Circulatory Changes

In utero, a series of central shunts moves oxygenated blood coming from the placenta through the umbilical vein to supply the brain and other organs. Most oxygenated blood bypasses the liver via the ductus venosus and enters the left side of the heart (bypassing the lungs) through the foramen ovale. The poorly oxygenated blood returning from the lungs bypasses the heart through the ductus arteriosus and returns to the placenta to obtain oxygen from the maternal circulation (Moore and Persaud, 1993).

On delivery, the fetal circulation must adapt to self-oxygenation, and the central shunting must cease. When the umbilical cord is clamped and cut, absent blood flow within the umbilical vein leads to the closure of the ductus venosus. The umbilical vessels functionally close by 2 to 3 days of life.

With the initial newborn breath, the aeration of the lungs drives fluid within the air space of the lungs into the pulmonary interstitium. An increase in the partial pressure of oxygen (Po2) within the pulmonary vasculature causes vaso-dilation and a progressive decrease in pulmonary vascular resistance. Increasing Po2 , along with circulating prostaglan-dins, stimulates the constriction and closure of the ductus arteriosus. Pulmonary blood now flows preferentially to the heart, rather than directly into the aorta through the ductus arteriosus (Moore and Persaud, 1993; Thureen et al., 2005).

In the normal infant the ductus arteriosus might remain partially open during the first several hours of life and cause a soft systolic murmur. It can take the normal infant a few hours to clear the lungs of excess fluid, causing fine crackles to be heard on lung examination and a transient elevation of the respiratory rate. As pulmonary vascular resistance falls, systemic vascular resistance increases. When left atrial and ventricular pressures increase above right atrial pressures, the foramen ovale closes. Poorly oxygenated blood from the superior and inferior vena cava can now flow into the pulmonary arteries (Moore and Persaud, 1993). The foramen ovale typically becomes fully sealed within the first month of life.

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