Foetal Circulation and Adaptation at Birth Flashcards
Resistance in the placenta?
A low resistance vascular bed
Functions of the placenta?
Carries out all the functions that will later be taken over by the organs:
Fetal homeostasis
Waste product transport from the fetus
Nutrient transport to the fetus
Gas exchange
Acid-base balance
Hormone production
Transport of IgG
PGE2 (prostaglandin) production - maintains patency of ductus arteriosus
Lungs function in the foetus?
Not in use; the lungs are fluid-filled, unexpanded and small. The pulmonary vessels and capillaries have high resistance so there is little blood flow
Liver function in the foetus?
Little role in nutrition and waste management
Gut function in the foetus?
Gut is not in use
Blood flow in the adult compared to the foetus?
Adult: blood flows from heart to lungs, back to heart, and then to the rest of the body
Foetus: foetal heart pumps blood to the placenta, via the umbilical arteries; blood from the placenta returns to the foetus via the umbilical veins. Oxygenated, nutrient-rich blood returns from the placenta and must be distributed to the growing foetus
Note: in foetal circulation, the pressure in the right side of the heart exceeds the pressure in the left side (opposite from the adult circulation)
Another point is that, in the foetus, the IVC carries oxygenated blood from the placenta
How many vessels are in the umbilical cord?
Normally:
2 umbilical arteries
1 umbilical vein
What are the 3 foetal shunts?
- Ductus venosus
- Foramen ovale
- Ductus arteriosus
Function of the ductus venosus?
Connects the umbilical vein to the IVC
What is the path of the ductus venosus through the foetus?
Nutrients come from the placenta and so require no further processing in the liver; so, the ductus venosus carries the majority of the placental blood straight into the IVC, bypassing the portal circulation
Function of the ductus arteriosus?
Connects pulmonary bifurcation to the descending aorta, allowing oxygenated blood delivery to the rest of the foetus (only a small amount of the right ventricle output goes to the lungs because of this)
How is the patency of the ductus arteriosus maintained?
Patency is maintained by circulating prostaglandin E2 (PGE2), produced by the placenta
Function of the foramen ovale?
Opening in atrial septum connecting the right atrium to the left atrium, allowing oxygenated blood to enter the left atrium, then the left ventricle, ascending aorta and the carotids
Structure of the foramen ovale?
Membrane flap on the left atrium side to allow blood to enter the left atrium from the right
3 adaptation in the first few minutes after birth?
First breath
Goes from blue to pink
Cord is clamped and shut
Describe changes with first breath
Baby inflates lungs and cries; pO2 increases and oxygen causes pulmonary vasodilatation and decrease in resistance of lungs
Describe changes with the baby going pink
More blood circulating to lungs and progressively increasing pO2
Describe changes with cord clamping
Large, low resistance vascular bed is removed from the circulation and increases systemic vascular resistance
How does the foramen ovale close?
As pulmonary vascular resistance falls and systemic vascular resistance rises, the left atrial pressure exceeds the right atrial pressure; flap is pushed closed
Consequences of patent foramen ovale?
In 1/3 of people, the foramen ovale may remain as a small hole; this is implicated in stroke (shunt of clot from right to left atrium) and migraine
3 factors causing ductus arteriosus closure?
- Fall in pulmonary vascular resistance reduces blood flow through the duct
- Rising oxygen tension has a direct effect on the duct smooth muscle, causing vasoconstriction (UNIQUE response of the duct)
- Loss of circulating PGE2 from the placenta and increase PGE2 metabolism in the lung
When is the closure period of the ductus arteriosus?
Functional closure within hours-days
Anatomical closure within 7-10 days
End result of ductus arteriosus closure?
Changes to fibrous ligament (ligamentum arteriosum)
Complication of failure of ductus arteriosis closure?
Very high incidence of patent ductus arteriosus (PDA) in pre-term infants
May contribute to other pre-term complications
Treatment options include waiting and seeing, NSAIDs and surgery
NSAIDs decrease PGE2 production, causing duct closure (this is why pregnant women are told to avoid Ibuprofen)
What is duct-dependent circulation?
Some congenital heart disease cause the whole circulation to be dependent on the ductus arteriosus remaining open; if the duct closes, the baby goes into shock and dies
How to keep the ductus arteriosus open?
IV PGE2 can be used to the keep the duct open until an alternative shunt is established or definitive surgery is carried out
Most common consequence of failure of adaptation?
Persistent Pulmonary Hypertension of the Newborn (PPHN) - failure of adaptation:
Lung vascular resistance fails to fall
Shunts remain, so there is right to left flow due to a PFO and due to a PDA
Causes of PPHN?
Sick babies, e.g: Sepsis Hypoxic ischaemic shunt Meconium aspiration syndrome (lung disease) Cold stress
Can also be related to underlying anatomical abnormality, such as congenital diaphragmatic hernia
Symptoms and examination signs of PPHN?
Blue baby
Large difference between pre and post-ductal saturation (measure O2 sats on right hand and on a foot and compare)
Treatment of PPHN?
Ventilation
Oxygenation causes pulmonary vasodilatation to decrease resistance; could also use inhaled NO
Exracorporeal Membrane Oxygenation (ECMO) - oxygenating blood externally
