Transition from Fetal to Extrauterine Circulation Flashcards
Where does the output from each ventricle go in fetal circulation?
The right ventricle delivers the majority of its output to the placenta for oxygenation, and the left ventricle delivers the majority of its output to the heart, brain, and upper part of the body
During fetal circulation, __% of right ventricle output flows through the low-resistance ductus arteriosus over the high-resistance collapsed fetal lung.
During fetal circulation, 90% of right ventricle output flows through the low-resistance ductus arteriosus over the high-resistance collapsed fetal lung.
Oxygenation of blood returning from the placenta
PO2 32–25 mm Hg
O2 saturation 70% (remember, this is fetal hemoglobin, higher affinity)
Fetal cardiac output regulation
Primarily via sympathetic vs parasympathetic output control of heart rate
Changes in preload have little effect on the fetal heart, as the fetal ventricles function at their plateau regions at baseline.
The right ventricle performs approximately ___ of the cardiac work before birth
The right ventricle performs approximately two thirds of the cardiac work before birth
This is reflected in the size and thickness of the right ventricle before and after birth
Because the pulmonary flow in utero is very small compared with that immediately after birth, ____ may be masked in utero.
Because the pulmonary flow in utero is very small compared with that immediately after birth, anomalies preventing normal pulmonary return (total anomalous pulmonary return, mitral stenosis, etc.) may be masked in utero (when pulmonary venous return is so low anyway).
Birth is a time of stress for ___
Birth is a time of stress for the left ventricle
4 Failure of any one of a complex series of pulmonary or cardiac events that take place within minutes of birth leads to ___
4 Failure of any one of a complex series of pulmonary or cardiac events that take place within minutes of birth leads to generalized hypoxemia and brain damage or death
Major immediate changes in circulation following birth
- Filling of lungs and their capillaries, combined with oxygen-mediated vasodilation. Drop in pulmonary resistance.
- Clamping of the umbilical cord, increasing the SVR by removing a low-resistance circuit from circulation. Increase in SVR.
What do the two immediate postpartum changes do to fetal circulation?
The drop in PVR and increase in SVR cause:
- reversal of the flow through the ductus arteriosus
- an increase in pulmonary flow
- drop in systemic venous return -> decreased RA pressure
- rise in pulmonary venous return -> increased LA pressure
How do the immediate postpartum circulation changes close the foramen ovale (in 90% of people)?
The increased pulmonary return increases LA pressure, while the decreased systemic return decreases RA pressure.
Combined, this makes LA > RA pressure, and this closes the flap-valve that keeps the foramen ovale open, eliminating left-to-right or right-to-left shunting.
What usually to the ductus arteriosus after birth?
Usually, the ductus arteriosus remains patent for several hours or days after birth. There is small ‘physiologic’ left to right shunt from aorta to pulmonary artery due to this anastomosis.
The mechanism of closure of the ductus arteriosus is not completely understood, but its patency in fetal development is maintained by PGE2, and patent ductus arteriosus can be cured in some patients by administering PGE2 isomerase inhibitor.
Ductus venosus
