Cardiovascular Changes at Birth

• Two umbilical arteries carry deoxygenated blood from the foetus to the placenta. It is oxygenated in the placenta. Oxygenated blood returns to the foetus via the umbilical vein.
• The ductus arteriosus acts as a shunt. It allows the flow of blood from the pulmonary trunk to the aorta. This provides a bypass, avoiding the lungs. Need to ensure higher pressure in the pulmonary arteries than the aorta to ensure the blood flows via the ductus arteriosus. This is achieved by pulmonary vasoconstriction mediated by hypoxia.
• The foramen ovale acts as a shunt from the right atrium to the left atrium. Through streaming in the right atrium this can allow most oxygenated blood to bypass the pulmonary circulation entirely. It relies on a high pressure in the right atrium - pulmonary vasoconstriction helps to maintain this.
• The ductus venosus allows oxygenated blood from the umbilical vein to bypass the liver, shunting it into the IVC. 30% of the oxygenated blood takes this route. Prevents the liver stealing lots of oxygen.
• Foetal haemoglobin consists of 2 alpha and 2 gamma chains. It has greater affinity for oxygen binding.
• Polycythaemia

  • Foetuses have a greater concentration of RBCs than adults
  • the breakdown of these causes a transient jaundice in the first few days of life, which usually resolves itself within 10 days of birth
• Changes after birth

  • first breath
  • oxygen levels increase
  • pulmonary vasodilation
  • ductus arteriosus bypass becomes less appealing, increasing flow in the pulmonary arteries
  • placental circulation ceases, stopping flow in the ductus venosus
  • systemic vascular resistance increases
• The foramen ovale closes after birth as the left atrium pressure is greater than the right atrium.
• Right atrial dynamics after birth - IVC blood (now deoxygenated) joins SVC blood to flow through the RV and pulmonary circulation.
• Ductus arteriosus at birth - the pulmonary and systemic circulations become separate, with the whole output of the right ventricle passing through the pulmonary circulation. Closure:

  • patency in utero is maintained by prostaglandin E2
  • closure normally occurs after birth within 2-3 days
  • it is the final event in the sequence
• remnants of ducts after birth:

