Inadequate systemic perfusion is the second most common manifestation of symptomatic heart disease in newborn infants. Affected infants present with moderate to severe respiratory distress in addition to signs of decreased systemic perfusion. Respiratory distress is caused by increased pulmonary venous pressure causing pulmonary edema. Pulmonary venous pressures are increased because (1) there is obstruction to the egress of blood from the lungs or from the left atrium into the left ventricle or (2) the left ventricle cannot adequately eject blood. In some infants, the decrease in systemic perfusion is profound, with decreased to absent peripheral pulses, cool extremities, hypotension, and severe metabolic acidosis. In these cases, the compromise of systemic blood flow is life threatening and requires urgent diagnosis and therapy. In other infants, respiratory distress is the most impressive finding, and the signs of decreased systemic perfusion are subtle, often leading to the erroneous conclusion that the infant has primary pulmonary disease rather than heart disease. This is particularly true when the infant does not have a heart murmur, which may occur in this group of cardiac defects. Signs of decreased systemic perfusion, which may be indicated solely by mildly decreased pulses or by a mild metabolic acidosis, should be carefully sought and considered in all infants with significant respiratory distress.
The two hemodynamic categories of cardiovascular pathophysiology that cause decreased systemic perfusion are left heart obstruction and cardiomyopathy. This chapter will review the various anatomic defects that cause left heart obstruction. Cardiomyopathies in newborn infants are reviewed in Chapter 9.
PATHOPHYSIOLOGY OF INADEQUATE SYSTEMIC PERFUSION
The primary pathophysiologic abnormality in the infant with inadequate systemic perfusion is the inability of the heart to supply an adequate amount of oxygen to the tissues to meet metabolic needs. In this context, the onset is more acute and severe as compared with the chronic heart failure syndrome discussed in Chapter 11. Furthermore, in contrast to cyanotic infants (Chapter 6), oxygen saturation is usually normal or only mildly decreased in infants with decreased systemic perfusion. Instead, the overriding problem is inadequate systemic blood flow.
Fetal Physiology and the Transition at Birth
In the normal fetus, different ventricles perfuse the upper and lower portions of the body. The right ventricle supplies the lower body, and the left ventricle supplies the upper body. During fetal life, obstruction to one ventricle, or a myopathic process isolated to that ventricle, does not lead to decreased systemic perfusion. Inflow can be diverted to the healthy unobstructed ventricle via the foramen ovale, and a portion of the outflow of the healthy ventricle can be diverted to the other vascular bed via the ductus arteriosus (Chapter 3, Figure 3-5). Left-sided obstruction causes decompensation after birth because the postnatal changes in the circulation prevent the right ventricle from performing the work of the left ...