foetal circulation and adaptations at birth

Question 1

foetus in uterus - circulation (1)

Answer 1

> placenta is included in circulation (responsible for gas exchange, nutrition, waste excretion, homeostasis)
lungs are fluid filled and unexpanded
liver has little role in nutrition and waste management
gut not in use

Question 2

foetal circulation (2)

Answer 2

the heart pumps blood to the brain and body which then travels through the placenta and then comes back to the foetal heart again
>don’t receive full cardiac output like they do in adult circulation
»foetal heart pumps blood to the placenta via the umbilical arteries
»blood from the placenta returns to the foetus via the umbilical vein

Question 3

what are the 3 shunts that are specific to foetal life

Answer 3

ductus venosus
foramen ovale
ductus arteriosus

Question 4

describe foetal circulation

Answer 4

blood enters baby through umbilical cord -blood circulates through the umbilical vein to the baby (oxygenated form maternal circulation) and travels through anterior abdominal wall until it gets to the liver, and most of this blood bypasses the liver via the ductus venosus, and travels through the IVC just before entering the RA, from the RA (via the flow) is shunted into the LA via the PFO, the blood then travels through the LV to where it then travels through the ascending aorta, where it then supplies the coronary arteries (thus head and neck)
the other blood coming into the RA is the venous return from the upper body and mixes with some of the oxygenated blood to go then through the RV, which is then pumped out into the pulmonary artery and then reaches the pulmonary bifurcation (supplying L and R lung) there is also third shunt - the ductus arteriosus which connects the pulmonary bifurcation to the descending aorta to supply the lower body and the umbilical arteries

Question 5

babies are relatively hypoxic

Answer 5

saturation is about 88%

Question 6

ductus venosus

Answer 6

nutrients come from the placenta - doesn’t need further processing in the liver
>carries the majority of the placental blood straight into IVC bypassing portal circulation

Question 7

PFO

Answer 7

> a hole between RA and LA
allows blood to flow from right to left atrium
allows the best oxygenated blood to enter LA then on to LV amending aorta, carotids
membrane flap on LA side

Question 8

ductus arteriosus

Answer 8

small fraction of RV output goes to the lungs
the rest goes via ductus arteriosus to join descending aorta
maintained by circulating prostaglandin E2 produced by placenta

Question 9

adaptation - at the point of birth

Answer 9

• massive changes in the few minutes following birth
• baby inflates lungs and cries
• goes from blue (cyanosed) > pink
• cord clamped and cut

Question 10

changes in resistance -

SVR

Answer 10

> flow in umbilical cord stops
cord clamped and cut
removal of large low resistance vascular bed from the systemic circulation
sudden increase in SVR

Question 11

changes in resistance - PVR

Answer 11

> baby breathes and cries
lungs expand and fill with air
air filled lungs lead to higher oxygen tension in pulmonary circulation
oxygen is a very effective vasodilator in the pulmonary arteries
dramatic drop in pulmonary resistance

Question 12

the PFO closes

Answer 12

in utero the pressure in the RA is higher than the LA and that’s how blood moves
» at birth the LA pressure becomes higher and so pushes the flap closed so there is no longer a transition from RA > LA

Question 13

ductus arteriosus

Answer 13

left side pressure becomes higher than right
flow changes direction in the duct - now flows from aorta to pulmonary artery
>starts to constrict
>there is a functional closure within hours to days - anatomical closure within 7-10 days
>ends up as a fibrous ligament - ligamentum arteriosum
»also placenta is removed and the prostaglandin E2 is what helps keep this open so without that as well it closes

Question 14

what if the ductus arteriosus fails to close?

Answer 14

there is a very high incidence of patent ductus arteriosus in preterm infants
>may contribute to other preterm complications
>treatment options include wait and see - NSAIDS and surgery

Question 15

when do we need to keep the ductus arteriosus open?

Answer 15

some congenital heart diseases cause a duct dependent circulation
IV prostaglandin E2 can be used to keep the duct open until an alternative shunt is established or definitive surgery is carried out

Question 16

pulmonary resistance

Answer 16

> continues to change for some time after birth
continues to drop
reaches ‘normal’ adult type levels by 2-3 months

Question 17

failure of adaptation

Answer 17

persistent pulmonary hypertension of the newborn:
when PVR doesn’t drop as normal but stays high as in utero
>more likely to happen in sick babies ie sepsis, hypoxic ischaemic insult, meconium aspiration syndrome, cold stress
>can be related to underlying anatomical abnormality such as congenital diaphragmatic hernia

Question 18

persistent pulmonary hypertension of the newborn

Answer 18

pulmonary pressure stays high
flow continues from RA>LA across PFO
flow continues to be from right to left across duct
>get a hypoxic baby

Question 19

pre and post ductal saturation measurement

Answer 19

put a saturation monitor on the right hand and on a foot

see a 10-20% difference in pre and post ductal