Foetal Circulation and Adaptation at Birth

Question 1

Resistance in the placenta?

Answer 1

A low resistance vascular bed

Question 2

Functions of the placenta?

Answer 2

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

Question 3

Lungs function in the foetus?

Answer 3

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

Question 4

Liver function in the foetus?

Answer 4

Little role in nutrition and waste management

Question 5

Gut function in the foetus?

Answer 5

Gut is not in use

Question 6

Blood flow in the adult compared to the foetus?

Answer 6

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

Question 7

How many vessels are in the umbilical cord?

Answer 7

Normally:
2 umbilical arteries
1 umbilical vein

Question 8

What are the 3 foetal shunts?

Answer 8

1. Ductus venosus
2. Foramen ovale
3. Ductus arteriosus

Question 9

Function of the ductus venosus?

Answer 9

Connects the umbilical vein to the IVC

Question 10

What is the path of the ductus venosus through the foetus?

Answer 10

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

Question 11

Function of the ductus arteriosus?

Answer 11

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)

Question 12

How is the patency of the ductus arteriosus maintained?

Answer 12

Patency is maintained by circulating prostaglandin E2 (PGE2), produced by the placenta

Question 13

Function of the foramen ovale?

Answer 13

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

Question 14

Structure of the foramen ovale?

Answer 14

Membrane flap on the left atrium side to allow blood to enter the left atrium from the right

Question 15

3 adaptation in the first few minutes after birth?

Answer 15

First breath

Goes from blue to pink

Cord is clamped and shut

Question 16

Describe changes with first breath

Answer 16

Baby inflates lungs and cries; pO2 increases and oxygen causes pulmonary vasodilatation and decrease in resistance of lungs

Question 17

Describe changes with the baby going pink

Answer 17

More blood circulating to lungs and progressively increasing pO2

Question 18

Describe changes with cord clamping

Answer 18

Large, low resistance vascular bed is removed from the circulation and increases systemic vascular resistance

Question 19

How does the foramen ovale close?

Answer 19

As pulmonary vascular resistance falls and systemic vascular resistance rises, the left atrial pressure exceeds the right atrial pressure; flap is pushed closed

Question 20

Consequences of patent foramen ovale?

Answer 20

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

Question 21

3 factors causing ductus arteriosus closure?

Answer 21

1. Fall in pulmonary vascular resistance reduces blood flow through the duct
2. Rising oxygen tension has a direct effect on the duct smooth muscle, causing vasoconstriction (UNIQUE response of the duct)
3. Loss of circulating PGE2 from the placenta and increase PGE2 metabolism in the lung

Question 22

When is the closure period of the ductus arteriosus?

Answer 22

Functional closure within hours-days

Anatomical closure within 7-10 days

Question 23

End result of ductus arteriosus closure?

Answer 23

Changes to fibrous ligament (ligamentum arteriosum)

Question 24

Complication of failure of ductus arteriosis closure?

Answer 24

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)

Question 25

What is duct-dependent circulation?

Answer 25

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

Question 26

How to keep the ductus arteriosus open?

Answer 26

IV PGE2 can be used to the keep the duct open until an alternative shunt is established or definitive surgery is carried out

Question 27

Most common consequence of failure of adaptation?

Answer 27

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

Question 28

Causes of PPHN?

Answer 28

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

Question 29

Symptoms and examination signs of PPHN?

Answer 29

Blue baby

Large difference between pre and post-ductal saturation (measure O2 sats on right hand and on a foot and compare)

Question 30

Treatment of PPHN?

Answer 30

Ventilation

Oxygenation causes pulmonary vasodilatation to decrease resistance; could also use inhaled NO

Exracorporeal Membrane Oxygenation (ECMO) - oxygenating blood externally