What are the main shunts that avoid the fetal lungs?
Ductus venosus, ductus arteriosus and foramen ovale.
How do the locations of the inferior and superior vena cavae determine what level of oxygenated blood the brain gets?
The IVC has the highest level of oxygenated blood and points in a direction that opens the foramen ovale, shunting that blood straight to the brain. The SVC points downward and blood is pushed more towards the right ventricle, where it will be taken out through the pulmonary artery and the PDA.
Why is it that fetal blood preferentially avoids the lungs and goes through the patent ductus arteriosus (60% of fetal cardiac output goes through the PDA)?
The lung is fluid filled and the pulmonary arteries are contracted because fetal [O2} is lower than adults. This creates a path of higher resistance to the lungs and lower resistance through the PDA.
How is blood preferentially shunted towards the lungs once the baby is born?
With the 1st breath, the lungs fill with air, decreasing resistance to blood flow. Oxygen saturation goes from 80%-100%, causing NO production which vasodilates pulmonary arteries, resulting in an overall decrease in resistance to blood flow through the lungs.
How does the foramen ovale close at birth?
With increased blood flow to the lungs, you have increased left atrial pressure which closes the one-way valve foramen ovale. Over time fibrocytes seal the flap. You also clamp the umbilical cord at birth, which closes off the ductus venosus, decreases venous return and thus decreases right atrial pressure.
What are clinical issues the 20% of people who have patent foramen ovales could be at risk for?
Paradoxical emboli, migraines and high altitude cerebral edema.
What is responsible for the low systemic and high pulmonary circuits in utero? How is this reversed at birth?
High pulmonary = contracted pulmonary arteries and fluid filled lungs. Low systemic = placenta has a high capacity to accept blood. At birth, the baby takes its first breath and resistance in the lungs decreases. The umbilical cord is clamped, causing increased systemic resistance.
Why does it take 4-6 weeks for the newborn's lungs to gain full function?
At birth, the ductus arteriosus is still open. This causes right ventricular pressure to be equal to left ventricular pressure. Closure of the PDA is what creates the normal pressure differences.
Usually by the 4th day of life, a newborn's ductus arteriosus is closed. What causes it to close?
Clamping the umbilical cord cuts off PGE supply. Opening of the lungs increases PGE break down. Higher oxygen content causes smooth muscle contraction of PDA. Ultimately fibrocytes infiltrate the lumen and you get hypertrophy of the endothelium and the lumen is obliterated in 2-3 weeks.
A child is born an is severely hypoxic. How is this child's ductus arteriosus affected? How do you treat this child?
Since the child is hypoxic, its pulmonary arterioles remain constricted with resistance still high in the pulmonary circuit. Blood is still shunted through the PDA. You vasodilate the lungs with NO and O2. You also give vasopressors to increase systemic resistance and shunt more blood toward the lungs.