Fetal CVS and Cardiovascular Transition at Birth

Question 1

What are the main differences between fetal and newborn/adult circulation?

Answer 1

Fetal:

• Ventricles pump in parallel
• Placenta is gas exchange organ
• Foramen ovale
• Ductus arteriosus
• Ductus venosus
• Artterial PO2 is ~35mmHg
• Lungs vasoconstricted so pulmonary arterial pressure > systemic arterial pressure
• There is retrograde flow during diastole from pulmonary circulation back out into the MPA and then through the DA

Newborn/Adult:

• Ventricles pump in series
• Lungs are gas exchange organ
• Fetal shunts closed
• Arterial PO2 is ~85-100mmHg
• Pulmonary blood flow is increased 25-30 fold
• Pulmonary pressure decreases
• Systemic MAP increases from 40-100mmHg
• HR lower
• Cardiac output significantly lower

Question 2

Explain the function of the ductus venosus:

Answer 2

• The most highly oxygenated blood in the fetus enters fetal circulation via the fetal vein
• The ductus venosus allows ~50% of blood flow from the umbilical vein to bypass hepatic circulation and flow directly into the IVC to conserve oxygen so it can delivered to the brain

Question 3

Explain the function of the foramen ovale:

Answer 3

• The role of the foramen ovale is to shunt blood coming into the right atrium into the left atrium
• By allowing blood to bypass the right heart, it also allows the blood to immediately bypass pulmonary circulation and be pumped into the left heart where it can be pumped into systemic circulation (primarily to the upper body to supply the brain)
• The FO is kept open as the right atria is at much higher pressure than the left atria

Question 4

Explain the function of the ductus arteriosus:

Answer 4

• 90% of the blood that passes through the right is shunted by the ductus venosus into systemic circulation (the descending aorta) so it does not enter pulmonary circulation
• The DA shunts blood from the main pulmonary artery into the descending aorta
• The DA is kept open by low O2 and high PGE2 and a pressure gradient (pulmonary pressure higher than systemic)

Question 5

What factors affect fetal combined ventricular output?

Answer 5

Heart Rate:

• Increases with activity
• Increases with acute hypoxia due to sympathetic activation
• Decreases with severe asphyxia due to parasympathetic activation

Stroke Volume:

• Increases during gestation due to increase in ventricular size and contractility
• Is reduced with reduced umbilical cord flow (venous return comprimised)

Question 6

What are the two main events at birth that affect the cardiovascular transition?

Answer 6

1. Lung aeration and initiation of air breathing:
   - Triggers 25-30 fold increase in pulmonary blood flow
2. Clamping of the umblical cord
   - Left side of blood is deprived for venous return
   - MAP increases
   - If lungs aerated this effect is minimised

Question 7

How does pulmonary blood flow change at birth?

Answer 7

1. Pulmonary blood flow increases by 25-30 fold
2. Decrease in pulmonary arterial pressure (from 45-50 to 10-15mmHg)
   - Thus massive decrease in resistance of pulmonary arteries

Question 8

Explain the main features of the cardiorespiratory transition at birth:

Answer 8

1. Lungs fill with air
2. Massive increase in pulmonary circulation and decrease in pulmonary arterial pressure
3. Increase in arterial pressure
4. Increase in systemic pressure and decrease in pulmonary arterial pressure = foramen ovale closes
5. If cord is clamped and lungs are aerated, blood will flow from the aorta, through the DA and into the pulmonary circulation (prevents drop in cardiac output)
6. Closure of DA: due to increase in pulmonary blood flow, reverse pressure gradient (pulmonary pressure < systemic pressure) causing turbulent blood flow through the DA and potentially PEG2
7. Closure of ductus venosus; less blood flow due to clamping of umbilical cord causes it to close and become ligament
8. Eventually there will be hypertrophy of left ventricular wall due increased workload