Question 11

College Answer

The transfer from the fetal to the neonatal state is complex.  There is a close relationship between the simultaneously occurring cardiovascular and respiratory changes.  Closure of umbilical vessels results in an increase in peripheral resistance and blood pressure. Respiratory centre activation (clamping of umbilical vessels, and cold) results in expansion of previously collapsed lungs.   The resultant dramatic decrease in pulmonary vascular resistance increases blood flow through the lungs, and increases return to the left atrium. This, plus the reduced return to the right atrium (clamped umbilical vein) and the increased resistance to left ventricular outflow reverse the pressure gradient across the atria (closing the valve over the foramen ovale.  The fall in pulmonary artery pressure (decreased PVR) and the increased aortic pressure results in flow reversal through the ductus arteriosus. Constriction and closure of the ductus arteriosus appears to be initiated by the high arterial oxygen tension which is now in the aortic blood. The neonate is still at risk of reversion to a foetal circulation early after birth, especially in the presence of physiological stresses and congenital abnormalities.

Discussion

These diagrams are from van Vonderen et al (2014):

In textual long form, which defeats the point of point form:

• Foetal lungs are cleared of fluid and aerated. 
  • ​With the first breaths, the lungs are aerated, creating an FRC. The foetus is capable of generating negative pressures in excess of 30 cm H₂O, and these are triggered by light, warm temperature and handling.
  • Transpulmonary pressures generated by the first breaths probably play the dominant role. The pressure generated by the first breaths causes the interstitial space pressure to become subatmospheric, attracting the fluid into that space.
  • Adrenaline released during birth stimulated the lung endothelium to activate sodium channels which then reabsorb sodium out of lung water. This causes an osmotic shift of fluid out of the lung.
  • There is also the theory that passing through the vagina somehow squeezes water out of the foetal lung. Direct measurements have found that this squeeze equates to around 70 cm H₂O. However, the foetal chest does not get much of that pressure - most of it is squandered on deforming the foetal skull. 
• FRC is created and maintained
  • First breaths create and maintain the FRC by being expiration-limited, like a sort of intentional gas trapping. The infant ends up finishing the prolonged expiration on a closed glottis with abdominal muscles still forcefully contracting. 
  • Crying, grunting etc - all these manoeuvres serve this principle
  • The effect of this is that the alveoli are splinted
  • Surfactant also serves to reduce lung recoil, maintaining open alveoli
• Aeration of lungs leads to decreased pulmonary arterial resistance
  • ​The sudden drop in the pulmonary vascular resistance makes the lungs a path of least resistance for right ventricular blood.
  • Right ventricular output is therefore directed into the pulmonary circulation, increasing left ventricular preload.
  • Some of the pulmonary blood flow also consists of oxygenated blood rom the ductus arteriosus.
  • Aeration is not the only factor contributing to changes in pulmonary vascular resistance: Oh's Manual also mentions gradual postnanatal regression of smooth muscle in the pulmonary vessel walls.
• Foramen ovale shunt is reversed.
  • Pre-birth, much of LV preload consist of venous return through the foramen ovale.
  • There is an inverse relationship between pulmonary blood flow and flow though the foramen ovale.
  • As pulmonary blood flow contributes more and more of LV venous return, so the foramen ovale is forced closed.
• Ductus arteriosus shunt is reversed. 
  • Ductus arteriosus is a large shunt from the pulmonary arteries to the aorta, and its diameter is approximately the same as that of the descending aorta.
  • It shunts right ventricular blood into the systemic circulation, bypassing the lungs. About 10% of the RV output still goes into the pulmonary circulation.
  • With decreased pulmonary vascular resistance, this shunt is reversed. Then, about 50% of the pulmonary blood flow ends up being oxygenated blood from the aorta, shunting back into the pulmonary circulation via the ductus arterisus
• Ductus venosus will remain patent for days, but will eventually close.
  • ​Ductus venosus sends some of the left umbilical vein blood flow directly to the inferior vena cava. About 50% of the blood in the IVC passes through the liver and the rest bypasses the liver via the ductus venosus.
  • Functional closure occurs very shortly after birth, but this ductus ends up being anatomically patent for some number of days. If it fails to close, it turns into an intrahepatic portosystemic shunt. If it closes politely, it becomes the ligamenum venosum.
• Systemic vascular resistance is increased by clamping of the umbilical cord
  • The umbilical/placental circulation is a high-flow, low-resistance system. 
  • Before birth the left ventricular preload is mostly dependent on umbilical venous blood flow, i.e. blood returning from the placenta.
  • After the cord is clamped, LV preload depends mainly on venous return via pulmonary blood flow.
  • Clamping the cord ends up increasing systemic vascular resiastance and improving venous return to the heart by 30-50%.

Fishman, Alfred P., and Dickinson W. Richards. "Physiological changes in the circulation after birth." Circulation of the Blood. Springer New York, 1982. 743-816.

van Vonderen, Jeroen J., et al. "Measuring physiological changes during the transition to life after birth." Neonatology 105.3 (2014): 230-242.

Koos, Brian J., and Arezoo Rajaee. "Fetal breathing movements and changes at birth." Advances in Fetal and Neonatal Physiology. Springer New York, 2014. 89-101.

Hooper, Stuart B., et al. "Cardiovascular transition at birth: a physiological sequence." Pediatric research (2015).

D’cunha, Chrysal, and Koravangattu Sankaran. "Persistent fetal circulation." Paediatrics & child health 6.10 (2001): 744.