A newborn with a large ductus arteriosus develops persistent pulmonary hypertension of the neonate. Oxygen saturation measured in the right hand is 96% but in either foot is 82%. This differential cyanosis occurs because:
- A The patent ductus carries oxygenated aortic blood into the pulmonary artery, desaturating the lower body
- B Deoxygenated ductal blood flows from the pulmonary artery into the aorta distal to the left subclavian artery ✓
- C Right-to-left shunting occurs through a foramen ovale preferentially perfusing the upper limbs
- D Pulmonary hypertension reverses flow in the brachiocephalic arteries
Correct answer: B. Deoxygenated ductal blood flows from the pulmonary artery into the aorta distal to the left subclavian artery
Explanation
With suprasystemic pulmonary pressures, blood shunts right to left across the ductus into the descending aorta just distal to the left subclavian origin, desaturating the lower body while the preductal vessels supplying head and right arm keep high saturations. Left hand saturation may be intermediate since the left subclavian arises proximally. Preductal versus postductal sampling is the standard bedside test for this diagnosis.
Reference: Nelson Textbook of Pediatrics, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.