A comprehensive understanding of fetal cardiovascular physiology and of the changes that occur at birth is essential for developing a systematic approach to the diagnosis and treatment of a newborn with congenital cardiovascular disease. Complex congenital cardiovascular disease rarely causes symptoms in the fetus but within hours or days after birth may cause the newborn infant to become critically ill. Specific defects lead to predictable cardiac and vascular alterations, and knowledge of such associations assists the clinician in the evaluation, diagnosis, and treatment of the critically ill newborn. This chapter reviews important physiologic aspects of the fetal circulation, how the fetal circulation can be monitored for hemodynamic stability, and the changes in circulatory physiology that occur at birth.

Overview of Essential Facts of Fetal Cardiovascular Function

There are four essential facts about the fetal circulation on which to base an understanding of fetal cardiovascular physiology and its impact on congenital cardiovascular defects:

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  Fact 1. The right and left ventricles function in the fetus as they do postnatally, supplying blood for oxygen uptake and delivery, respectively.

  
  
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  Fact 2. Only one ventricle is required for cardiovascular stability in the fetus.

  
  
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  Fact 3. The right ventricle is the dominant ventricle in the fetus.

  
  
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  Fact 4. After embryogenesis, the size and orientation of a cardiovascular structure (cardiac chamber, valve, or blood vessel) is determined by the blood flow pattern and volume.

Tasks of the Fetal Ventricles

Central shunts, or vascular communications, between the major vascular beds (systemic, pulmonary, and placental circulations) and the two sides of the heart (Figure 3-1) are present in the normal fetus. The ductus venosus joins the placental venous return to the systemic venous return, the ductus arteriosus connects the pulmonary arterial circulation directly to the systemic arterial circulation, the umbilical arteries join the systemic arterial circulation with the placental arterial circulation, and the foramen ovale joins the left and right sides of the heart. Many investigators argue that these shunts create a parallel circuit such that the left and right ventricles perform the same tasks, receiving the same venous return and ejecting into the same vascular beds. This would be dramatically different than the postnatal state, in which the two ventricles perform very different tasks. However, careful analysis of fetal blood flow patterns indicates that the two fetal ventricles do not function in a completely parallel fashion, and, in fact, they function in a similar manner as in the postnatal circulation: oxygen uptake (right ventricle) and oxygen delivery (left ventricle). Fetal blood flow patterns promote the effective distribution of poorly oxygenated blood to the right ventricle, which then directs this blood primarily to the placenta for oxygen uptake. The well-oxygenated blood flows to the left ventricle, which then directs the majority of its output systemically to the highly metabolic organs (Figure 3-1). To understand this phenomenon, it is necessary to understand venous blood flow patterns to the heart and arterial flow patterns to the various vascular beds.

The fetal central venous system can be divided roughly into six components: (1) the superior vena cava, which receives upper body blood flow; (2) the coronary sinus, which receives myocardial flow; (3) the ductus venous, which receives most of the placental blood flow from the umbilical vein; (4) the inferior vena cava below the hepatic veins, which receives lower body flow; (5) the hepatic veins, which receive portal venous and hepatic arterial flow; and (6) the pulmonary veins, which receive pulmonary blood flow. The approximate percentage of total venous return from each of these components is presented in Figure 3-2.

Almost all of the blood flowing through the superior vena cava and coronary sinus returns to the right ventricle (Figure 3-3). The superior vena cava courses anteriorly and inferiorly as it enters the right atrium. The anatomy of the sinus venosus and of the eustachian valve promotes streaming so that almost all of the flow from the superior vena cava crosses the tricuspid valve into the right ventricle. Similarly, the coronary sinus enters the right atrium just above the medial aspect of the tricuspid valve annulus. The position of the coronary sinus ostium directs flow across the tricuspid valve into the right ventricle. Most of the superior vena caval blood is derived from the brain with lesser amounts from the upper extremities, and thus the blood is poorly oxygenated (Figure 3-4). Similarly, the coronary sinus blood is derived from the myocardium, and thus it is even more desaturated (Figure 3-4). Consequently, almost all of the poorly oxygenated blood from these two venous compartments enters the right ventricle.

