What is the pathophysiology described by Eisenmenger syndrome?

• A. Worsening left-to-right shunt
• B. Reversal of left-to-right shunt to right-to-left shunt
• C. Development of coarctation of the aorta
• D. Pulmonary embolism causing dyspnea

Answer: (B)

Eisenmenger syndrome arises when a long-standing left-to-right shunt from a congenital defect (like VSD, ASD, or PDA) causes progressive remodeling and narrowing of the pulmonary vessels, leading to increased pulmonary vascular resistance. Once the pulmonary resistance exceeds systemic vascular resistance, the blood flow reverses direction across the defect, becoming a right-to-left shunt. This reversal causes systemic desaturation and cyanosis, which defines the syndrome. The other scenarios describe either the opposite direction of shunting, a nonrelated vascular problem, or an embolic cause of dyspnea, none of which capture the shunt reversal central to Eisenmenger.