  • foramen ovale fuses to fossa ovale
  • ductus arteriosus leaves ligamentum arteriosum
  • ductus venosus leaves ligamentum venosum
• Pulmonary vasoconstriction is mediated by hypoxia (in the blood and the tissues). It ensures that the pulmonary arteries remain vasoconstricted (increasing pulmonary arterial pressure but reducing flow). It can occur later in life as a result of chronic hypoxia, causing pulmonary hypertension.
• Ohm's law: pressure = flow x resistance
• NSAIDs inhibit cyclooxygenase enzymes, which are responsible for the production of prostaglandins. Prostaglandin E2 is used to keep the ductus arteriosus open in utero, therefore care should be taken with NSAIDs during pregnancy.
• In the fetus, blood needs to go via the placenta to collect oxygen and nutrients and to dispose of waste products such as carbon dioxide and lactate (via the mother). Therefore, there must be a way for blood to travel to the placenta and back. As the fetal lungs are not fully developed or functional, it does not make sense for the fetal blood to pass through the pulmonary circulation. Therefore there are three shunts that allow blood to bypass the lungs.
• Ductus venosus: this shunt connects the umbilical vein to the inferior vena cava and allows blood to bypass the liver
• foramen ovale - this shunt connects the right atrium with the left atrium and allows blood to bypass the right ventricle and pulmonary circulation
• ductus arteriosus - this shunt connects the pulmonary artery with the aorta and allows blood to bypass the pulmonary circulation
• The first breaths the baby takes expands the alveoli, decreasing the pulmonary vascular resistance. The decrease in pulmonary vascular resistance causes a fall in pressure in the right atrium. At this point the left atrial pressure is greater than the right atrial pressure, which squashes the atrial septum to cause functional closure of the foramen ovale, similar to a closed valve with nothing flowing through it. This then gets sealed shut structurally after a few weeks and becomes the fossa ovalis.
• Prostaglandins are required to keep the ductus arteriosus open. Increased blood oxygenation causes a drop in circulating prostaglandins. This causes closure of the ductus arteriosus, which becomes the ligamentum arteriosum.
• Immediately after birth the ductus venosus stops functioning because the umbilical cord is clamped and there is no flow in the umbilical veins. The ductus venosus structurally closes a few days later and becomes the ligamentum venosum.
• A fetus is surrounded by amniotic fluid and will use the placenta as its source of oxygen and nutrients. While the lungs are developing, they are not functional and provide no oxygenation. There are also high energy demands of developing tissue, in particular the brain. As a result, the fetal circulation must direct blood away from non-functional organs and ensure that growing tissues receive their oxygen requirements.
• The umbilical vein delivers oxygenated blood from the placenta to the fetus, providing oxygen and nutrients
• the umbilical arteries are used to transport deoxygenated blood away from the fetal tissue and back towards the placenta for re-oxygenation
• the ductus venosus allows blood from the placenta to bypass the highly demanding but relatively inactive liver
• the ductus arteriosus is the fusion of the primitive pulmonary artery to the aorta, therefore allowing blood to pass straight from the right ventricle into the aorta and bypass the inactive lungs
• the foramen ovale creates a shunt between the right atrium and the left atrium so oxygenated blood from the placenta can move to the left atrium. This allows for the oxygenated blood to pass through the left ventricle and into the ascending aorta, oxygenating the brain
• The highest partial pressure of oxygen in the feto-placental circulation is approximately 4 kPa. This is compared to 13 kPa in an adult. However, the fetus is able to maintain adequate oxygen delivery to tissues through the use of shunts, assisted by a relative polycythaemia and the properties of fetal haemoglobin.
• Fetal haemoglobin has a different quaternary structure to adult haemoglobin. Adult haemoglobin (HbA) is formed of 2 alpha subunits and 2 beta subunits. However, fetal haemoglobin contains different subunits, namely 2 alpha subunits and 2 gamma subunits. This change in three-dimensional structure of the protein means that fetal haemoglobin can bind more readily to oxygen from maternal circulation. This allows for adequate oxygenation of tissues.
• 1. The first breath causes a rise in the partial pressure of oxygen
  2. An increase in the partial pressure of oxygen causes pulmonary vasodilation
  3. Pulmonary vasodilation leads to a drop in right heart pressure
  4. Simultaneously, placental circulation ceases, causing left heart pressure to rise
  5. These factors combine to cause the foramen ovale to shut
  6. The pulmonary and systemic circulations become separate, and the whole output of the right ventricle passes through the pulmonary circulation
  7. The final step in the sequence is the closure of the ductus arteriosus, occurring 2-3 days after birth.
• If the ductus arteriosus remains open beyond 3 months of life in preterm infants and after 1 year of life in full-term infants, it is a persistent patent ductus arteriosus (PDA). 5-10% of congenital heart defects in term infants are PDAs, but they are far more common in preterm neonates. Prostaglandin-E2 is responsible for keeping PDAs open.
• Risk factors for persistent PDA include trisomy 21, Holt-Oram Syndrome, exposure to sodium valproate in-utero, and asphyxia during birth. Rubella infections during pregnancy can also predispose to PDAs.
• We recognise a PDA by a continuous machinery murmur over the upper left sternal border. Generally, they are asymptomatic but large shunts can lead to recurrent lower respiratory tract infections, feeding difficulties, failure to thrive and even heart failure. We identify these via echocardiograms and can give indomethacin in preterm infants, or use surgical methods to close in-term, symptomatic infants.
• In up to 20% of healthy adults, the foramen ovale will not completely close. We refer to this as a patent foramen ovale. Large, or slightly displaced holes in the septum are called atrial septal defects. While these may be asymptomatic, they can cause tachypnoea, poor weight gain and recurrent chest infections in children and adults. We can detect them by listening for widely split-second heart sounds alongside a soft systolic ejection murmur at the upper left sternal border. If necessary, surgical closure can take place.
• Oxygenated blood from the placenta travels via the umbilical vein, which branches into the left and right umbilical veins at the liver. The right umbilical vein provides oxygenated blood to the liver via the portal vein, while the left umbilical vein branches into the ductus venosus, which bypasses the liver to carry oxygenated blood directly into the inferior vena cava. A mix of oxygenated and deoxygenated blood then enters the right atrium via the IVC, also mixing with the SVC.
• Fetal lungs have no role in gas exchange so pulmonary arteries are hypoxic. Hypoxia causes pulmonary vasoconstriction, increasing pulmonary vascular resistance and pressure within pulmonary circulation. Pressure is higher in the right side of the heart. Right ventricular afterload increases, causing blood to preferentially shunt away from the right ventricular outflow tract, via ductus arteriosus/foramen ovale. Most blood bypasses right ventricle and lungs, entering left atrium or directly into aorta (systemic circulation). This allows relatively oxygen-rich blood to be pumped to the body.
• After birth, air flows into lungs to replace water in alveoli, resulting in significant rise in oxygen levels in the lungs. Pulmonary vascular resistance falls due to reductions in hypoxic pulmonary vasoconstriction, meaning a lower pulmonary resistance and decreased afterload in the right side of the heart.
• A change in pressure gradients between the left and right side of the heart results in closure of the foramen ovale. Decrease in pulmonary pressure means that blood flow across the ductus arteriosus is also reversed, with blood initially shunted from the aorta to pulmonary artery. As oxygen levels rise, the smooth muscles in the walls of ductus arteriosus constrict, eventually closing completely.
• After birth, the umbilical vessels constrict, forming the round ligament of the liver (umbilical vein), ligamentum venosum of the liver (ductus venosus), and superior vesical arteries (umbilical arteries).
• Fetal haemoglobin has a higher affinity for oxygen compared to maternal haemoglobin. This means fetal haemoglobin binds to oxygen more strongly and enables the transfer of oxygen from mother to fetus prenatally.