3. Foetal Circulation and CHD

Question 1

Fetal circulation

Answer 1

Umbilical venous blood (SpO2 80%) passes into the IVC (SpO2
~67%) via the ductus venosus (which traverses the liver).

Most of this blood crosses into the left atrium via the foramen ovale.

By the time this blood reaches the ascending aorta
its saturation has fallen but some of this flow is
destined for the cerebral circulation,
and its saturation of around 62% is still higher than that in the
ductus arteriosus (50%) and descending aorta (58%).

These figures emphasize the fact that the fetus exists in a relatively hypoxic environment.

Only about 10% of the cardiac output traverses the pulmonary vascular bed.

Question 2

In utero pump and pulmonary circulation

Answer 2

In utero the right and left hearts pump in parallel.

There are connections between the
systemic and pulmonary circulations
via the ductus arteriosus
(which links the pulmonary artery to the aorta)

and the foramen ovale
(which is a communication between the left and right atria).

The pulmonary circulation has high resistance,
and the right and left ventricular pressures are equal,

although the right ventricle ejects 66% of the combined ventricular output

Question 3

Change at birth

Answer 3

With clamping of the umbilical cord there is a sudden rise in systemic vascular
resistance (SVR) and aortic pressure.

Respiration expands the lungs,
and pulmonary vascular resistance (PVR) decreases
in response to expansion,
respiratory movements,
increased pH and increased oxygenation.

(PVR continues to decrease with recruitment of small arteries,
and the reduction over weeks of pulmonary vascular smooth muscle.)

Pulmonary blood flow increases. E
nhanced pulmonary venous return into the left atrium raises the left
atrial pressure above the right, and the foramen ovale closes by a flap valve effect. It
is a functional closure which can be reversed if there is a sudden increase in right
atrial pressure.