A day-old neonate with transposition of the great arteries on prostaglandin infusion develops worsening respiratory distress. Preductal SpO2 is 88% and postductal SpO2 is 72%. This pattern of saturation difference is best explained by:

• A Right-to-left ductal shunting due to pulmonary hypertension ✓
• B Left-to-right ductal shunting with streaming into the descending aorta
• C Coexisting coarctation of the aorta
• D Obstructed total anomalous pulmonary venous connection

Explanation

In d-TGA the aorta receives systemic venous blood. Normally a ductal left-to-right shunt sends oxygenated blood from the aorta into the pulmonary artery, giving higher postductal than preductal saturation (differential cyanosis). When pulmonary hypertension reverses the ductal shunt, deoxygenated right ventricular blood enters the descending aorta, dropping postductal saturation below preductal values, called reverse differential cyanosis. Coarctation does not produce this pattern in TGA.

Reference: Park's Pediatric Cardiology for Practitioners, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.