A preterm neonate born at 28 weeks gestation is intubated and given surfactant. Despite improvement in oxygenation, PaO2 remains 45 mmHg on FiO2 0.6. Echocardiography reveals a large patent ductus arteriosus with left-to-right shunt. Which mechanism explains why surfactant therapy can paradoxically unmask or worsen a PDA in preterm infants?
- A Surfactant increases systemic vascular resistance, reversing shunt direction
- B Surfactant causes direct ductal vasodilation via prostaglandin release
- C Surfactant reduces pulmonary vascular resistance, increasing left-to-right shunt across the PDA ✓
- D Surfactant increases pulmonary blood flow by expanding alveoli, raising PVR
Correct answer: C. Surfactant reduces pulmonary vascular resistance, increasing left-to-right shunt across the PDA
Explanation
Surfactant therapy improves lung compliance and reduces pulmonary vascular resistance (PVR). In a preterm infant with a PDA, lowering PVR increases the pressure gradient driving left-to-right flow across the ductus, thereby unmasking or worsening the shunt. This phenomenon is well recognised — infants who show sudden deterioration after initial improvement post-surfactant should be evaluated for haemodynamically significant PDA. Options B, C, and D are physiologically incorrect.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.