A day-old term neonate has SpO2 of 96% in the right upper limb but 78% in both lower limbs, with mild tachypnoea. The baby was born by normal delivery with good Apgar scores. This pattern of oxygen saturation is BEST explained by:
- A Left-to-right shunt through a ventricular septal defect
- B Right-to-left ductal shunting into the descending aorta with elevated pulmonary vascular resistance ✓
- C Coarctation of the aorta proximal to the left subclavian artery
- D Total anomalous pulmonary venous connection with obstruction
Correct answer: B. Right-to-left ductal shunting into the descending aorta with elevated pulmonary vascular resistance
Explanation
Pink upper body with cyanosed lower body (differential cyanosis) occurs when deoxygenated right ventricular blood shunts right-to-left across the ductus into the descending aorta while oxygenated left ventricular output supplies the preductal vessels. This is seen with persistent pulmonary hypertension of the newborn or ductus-dependent lesions such as interrupted aortic arch. An ASD or VSD shunt cannot create limb-to-limb saturation differences, and TAPVC produces uniform cyanosis.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.