Changes to CVS that occurs at birth

Step 1 - Umbilical cord clamped

Loss of umbilical circulation

--> Increase in systemic vascular resistance (SVR)

Step 2 - First breath

First breath causes

• Increase in lung volume
  * Main reason
• Increase in PAO2
  * Removal of hypoxic pulmonary vasoconstriction

--> Both leads to decrease in pulmonary vascular resistance (PVR)
* Reduced to about 1/10th

Step 3 - Reversal in atrial pressure gradient

Increased SVR and venous return to LA

--> Increase in LA pressure

Decreased PVR and venous return to RA

--> Decrease in RA pressure

Therefore,

Pressure in LA>RA

--> Foramen ovale closes

Step 4 - Closure of other shunts

Closure of ductus arteriosus

• Mainly due to increase in PaO2
• Decreased circulating PGE2 may also contribute
• Progressive. Complete within 1-2 days

Closure of ductus venosus

• Mechanism UNKNOWN
• Cessation of blood flow
  --> Functional closure

Summary

1. Loss of umbilical circulation
2. Large increase in pulmonary circulation
3. Closure of forman ovale
4. Closure of ductus arteriosus
5. Closure of ductus venosus