Fetal Circulation

Question 1

Describe the fetal circulation

Answer 1

Oxygenated blood from placenta –> umbilical vein to IVC via portal system or venous duct. High oxygenated IVC blood enters RA and goes to either PFO into LA –> LV to developing brain. SVC - RA - RV - arterial duct into systemic circulation.

Question 2

Proportion of pulmonary blood flow changes with gestation. When does it increase?

Answer 2

Increases during 3rd trimester

Question 3

How can fetal pulmonary blood flow be increased?

Answer 3

By pulmonary vasodilator agents (such as o2) administered to mother

Question 4

What is the maximum cardiac output at term?

Answer 4

250ml/kg/min

Question 5

How much of the fetal cardiac output does the RV and lV contribute?

Answer 5

RV 55%

LV 45%

Question 6

Of the combined output (RV + LV) how much returns to the placenta and how much goes to the fetal organs and tissues?

Answer 6

65% returns to the placenta

35% to fetal organs and tissues

Question 7

Characteristics of the immature fetal heart

Answer 7

Less compliant

Less able to generate contractile force for the same degree of stretch

Question 8

Effects of advancing gestation on fetal heart

Answer 8

Allow maturation of excitation-contraction coupling as well as increasing autonomic innervation

Question 9

What happens to the circulatory system at birth?

Answer 9

Shifts from parallel circulation to ‘series’ circulation
Increase in CO from both ventricles
With inspiration –> rapid fall in PVR (due to lung expansion allows new vessels to open and existing vessels to enlarge)
Reduced resistance and decreased PA pressures –> increase Pulmonary blood flow
Lower-resistance placental circulation is removed from systemic circulation (cut cord)
Sudden Increase in O2 tension produced by breathing - alters local PG synthesis –> constriction of arterial and venous ducts

Question 10

When does functional closure of PDA occur?

Answer 10

Within 24-72 hrs

Question 11

When does anatomical closure of PDA occur?

Answer 11

1-2 weeks

Question 12

When does PFO functionally close?

Answer 12

In most cases by 3rd month of life

Question 13

What could mask signs of underlying structural congenital CV malformations?

Answer 13

PFO and venous duct which can potentially allow shunting after birth.

Question 14

Three principle mechanisms of cyanosis

Answer 14

1) Obstruction to PBF, associated with a right-to-left shunt
2) Discordant VA connections with adverse streaming of blood through the heart
3) Mixing of blood which may occur at atrial, ventricular or great artery level (miseducating systemic and Pulmonary venous return is distributed to both PA and aorta)

Question 15

Where does obstruction occur in patients with cyanosis and reduced pulmonary blood flow (PBF)?

Answer 15

May occur at TV, RV, PV or PA level with obligatory communication within the heart, allowing shunting of the blood returning via the systemic veins from the right to the left side of the heart

Question 16

What are examples of ACHD in patients with cyanosis and reduced PBF?

Answer 16

ToF
Pulmonary atresia (with or without VSD)
Tricuspid atresia

Question 17

What causes cyanosis with normal/increased PBF?

Answer 17

Complete transposition or complete mixing.

Question 18

At what levels can mixed blood occur?

Answer 18

Atrial (Total anomalous PV connection)
Ventricular (functionally single ventricle)
Great arteries (Common arterial trunk)

Question 19

What are some complications of long term cyanosis?

Answer 19

Impaired growth
Developmental delay
Chronic hyperaemia –> finger clubbing and polycythaemia, renal dysfunction, hyperuricaemia and acne