Respiratory distress in the setting of normal peripheral perfusion and without overt cyanosis is the least common manifestation of symptomatic cardiovascular 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 the respiratory symptoms, such as tachypnea with feeding, are often 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 with normal systemic perfusion; obstructive structural heart disease is discussed in Chapter 8; cardiomyopathies and arrhythmias are discussed in Chapters 9 and 10, respectively; and heart failure is discussed in Chapter 11.
PATHOPHYSIOLOGY OF INCREASED PULMONARY BLOOD FLOW
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 signs and symptoms caused by this group of defects. The arterial oxygen saturation, although sometimes mildly decreased, is not so low that either cyanosis is present or systemic oxygen delivery is compromised.
The primary symptom in these infants is tachypnea, often accompanied by mildly increased work of breathing. 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 systemic and pulmonary venous filling pressures are usually normal. 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.
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 ...