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Respiratory distress in the setting of normal peripheral perfusion
and without overt cyanosis is the least common manifestation of
symptomatic heart disease in the newborn. Particularly in the absence
of a murmur, the diagnosis of heart disease is often delayed or
missed entirely because respiratory distress alone in an acyanotic
infant with normal perfusion is most often caused by lung disease
rather than intrinsic cardiac disease. Furthermore, symptoms usually
develop gradually over the first few days or weeks of life and are
often rather subtle. It may take several weeks or more to recognize
that the infant is growing poorly and that heart disease may be
the cause. This chapter reviews structural cardiovascular defects
that can cause respiratory distress; cardiomyopathies and arrhythmias
are discussed in Chapters 9 and 10, respectively and heart failure
is discussed in Chapter 11.
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A diverse group of congenital structural cardiovascular defects
share the common feature of increased pulmonary blood flow as the
main pathophysiologic process. It is this common characteristic
that is the basis for the majority of symptoms caused by this group
of defects. The arterial oxygen saturation, although sometimes mildly
decreased, is not so low that either cyanosis is appreciated or
systemic oxygen delivery is compromised. The primary symptom in
these infants is tachypnea, often
accompanied by mildly increased work of breathing.
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In addition to tachypnea, many of these infants exhibit other
signs and symptoms of the heart failure syndrome (Chapter 11). These
infants have heart failure with high cardiac output (“high
output failure”), which is very different than the low
output failure that occurs in adults with acquired heart disease
and in neonates with decreased systemic perfusion (Chapters 8 and 11). In addition to increased pulmonary blood flow, systemic blood
flow is often increased in response to the increased metabolic demands
resulting from the greater respiratory effort. The increased cardiac output
leads to greater circulating blood volume to maintain normal filling
pressures. The heart is hypercontractile and venous filling pressures
are usually normal on both sides of the heart. Peripheral edema
does not occur because venous pressures are not increased. However, hepatomegaly is a fairly constant
finding because the liver and hepatic veins are very compliant and
enlarge to accommodate the increased circulating blood volume.
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Oxygen consumption or metabolic demand is increased for a variety
of reasons. The major contributor is the increased work of breathing.
In a normal infant, breathing is a large component of basal oxygen
consumption (20%), which is similar to the metabolic requirements
for growth. As the work of breathing increases, it may comprise
30% to 40% of oxygen consumption. An increase
in adrenergic drive is necessary to maintain the increased combined
ventricular output and this too increases oxygen consumption, particularly
by its effect on brown fat metabolism. This increased adrenergic
drive is mediated by both neural and hormonal mechanisms and causes two ...