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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 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.
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Essential Facts of Fetal
Cardiovascularfunction: An Overview
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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.
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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 ...