NORMAL PULMONARY VASCULAR TRANSITION:
At birth, physiologically high PVR decreases and there is 10 to15- fold increase in
pulmonary blood flow. The initial decrease in PVR is fairly steep at birth, followed by a
gradual decrease over the 2 months of life.
Fetal Circulation:
- Pulmonary and Systemic pressures are nearly equal in utero.
- High PVR ensures that 90% of right ventricular output flows across the PDA to
descending aorta and then to placenta for gas exchange. - Low fetal oxygen tension (PaO2 of 20-25 torr) contributes to fetal pulmonary
vasoconstriction - Fetal Hb with high O2 affinity and high systemic blood flow (both ventricles
pumping in parallel into aorta) ensure that O2 delivery is optimal - Streaming of umbilical venous blood, which has the highest O2 saturation in the
fetus, across PFO to left atrium ensures that O2 delivery is maximized to brain and
heart - Placenta is a low resistance/low pressure vascular bed, which favors right
ventricle pumping into the systemic instead of pulmonary circulation
Postnatal transition: Pulmonary artery pressure drops from removal of lung
liquid, expansion of the lungs, increase in arterial oxygen tension and release of
vasoactive mediators from vascular endothelium. Oxygen, increases in pH and
decreases in PaCO2 also directly cause vasodilation.