The lower inferior vena cava joins with the right and left hepatic veins and the ductus venosus near the right atrium to form the upper portion of the inferior vena cava, which delivers all of the venous return from the lower body and placenta to the heart (Figure 3-3). Although these venous systems join to form a single connection with the right atrium, the upper inferior vena cava is relatively short and exhibits fascinating streaming patterns. These patterns allow for the effective distribution of the different venous systems to the two ventricles. The lower inferior vena cava carries blood from the lower body, which, although not as desaturated as that of the upper body or coronary sinus, is much more desaturated than umbilical venous blood (Figure 3-4). This blood courses along the lateral wall of the upper inferior vena cava. It remains separate from the other sources of blood except for that from the right hepatic veins, which also enters the lateral wall of the upper inferior vena cava. The right hepatic veins primarily receive poorly saturated portal sinus blood from the splanchnic circulation and from the right hepatic arteries (Figure 3-4). The lower body and right hepatic vein streams join and course into the right atrium along the inferior margin of the eustachian valve, which directs most of that flow, along with that of the superior vena cava, across the tricuspid valve into the right ventricle.

Conversely, although umbilical venous blood is also delivered to the upper inferior vena cava, its course into the heart is quite different. The umbilical vein enters the portal sinus. From there, umbilical venous blood splits into the ductus venosus (60% of umbilical venous blood) and the left portal venous streams (Figure 3-3). Ductus venosus blood is entirely derived from the umbilical vein and is thus well saturated (Figure 3-4). Left portal venous blood is mostly derived from the umbilical vein but joins with hepatic arterial flow in the left lobe of the liver to exit via the left hepatic vein. Because hepatic arterial blood represents only a small portion of hepatic blood flow, it does not appreciably decrease the saturation exiting the liver via the left hepatic veins. Blood from both the ductus venosus and the left hepatic vein enter the upper inferior vena cava along its medial margin. These two highly oxygenated streams flow together into the right atrium along the superior margin of the eustachian valve and are directed toward the foramen ovale. The foramen acts as a windsock, directing blood into the body of the left atrium and then across the mitral valve into the left ventricle (Figure 3-3).

Finally, pulmonary venous blood returns to the left atrium directly. Because the lungs are not very metabolically active in utero, this blood is not as desaturated as blood returning to the superior vena caval. Moreover, this relatively poorly saturated blood passing to the left ventricle represents only 8% of combined venous return (Figure 3-2) and thus does not appreciably decrease left ventricular oxygen saturation.

The result of these circulatory patterns is that the right ventricle receives almost all of the blood with the lowest oxygen saturation (venous return from the superior vena cava, coronary sinus, lower body, and right hepatic veins). Conversely, the left ventricle receives most of its blood from the ductus venosus and the left hepatic vein, the two sources of the most highly saturated umbilical venous blood. The oxygen saturation of blood in the left ventricle is estimated to be about 28% higher than that in the right ventricle. This difference is similar to that seen between the ventricles in the postnatal state.

In order for the ventricles to accomplish their tasks effectively, the left ventricle must not only receive more highly oxygenated blood but also deliver the majority of its blood to the organs with high metabolic rates (heart, brain, and adrenal glands), while the right ventricle must deliver the majority of its blood to the placenta for oxygen uptake (Figure 3-2). Although the adrenal glands use a great deal of oxygen per gram of tissue, they are small and thus receive only a very small portion (<1%) of combined ventricular output. The heart and the brain receive all of their blood from the left ventricle. Approximately 7% of left ventricular output is delivered to the heart via the coronary arteries, and 55% is delivered to the brain via the carotid and vertebral arteries. Of the remaining left ventricular output, 15% is delivered to the upper extremities, and only 23% is delivered across the aortic isthmus to the descending aorta. Note that the aortic isthmus is very narrow in the fetus. The relative narrow isthmus functions as a resistor, providing further evidence that the ventricles do not eject blood in parallel.

