Chapter 3

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 heart disease. The fetus with complex congenital heart disease is rarely symptomatic, yet many newborn infants with the same defects are critically ill within hours or days after birth. In addition, specific cardiovascular abnormalities are associated with specific cardiac defects, 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 which occur at birth.

Essential Facts of Fetal Cardiovascularfunction: An Overview

Four essential facts about the fetal circulation upon which to base an understanding of fetal cardiovascular physiology and its impact on congenital heart defects are listed here and discussed further in the following text.

1. The right and left ventricles perform the same tasks in the fetus as they do postnatally. Much has been written about the differences between the fetal and postnatal circulations. Most particularly, the case is frequently made that the former is a circulation in parallel, with the ventricles sharing the tasks of ejecting blood of similar oxygen content for oxygen uptake and delivery, while the latter is a circulation in series, with the right ventricle ejecting poorly oxygenated blood to the lungs for oxygen uptake and the left ventricle ejecting more highly oxygenated blood to the highly metabolic organs for oxygen delivery. However, the fetal ventricles actually perform their normal postnatal tasks quite efficiently, achieving this by remarkable venous and intracardiac flow patterns, and central shunts that are unique to the fetal circulation.

Only one ventricle is required for cardiovascular stability in the fetus. Despite the separation of functions in the normal fetus, in the absence of two normal ventricles, in most instances the single functional ventricle is able to take over the function of the other ventricle to maintain a normal hemodynamic status in the fetus.

The right ventricle, not the left, is the dominant ventricle in the fetus. Postnatally, the left ventricle is dominant (has a greater mass) because it ejects an equal amount of blood as the right ventricle but does so under higher pressure. In the normal fetus, the right ventricle is dominant because it ejects blood at the same pressure as the left ventricle, but it ejects more blood.

After embryogenesis, the size and orientation of a cardiovascular structure (cardiac chamber, valve, or blood vessel) is determined by the flow pattern and volume of blood passing through it. Genetic and environmental determinants affect embryogenesis and can cause abnormal development of cardiac and vascular structures. These primary abnormalities may alter blood flow throughout fetal life which in turn may cause further abnormalities of structure and function. These secondary flow-determined abnormalities are often ...

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