PERINATAL AND NEONATAL CIRCULATION
Labor benefits the fetus’ impending transition to life by moving fluid present in the alveoli during fetal development into the interstitial space and speeding up its absorption. In addition, by 34 weeks of gestation surfactant is produced, which prevents the alveoli from collapsing with expiration.
The first breath is taken in response to hypoxia at birth, reexpansion of the chest when vaginal delivery is complete, and changes in skin temperature when the neonate emerges from the womb. This initial breath causes the pulmonary artery blood flow to increase and the pulmonary vascular resistance to decrease. As the pulmonary vascular resistance falls below that of the systemic circulation, blood flow across the ductus arteriosus shifts from left to right. This initiates the functional closure of the ductus arteriosus. By the time the neonate is 8 hours old, right-to-left shunting has stopped, and the vessel closes completely by the time the infant is 1 to 3 weeks old.
During fetal life a one-way valve, the
foramen ovale, permits shunting of blood from the inferior vena cava through the right atrium to the left atrium. At birth, left atrial pressure rises above right atrium pressure and closes the foramen ovale flap.
By 2 months of age the infant’s pulmonary artery has reverted from a thick-walled vessel to a thin-walled vessel and pulmonary artery pressure drops. If the infant continues to have left-to-right shunting, causing pulmonary overload, the signs and symptoms become apparent by 2 months of age.
Table 25-1 lists assessments and their significance for cardiac structural defects in the newborn.
By the time the neonate is 8 hours old, right-to-left shunting has stopped.
CONGENITAL HEART DISEASE
