Fetal Circulation Flashcards
What are the intra and extra cardiac shunts?
Intra - Foramen Ovale
–connects the RA and LA
Extra - Ductus Arteriosus and Ductus Venosus
–Connects to Pulmonary artery to Aorta
–Bypasses the liver and diverts oxygenated blood from the umbilical vein to the IVC
Which organ is the first to receive blood from the placenta?
The liver
What type of cardiac circulation is the fetus?
Parallel
CO = RV +LV
What are four special features of the fetal circulation?
- RV pumps against higher pressure
- BF to lungs is a fraction of RV output
- Lungs extract O2
- Lungs secrete fluid
The placenta does what the lung should be doing, getting rid of CO2 and taking up O2
Where does the largest percent of fetal CO go?
55% goes to the placenta
How much blood goes into the RA? The FO? The RV?
91% goes into the RA
of that, 46% cross to the LA from the FO
The other 45% goes into the RV
Of that, only 9% goes into the lungs due to its high resistance, the other 36% cross the DA and goes into the descending Aorta
The LV receives that 9% from the lungs for a total of 55% (46% + 9%)
Of this, 15% goes to the head, 3% to the coronary sinus, and the remaining 37% into the aorta
What happens if the FO closes early?
The RV would have to pump 91% of the blood to the PA, theres high pulmonary vascular resistance and this would cause RV hypertrophy
LV will receive less blood and be underdeveloped
What happens if the DA closes prematurely?
With high PVR more blood would be forced to pass through the FO causing the LA to grow bigger and the right ventricle will be underdeveloped
What is the purpose of the shunts?
Proper ventricle development
Directing umbilical blood to the heart and brain
Whats the difference between fetal and maternal blood O2?
Fetal and maternal blood do not mix
Max fetal arterial O2 is 80% - in the umbilical vein, know this number!!
Average O2 for adults is 95%
The 80% from the umbilical vein mixes with the 26% from the IVC resulting in about 67% in the IVC above the liver
about 62% PO2 is pumped in the aorta (as the lungs had extracted some of the O2)
How do shunts of the fetal circulation compensate for the low O2 of the blood?
Blood from the IVC preferentially enters the FO ensuring adequately oxygenated blood reaches the heart and brain
Blood from the SVC enters RV
What is the P50 for the fetus?
What PaO2 makes fetus blood 80% saturated?
P50 is about 20
It is around 30 for adults
Fetal Hgb has better capacity to bind oxygen
The highest PO2 a fetus can reach is lower than the most acceptable low for an adult
80% saturation is reached at PaO2 of 35
What are some other compensatory mechanism for the fetus?
Increased CO
HCT is greater (15 adults and 18 for fetus)
Increased blood flow to brain and heart
When baby takes first breath there is decrease in blood flow to cerebral and heart
How does the fetal heart compare to the adult heart on the pressure volume curve?
The fetal heart is less compliant, has less capacity, and usually is operating at maximum capacity
If fetal HR decreases then SV cannot increase as it would in an adult
What changes occur at birth?
PVR drops instantly after first breath
Pulmonary blood flow increases increases VR LA which raises LAP
Increased SVR, placenta is clamped so you lose low resistance circuit
BP on left side of heart rapidly increases
decrease venous return to RA so pressure decreases
greater blood flow to LA from lungs
Foramen Ovale shuts as LAP > RAP
Ductus arteriosus closes because of the high PO2 prostaglandins previously produced by the placenta not being there
Lower PO2 will initiate constriction and result in stronger contractions
