thereby dropping PVR considerably. At the same time, the umbilical vessels are clamped, no further flow to the placenta occurs, and systemic vascular resistance (SVR) increases. The lungs now have to act ...