Fetal to Neonatal Circulation Transition

Question 1

summary paragraph circulation before birth

Answer 1

• Umbilical vein to IVC vs through liver via ductus venosus, bringing mixed oxygenated blood to heart
  
  • Super high RA pressure d/t high resistance in lungs (hypoxic vasoconstriction of pulmonary arterioles) forces foramen ovale to open causing blood to bypass to LA
  • Vessel allowing blood flow b/w pulmonary artery and aorta (ductus arteriosus), most of the blood passes d/t pressure differential b/w RS vs LS of heart blood flow
  • Low resistance of placenta draws blood flow back towards it from fetal heart via umbilical arteries

Question 2

summary paragraph circulation after birth

Answer 2

• Clamping/wharton’s jelly contracting makes “placenta” have extremely high resistance, restricting flow causing clots. First few days stop using umbilical vein and ductus venosus
  
  • Taking in oxygen stops HPV, arterioles dilates which switches pulmonary circulation to be under low resistance … Low resistance drops pressures in RS of heart (RA and through pulmonary arteries) which brings oxygenated blood to heart … Pressures in RA fall, and pressures in LA increase which causes foramen ovale to fall (pressure of LA > RA). This occurs during first few minutes
  • Aortic pressures > pulmonary pressures, during first few hours of life have muscular constriction of ductus arteriosus which sense oxygen levels in blood; can also sense placenta has been removed (d/t decrease in prostaglandin levels). Combination of both factors, begins over course of hours. Umbilical arteries also constrict for same reasons which stop blood flow to umbilicus

Question 3

fetus increases ____ and ___ vasodilators

Answer 3

nitric oxide synthase and soluble guanylate cyclase

Question 4

placenta vs fetal lungs

Answer 4

placenta low resistance, lungs high resistance d/t hypoxic pulmonary vasoconstriction

Question 5

3 fetal shunts in-utero

Answer 5

Fossa Ovalis: right to left heart blood flow of o2 rich blood from placenta

Ductus Ateriosus: connects PA and aorta to redirect o2 rich/mixed blood flow to vital organs bypassing high resistance lungs

Ductus Venosus: allows most o2 rich blood from placenta to go around liver and enter heart via IVC

Question 6

stimuli for first breath

Answer 6

• cord clamping leads to transient hypoxemia and hypercarbia acting as chemoreceptor stimulation to breathe
• environmental change/stress
• drying off stimulates
• chest wall re-expansion

Question 7

first breath

Answer 7

need -60 to -100 mmHg to generate average Vt of 40mls

Question 8

circulatory transition

Answer 8

• cord clamping increases SVR
• breathing increases PaO2 which decreases PVR/reverses HPV; it also inactivates PgE2 which causes PDA to close enabling more blood flow to LA from lungs

Question 9

functional and anatomical closure of structures

Answer 9

foramen ovale
- functional closure minutes
- anatomical closure 4 weeks to 3 months
- d/t increased SVR, decreased PVR, inactivated PgE2

ductus arteriosus
- functional closure hours after delivery
- anatomical closure 2-4 weeks (PDA caused by hypoxia and acidosis)
- d/t increased PaO2 inhibiting PgE2

ductus venosus
- functional closure within days
- anatomical closure by 1 week (longer if preterm)
- d/t cord clamping ceasing perfusion via umbilical vein

Question 10

nomenclature changes with closure of fetal shunts

Answer 10

foramen ovale -> fossa ovalis

ductus arteriosus -> ligamentum arteriosum

ductus venosis -> ligamentum venosum

umbilical arteries -> medial umbilical ligaments

umbilical vein -> ligamentum teres