Immediately at birth, which haemodynamic change results PRIMARILY from the baby's first breath?

• A Closure of the ductus arteriosus due to a rise in PaO2
• B Closure of the foramen ovale due to a rise in left atrial pressure from increased pulmonary flow
• C Fall in pulmonary vascular resistance (PVR) due to lung expansion and rising PaO2, reversing right-to-left shunts ✓
• D Constriction of the ductus venosus due to cutting the umbilical cord

Explanation

The first breath initiates the transition from fetal to neonatal circulation. Lung expansion mechanically reduces PVR; rising alveolar PO2 releases hypoxic pulmonary vasoconstriction, further reducing PVR. This dramatically increases pulmonary blood flow, raises left atrial pressure above right atrial pressure (closing the foramen ovale functionally), and reverses the right-to-left ductal and foramen shunts. The ductus arteriosus constricts over hours to days in response to rising PaO2; the ductus venosus closes with cord clamping but this is a portal venous change. The FIRST and IMMEDIATE trigger is the reduction in PVR from the initial breath.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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