Eisenmenger syndrome develops in patients with left-to-right shunts that result in right heart volume overload.

The shunt is most commonly due to atrial septal or ventricular septal defects, although any intracardiac defect that results in left-to-right shunting of blood can result in volume and pressure overload. Increased pulmonary vascular resistance is a consequence of this volume overload, which results in right ventricular enlargement, pulmonary hypertension, and reversal of the left-to-right shunt into a bidirectional or right-to-left fixed shunt. Patients are clinically cyanotic, frequently have dyspnoea on exertion, decreased exercise tolerance, signs and symptoms of congestive heart failure including signs of right ventricular overload and failure, peripheral oedema, syncope, and may have alterations in end-organ function (hepatic congestion, abnormal bleeding, cerebral vascular accidents, renal failure, etc.). Patients may be normal in appearance or may be syndromatic. Pregnancy carries a particularly high peripartum risk for premature labour as well as mortality regardless of delivery method and is discouraged. Patients may present in the 3rd and 4th decades of life, although smaller shunts may not be problematic until later in life. Patients are often managed with pulmonary vasodilators to reduce the pulmonary vascular resistance, improve quality of life and exercise tolerance, as well as treat heart failure, if present.