Foetal Circulation

Explain the physiological changes during pregnancy, and parturition

In Utero

The foetal circulation has a number of structural differences:

• Two umbilical arteries
  The umbilical artery returns deoxygenated blood to the placenta.
  • PO₂ of 18mmhg (SpO₂ 45%)
  • Over 50% of the combined output of both foetal ventricles enters the placenta
• One umbilical vein
  The umbilical vein supplies oxygenated blood to the foetus.
  • Has a PaO₂ of 28mmHg (SpO₂ 70%)
  • 60% of blood from the umbilical vein enters the IVC
  • 40% of blood enters the liver
• Two ducts:
  • Ductus venosus
    Shunts blood from the umbilical vein to the IVC.
  • Ductus arteriosus
    Shunts blood from the pulmonary trunk to the descending aorta.
• A foramen ovale
  Shunts blood from the right atrium to the left atrium.
• Immature myocardium
  Foetal myocardium does not obey Starlings Law, and does not adjust contractility for any given preload. Therefore:
  • SV is fixed
  • CO is HR dependent
    Normal HR at term is 110-160 bpm.

These structural difference alter the pathway of blood circulation:

• Oxygenated blood returns via the umbilical vein
  • 40% flows to the liver
  • 60% is returned to the IVC
• Oxygenated blood in the IVC is directed via the Eustachian valve through the foramen ovale
• Blood returning from the SVC is directed into the RV, and then into the descending aorta by the ductus arteriosus
  • ~10% of RV output flows through the pulmonary circulation

This arrangement has several features:

• Blood with the most oxygen is delivered to the arch vessels to supply the brain
• Blood with the least oxygen is delivered to the umbilical arteries for gas exchange
• Both the RV and the LV eject into systemic circulations, and are of similar size and wall thickness

Changes at Birth

Several changes happen at birth:

• Placental circulation is lost
  There is a transition from circulation in parallel to circulation in series.
• An FRC is established
  Reversal of hypoxic pulmonary vasoconstriction results in a rapid drop in PVR.

• The cord is clamped
  The systemic vascular bed volume falls, and SVR increases due to the loss of the low-resistance placental circulation.

• The fall in PVR lowers RV afterload
  RAP falls due to the loss of hypoxic pulmonary vasoconstriction.

• The rise in SVR increases LV afterload
  LAP rises as the LV moves up the Starling curve.

• When LAP exceeds RAP, the foramen ovale closes
  A degree (~10%) of residual shunt remains. Shunt is:

  • Bidirectional
    • Left-to-right shunt is unconcerning
    • Right-to-left shunt has usually only minor effects on systemic SpO₂
      Will be increased with ↑ PaCO₂, excessive PEEP, ↓ pH.
      • Beware embolic material
        Don't forget the bubbles.

• Increased left sided afterload causes flow reversal in the ductus arteriosus
  There is progressive closure of the ductus over hours to days, under the influence of prostaglandins and oxygenated blood flowing through the duct.

• The ductus venosus progressively fibroses over a period of days to weeks

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
3. Brandis K. The Physiology Viva: Questions & Answers. 2003.