What leads to a right-to-left shunt in Eisenmenger Syndrome?

In Eisenmenger Syndrome, the development of a right-to-left shunt occurs primarily as a result of severe pulmonary hypertension. Initially, patients with congenital heart defects such as a ventricular septal defect (VSD) or an atrial septal defect (ASD) experience a left-to-right shunt due to the higher pressure in the left side of the heart compared to the right. Over time, as the volume overload leads to pulmonary vascular remodeling and elevated pressures in the pulmonary arteries, pulmonary hypertension can become severe enough to reverse the shunt direction.

When the pressure in the right side of the heart exceeds that in the left side, the shunt direction changes from left-to-right to right-to-left. This reversal results in deoxygenated blood being shunted into the systemic circulation, leading to cyanosis and associated complications. The critical factor here is the severity of the pulmonary hypertension, which ultimately drives the shunt reversal and contributes to the clinical manifestations of Eisenmenger Syndrome.

Other factors, while they may influence the overall cardiovascular status, do not directly lead to the right-to-left shunt in the context of Eisenmenger Syndrome.