Eisenmenger syndrome is the irreversible end-stage of a long-standing, unrepaired left-to-right cardiac shunt (most commonly ventricular septal defect, atrial septal defect, or patent ductus arteriosus). Initially, oxygenated blood flows from the high-pressure left side to the low-pressure right side, increasing pulmonary blood flow. Over years, this chronic excessive pulmonary flow causes progressive remodeling of the pulmonary vasculature: medial hypertrophy, intimal fibrosis, and eventually plexiform lesion formation, producing fixed irreversible pulmonary arterial hypertension. When pulmonary vascular resistance exceeds systemic vascular resistance, the shunt reverses to right-to-left, mixing deoxygenated blood into the systemic circulation and producing cyanosis. Chronic hypoxemia triggers compensatory erythropoietin-driven secondary erythrocytosis (polycythemia) to increase oxygen-carrying capacity, but elevated hematocrit increases blood viscosity and thrombotic risk. The right-to-left shunt also allows venous thrombi to cross directly into the systemic arterial circulation (paradoxical embolism). The critical nursing concept is that shunt reversal is IRREVERSIBLE -- surgical closure of the defect at this stage is contraindicated because the shunt serves as a pressure relief valve for the pulmonary hypertension; closing it would cause acute right ventricular failure and death.