What pathophysiological condition leads to pulmonary hypertension in Eisenmenger syndrome?

Eisenmenger syndrome arises as a direct consequence of an uncorrected left-to-right shunt, commonly associated with congenital heart defects like ventricular septal defects or atrial septal defects. In these conditions, blood flows from the left side of the heart to the right side, leading to increased volume in the pulmonary circulation.

Over time, the excessive blood flow to the pulmonary arteries causes vascular remodeling and increased pulmonary arterial pressure, leading to pulmonary hypertension. This elevated pressure eventually causes the shunt to reverse, resulting in a right-to-left shunt. However, the underlying pathophysiological condition that initiates this cascade is the uncorrected left-to-right shunt, which creates the initial hemodynamic burden on the pulmonary arteries, ultimately leading to the development of pulmonary hypertension characteristic of Eisenmenger syndrome.