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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.
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FETAL CARDIOVASCULAR PHYSIOLOGY
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Overview of Essential Facts of Fetal Cardiovascular Function
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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.
Fact 2. Only one ventricle is required for cardiovascular stability in the fetus.
Fact 3. The right ventricle is the dominant ventricle in the fetus.
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.
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Tasks of the Fetal Ventricles
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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 ...