Thus, the descending aorta receives most of the output of the right ventricle; only a small portion of left ventricular output joins that from the right ventricle in the descending aorta. The right ventricle delivers its output according to the relative resistances of the pulmonary, systemic, and placental vascular beds. In the fetal circulation, the resistance to blood flow in the pulmonary vascular bed is extremely high, much more so than that of the systemic and placental vascular beds. Thus, only a very small percentage of fetal right ventricular output (15%, or 8% of combined ventricular output) is delivered to the lungs. The remainder crosses the ductus arteriosus into the descending aorta. Of that flow, approximately one-third is delivered to the lower body and two-thirds to the placenta for oxygen uptake. Thus, the majority of the relatively desaturated right ventricular output goes to the placenta for oxygen uptake, and most of the remainder goes to organs of low metabolic activity. As discussed above, the majority of highly saturated left ventricular output is delivered to the highly metabolic heart and brain. Thus, the ventricles perform their tasks in an effective manner, albeit somewhat less so than in the postnatal state.

The efficiency of the circulatory system is apparent from analysis of the proportion of available oxygen that is actually used by the fetus. Fractional extraction of oxygen is defined as the fraction of oxygen removed by the tissues from the blood that is delivered by the arterial system. In the postnatal state, only 25% to 30% of available oxygen is extracted from blood. Thus, there is a large oxygen reserve for extraction during stress, even without an increase in cardiac output. In the fetus, despite the lower oxygen saturation of blood delivered to the fetal body, only about 30% of oxygen is extracted, leaving the fetus with almost the same oxygen extraction reserve. This reserve is achieved in part by the remarkable fetal blood flow pathways described in this section.

One Ventricle Can Maintain Fetal Cardiovascular Stability

The mixing of left and right ventricular output through central shunts decreases the efficiency of oxygen delivery somewhat, but this inefficiency is advantageous to the fetus with congenital cardiovascular disease when one ventricle is underdeveloped. Because these shunts allow both ventricles to eject into all three vascular beds, only one ventricle is necessary for cardiovascular stability.

This fact can be most readily demonstrated in the fetus with only one functional ventricle. Hypoplasia of one atrioventricular valve, usually in association with atresia of the corresponding semilunar valve, is a relatively common form of complex cardiovascular disease. Describing physiology of the fetus with one of the more common defects, hypoplastic left heart syndrome, is instructive in understanding the impact of congenital cardiovascular disease on fetal cardiovascular physiology (Figure 3-5). In a fetus with this condition, all of the venous return (except the very small amount of blood that passes through the pulmonary circulation) enters the right atrium normally. Rather than crossing the foramen ovale to the left atrium, blood in the ductus venosus and left hepatic veins crosses the tricuspid valve with the rest of the systemic venous return. Pulmonary venous return, instead of crossing the mitral valve, crosses the foramen ovale in a left-to-right direction and then enters the right ventricle. Thus, the right ventricle receives all of the venous return. As is normally the case, the right ventricle ejects its blood into the main pulmonary artery, and about 8% enters the pulmonary circulation. The remaining 92% crosses the ductus arteriosus to the descending aorta. Because the aortic valve is atretic, blood flow from the ductus arteriosus not only passes down the descending aorta to the lower body and placenta but also passes retrograde, up the aortic isthmus and around the arch, to supply the upper body and heart. Although cardiovascular stability is maintained in the vast majority of fetuses with hypoplastic left heart syndrome, brain development may be impaired, possibly to a decrease in either flow or its perfusion pressure due to the isthmic resistor. Furthermore, pulmonary problems can occur after birth if the foramen ovale is restrictive in utero, leading to pulmonary venous hypertension.