Paediatrics: Circulatory adaptation at birth

2021-03-05 12:00 AM

Oxygenated placental blood (PaO2 75kPa) returns to the foetus via the umbilical vein.

 

Circulatory adaptation at birth

Foetal circulation

Oxygenated placental blood (PaO2 75kPa) returns to the foetus via the umbilical vein. Blood bypasses the liver via the ductus venosus and flows into the inferior vena cava, and then the right atrium. This blood is then channelled to the left atrium and so to the left ventricle (via the foramen ovale). Oxygenated blood is then pumped to the cerebral and coronary vessels. The right ventricle mostly receives deoxygenated blood from the superior vena cava. About 15% is pumped to the lungs and the rest is diverted, via the ductus arteriosus, to the descending aorta so that it can go to the placenta via the umbilical arteries.

Postnatal circulation

At birth, oxygen inhalation leads to pulmonary arterial vasodilatation, leading to ‘fall’ arterial resistance and ‘raise pulmonary blood flow. At the same time systemic vascular resistance ‘rise due to loss of the low resistance placental circulation. The ductus arteriosus constricts as PaO2 i. The foramen ovale closes as a pulmonary venous return to the left atrium ‘rise and right atrial pres-sure d. Although initially rapid, these changes consolidate over 2–3wks.

 

Persistent pulmonary hypertension of the newborn (PPHN)

Failure of pulmonary vascular resistance to fall after birth causes decreased pulmonary blood flow; incidence 1/1000–1500 live births.

Causes Rarely primary/idiopathic due to disease of the pulmonary vasculature.

More commonly, it is a secondary complication of severe illness.

Presentation

  • Hypoxia disproportionate to any difficulty with CO2elimination.
  • The discrepancy between pre-and post-ductal arterial oxygen saturations >10%.
  • Mild breathlessness (as PaCO2, not PaO2, is the main physiological determinant of respiratory rate), acidosis, hypotension.
  • Loud single second heart sound.

Echocardiography shows ‘raise pulmonary arterial pressure, large right to left shunt at the level of the foramen ovale and ductus arteriosus.

Management

  • Treat causes; minimal handling.
  • Optimize BP, pH (aim high-normal), Hb, U&E, blood glucose.
  • Ventilate (aim for high PaO2and normal PaCO2). HFOV may be helpful.
  • Inhaled nitric oxide (results in selective pulmonary vasodilatation): dose 20ppm for 6hr initially and monitor for toxic levels of NO2and methaemoglobin.
  • ECMO, if severe.

Prognosis 

10–30% mortality. Risk of neurodevelopmental impairment in survivors.