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
- Loss of umbilical circulation
- Large increase in pulmonary circulation
- Closure of forman ovale
- Closure of ductus arteriosus
- Closure of ductus venosus