The Heart Structure is the Ductus Arteriosus
The answer is b. Ductus arteriosus. This premature infant presents with the classic clinical picture of a patent ductus arteriosus (PDA): a continuous murmur with maximum intensity at the second left intercostal space, respiratory distress requiring CPAP support, and prematurity at 32 weeks gestation 1.
Clinical Reasoning
Why the Ductus Arteriosus
- Premature infants have a 60% incidence of persistent PDA, with occurrence inversely related to gestational age and weight 2, 3
- The continuous murmur is pathognomonic for PDA, reflecting persistent left-to-right shunting throughout both systole and diastole 1
- The location at the second left intercostal space corresponds to the anatomic position where the ductus arteriosus connects the pulmonary artery to the descending aorta 1
- Respiratory distress in premature infants is frequently caused by a combination of lung immaturity, hyaline membrane disease, and persistence of the ductus arteriosus 1
Why Not the Other Options
Atrial septum (a) and Foramen ovale (c):
- These structures produce no murmur when patent, as the pressure gradient between atria is minimal 4
- Atrial septal defects cause a midsystolic murmur in the pulmonic area with fixed splitting of S2, not a continuous murmur 1
Ventricular septum (e):
- VSDs produce a holosystolic murmur, not a continuous murmur 1
- The murmur is typically heard best at the lower left sternal border, not the second intercostal space 1
Ductus venosus (d):
- This fetal vessel connects the umbilical vein to the inferior vena cava and closes functionally at birth 4
- It produces no audible murmur and is not associated with respiratory distress 4
Pathophysiology in This Case
- In utero, the ductus arteriosus is essential for fetal circulation, allowing right ventricular output to bypass the non-functioning lungs 5, 4
- Normal closure mechanisms fail in prematurity due to altered oxygen tension and decreased prostaglandin metabolism 2, 3
- The persistent left-to-right shunt increases pulmonary blood flow, causes left ventricular volume overload, and contributes to respiratory distress by pulmonary edema 1
- Echocardiography determines the patency, direction and degree of shunting at the ductal level and assesses ventricular compensation 1
Clinical Implications
- ACC/AHA guidelines classify this as a Class I indication for echocardiography: cyanosis, respiratory distress, or cardiac murmur in a neonate requires immediate echocardiographic evaluation 1
- The diagnosis must be confirmed before ductal closure is attempted (either pharmacologically with indomethacin/ibuprofen or surgically) to rule out ductal-dependent lesions 1
- A significant left-to-right shunt through the PDA increases morbidity and mortality in premature infants, making timely diagnosis and treatment essential 2, 3