Knowledge of fetal and perinatal circulation is an integral part of understanding the pathophysiology, clinical manifestations, and natural history of CHD, especially anomalies seen in the newborn period. Only a brief discussion of clinically important aspects of fetal and perinatal circulation is presented.
Fetal Circulation
Fetal circulation differs from adult circulation in several ways. Almost all differences are attributable to the fundamental difference in the site of gas exchange. In adults, gas exchange occurs in the lungs. In fetuses, the placenta provides the exchange of gases and nutrients.
Course of Fetal Circulation
There are four shunts in fetal circulation: placenta, ductus venosus, foramen ovale, and ductus arteriosus ( Fig. 8-1 ). The following summarizes some important aspects of fetal circulation:
- 1.
The placenta receives the largest amount of combined (i.e., right and left) ventricular output (55%) and has the lowest vascular resistance in the fetus.
- 2.
The superior vena cava (SVC) drains the upper part of the body, including the brain (15% of combined ventricular output), and the inferior vena cava (IVC) drains the lower part of the body and the placenta (70% of combined ventricular output). Because the blood is oxygenated in the placenta, the oxygen saturation in the IVC (70%) is higher than that in the SVC (40%). The highest Po 2 is found in the umbilical vein (32 mm Hg) (see Fig. 8-1 ).
- 3.
Most of the SVC blood goes to the right ventricle (RV). About one third of the IVC blood with higher oxygen saturation is directed by the crista dividens to the left atrium (LA) through the foramen ovale, and the remaining two thirds enters the RV and pulmonary artery (PA). The result is that the brain and coronary circulation receive blood with higher oxygen saturation (Po 2 of 28 mm Hg) than the lower half of the body (Po 2 of 24 mm Hg) (see Fig. 8-1 ).
- 4.
Less oxygenated blood in the PA flows through the widely open ductus arteriosus to the descending aorta and then to the placenta for oxygenation.
Dimensions of Cardiac Chambers
The proportions of the combined ventricular output traversing the heart chambers and the major blood vessels are reflected in the relative dimensions of these chambers and vessels (see Fig. 8-1 ).
Because the lungs receive only 15% of combined ventricular output, the branches of the PA are small. This is important in the genesis of the pulmonary flow murmur of newborns (see Chapter 2 ).
The RV is larger and more dominant than the left ventricle (LV). The RV handles 55% of the combined ventricular output, and the LV handles 45% of the combined ventricular output. In addition, the pressure in the RV is identical to that in the LV (unlike in adults). This fact is reflected in electrocardiograms (ECGs) of newborns, which show more RV force than adult ECGs.
Fetal Cardiac Output
Unlike the adult heart, which increases its stroke volume when the heart rate decreases, the fetal heart is unable to increase stroke volume when the heart rate falls because it has a low compliance. Therefore, the fetal cardiac output depends on the heart rate; when the heart rate drops, as in fetal distress, a serious fall in cardiac output results.
Changes in Circulation after Birth
The primary change in circulation after birth is a shift of blood flow for gas exchange from the placenta to the lungs. The placental circulation disappears, and the pulmonary circulation is established.
- 1.
The removal of the placenta results in the following:
- a.
An increase in systemic vascular resistance (SVR) results (because the placenta has the lowest vascular resistance in the fetus)
- b.
Cessation of blood flow in the umbilical vein results in closure of the ductus venosus
- a.
- 2.
Lung expansion results in the following:
- a.
A reduction of the pulmonary vascular resistance (PVR), an increase in pulmonary blood flow, and a fall in PA pressure ( Fig. 8-2 )
- a.
