Cyanosis is the most common manifestation of symptomatic cardiovascular disease in the newborn infant. Moreover, cyanosis in the absence of significant respiratory distress is almost always caused by structural cardiovascular disease because pulmonary disease severe enough to cause cyanosis is usually associated with severe respiratory distress. Congenital cardiovascular defects that cause primarily cyanosis in newborn infants are reviewed in this chapter. Infants who have decreased systemic perfusion as the primary symptom, even if cyanosis is also present, are discussed in Chapter 8.
PATHOPHYSIOLOGY OF CYANOSIS
Adequate oxygen delivery to meet metabolic needs is essential for healthy survival. The amount of oxygen delivered to the tissues is dependent on systemic blood flow, hemoglobin concentration, and hemoglobin oxygen saturation (Table 6-1). At birth, oxygen consumption increases nearly threefold to meet the energy costs of breathing, feeding and digestion, and thermoregulation. Immediately after birth, systemic blood flow at least doubles, and systemic arterial oxygen saturation increases from about 75% to 95% (reviewed in Chapter 3). Thus, despite the increase in oxygen consumption, oxygen delivery increases similarly, and the reserve to extract oxygen remains large in normal infants. The fractional extraction of oxygen is about 30% so that the mixed venous saturation is about 65% to 70%. In contrast, newborn infants with cyanotic congenital heart disease cannot increase systemic arterial oxygen saturation, and, in fact, oxygen saturation often falls precipitously soon after birth. These infants are therefore at risk for inadequate systemic oxygen delivery, which, if untreated, may result in anaerobic metabolism, metabolic acidosis, and death.
TABLE 6-1.Calculation of Oxygen Delivery and Blood Flows
Hemodynamic Categories of Cyanotic Cardiovascular Disease
Decreased pulmonary blood flow (Table 6-2) and malposition of the aorta over the systemic venous ventricle (transposition complexes) are the two main pathophysiological mechanisms responsible for severely decreased systemic arterial saturation in newborn infants with cyanotic heart disease. In the normal postnatal circulation, all of the poorly saturated systemic venous blood is directed through the right heart structures to the pulmonary arteries; the oxygen saturations of the blood in ...