eLFH - The Foetal Circulation

Question 1

Where does gas exchange occur in foetal circulation

Answer 1

Placenta

Placenta receives deoxygenated blood via umbilical arteries and returns oxygenated blood via umbilical vein

Question 2

Why is foetal circulation “shunt dependent”

Answer 2

Foetal circulatory system has preferential streaming of oxygenated blood and intracardiac and extracardiac shunts to ensure most highly oxygenated blood is received by brain and myocardium

Question 3

Describe foetal circulation

Answer 3

Deoxygenated blood to placenta via umbilical arteries

Oxygenated blood from placenta via umbilical vein

50-60% umbilical venous blood bypasses hepatic circulation via ductus venosus to enter IVC

Eustachian valve in IVC directs more oxygenated blood along dorsal IVC and across foramen ovale into LA

Highly oxygenated blood ejected via LV to ascending aorta, to brain and coronary circulation

Deoxygenated blood from SVC + anterior IVC flow directed across tricuspid into RV

RV ejected into pulmonary arteries

High pulmonary vascular resistance so only 12% of RV output enters pulmonary circulation
Remaining 88% RV output crosses ductus arteriosus into descending aorta

Descending aorta supplies lower half of body

Umbilical arteries arise from iliac arteries

Question 4

Venous blood which forms SVC deoxygenated blood

Answer 4

Jugular venous blood
Coronary sinus

Question 5

Venous blood which forms deoxygenated anterior IVC blood

Answer 5

Venous blood from extremities
Hepatic venous flow

Question 6

Partial pressure O2 of blood in umbilical vein

Answer 6

~4.7 kPa

Question 7

O2 saturations of blood in umbilical vein

Answer 7

~80%

Question 8

O2 saturations of blood in left atrium

Answer 8

~65%

Question 9

Partial pressure of blood in descending aorta which supplies lower half of the body (distal to ductus arteriosus)

Answer 9

~2.7 kPa

Question 10

Oxygen saturations as different points within the foetal circulation - in picture form

Answer 10

Question 11

Combined ventricular output definition

Answer 11

Combined cardiac output of both ventricles in one minute

Question 12

Use of combined ventricular output

Answer 12

Used to define and measure foetal cardiac output

In adults there are no shunts so RV and LV stroke volumes are equal. Therefore CO defined as volume of blood ejected by one ventricle in one minute

However intracardiac and extracardiac shunts in foetal circulation mean RV and LV stroke volumes are not equal

Question 13

Percentage of venous return received by RV

Answer 13

65%

Question 14

Percentage of venous return received by LV

Answer 14

35%

Question 15

Cause of high Pulmonary vascular resistance in foetus

Answer 15

Foetal pulmonary arterioles have high muscle mass and high resting tone

Foetal lungs are collapsed with low resting oxygen tension

Ductus arteriosus contains muscle that is sensitive to oxygen tension and vasoactive substances

Question 16

Mechanisms of maintaining patent ductus arteriosus in utero

Answer 16

Low oxygen tension

Vasodilation effect of prostaglandin E2

Question 17

Reasons gas exchange in the placenta is less efficient than gas exchange in the lung

Answer 17

Larger minimum diffusion distance in placenta

Blood-blood permeability lower in placenta

Question 18

How is less efficient gas exchange in placenta offset

Answer 18

Large surface area of gas exchange compared to the size of the foetus

Question 19

Maternal and foetal blood flow to placenta

Answer 19

Question 20

Haemoglobin oxygen dissociation curve of adult Hb compared to foetal Hb

Answer 20

HbF has lower content of 2,3-DPG shifting O2 dissociation curve to left

Question 21

2,3 DPG

Answer 21

2, 3 diphosphoglycerate

Question 22

Foetal Hb concentration at term and why

Answer 22

160 - 180 g/L

Increase oxygen content of blood

Question 23

Bohr effect definition

Answer 23

As level of CO2 in tissue rises, affinity of Hb for O2 decreases

Question 24

Double Bohr effect

Answer 24

CO2 excreted by foetus is removed in placenta into maternal intervillous sinuses

Higher PCO2 in maternal side increases O2 unloading of maternal Hb

Lower PCO2 in foetal side (as removed in placenta) leads to better oxygen loading of foetal Hb

Question 25

Rate of increased uterine blood flow during pregnancy

Answer 25

Increases 20 fold during pregnancy

Question 26

Foetal percentage of HbF vs HbA

Answer 26

75% HbF 25% HbA

Question 27

When does high HbF in foetus become disadvantage

Answer 27

After birth

Question 28

Adaptations that need to be made as foetus transitions to post natal life

Answer 28

Gas exchange transfers from placenta to lungs Foetal circulatory shunts must close Left ventricle output must increase

Question 29

How does pulmonary vascular resistance decrease at birth

Answer 29

Expansion of lungs Dramatic fall in PVR 8-10x increase in pulmonary blood flow Improved oxygenation of neonatal blood reverses hypoxic pulmonary vasoconstriction

Question 30

Effect of drop in pulmonary vascular resistance on circulatory system at birth

Answer 30

Increased pulmonary blood flow causes massive rise in venous return to LA Decrease in IVC flow reduces venous return to RA Therefore LA and RA pressures equalise Foreman ovale flap pushed against atrial septum and closes atrial shunt

Question 31

How long does it take for foreman ovale to close

Answer 31

Initially closes in minutes to hours after birth Anatomic closure occurs later by tissue proliferation

Question 32

Closure of ductus arteriosus

Answer 32

Simultaneous with drop in pulmonary vascular resistance, DA becomes bi-directional Increased PO2 in neonatal blood causes direct constriction of DA smooth muscle (exact mechanism not known) PGE2 produced by placenta also drops adding to constriction of duct

Question 33

Closure of ductus arteriosus timing

Answer 33

Functional closure of DA by 96 hours Anatomical closure later by endothelial and fibrous tissue proliferation

Question 34

Closure of ductus venosus

Answer 34

Placental circulation removed causing drop in flow through ductus venosus and fall in venous return through IVC DV closes passively 3-10 days after birth

Question 35

Persistent foetal circulation

Answer 35

Circumstances can occur which revert neonate back to foetal circulation - pathophysiological state termed persistent foetal circulation

Question 36

Patent ductus arteriosus

Answer 36

Failure of DA to close Left to right shunt Increased volume and workload of LV Eventually leads to left heart failure

Question 37

Ventricular septal defect

Answer 37

Well tolerated in foetus as LV and RV pressures are equal After birth, SVR increases and PVR decreases Causes left to right shunt Leads to congestive heart failure

Question 38

Tetralogy of Fallot features (two most important features first)

Answer 38

Pulmonary stenosis resulting in RV outflow obstruction Ventricular septal defect Right ventricular hypertrophy Overriding aorta

Question 39

Consequences of tetralogy of Fallot after birth

Answer 39

As foetus, depending on severity of pulmonary obstruction, pulmonary blood flow is dependent on ductus arteriosus After birth as DA closes, develop right to left shunt Severe cyanosis Prostaglandin infusion to re-establish DA flow is vital to stabilise these neonates

Question 40

Transposition of the great arteries origin and initial presentation

Answer 40

Abnormal rotation and separation of arterial truncus during embryogenesis Aorta arises from RV and pulmonary artery from LV To maintain arterial O2 sats compatible with life relies on PFO, VSD or PDA

Question 41

Management of Transposition of the Great Arteries

Answer 41

Re-establish ductus arteriosus patency with prostaglandin infusion Balloon atrial septoplasty under echo guidance on PICU Complete surgical repair electively at later date once neonate has been stabilised