Pulmonary hypertension (PH) is a consequence of an increase in pulmonary vascular resistance (PVR), pulmonary blood flow, pulmonary venous pressure, or a combination of these elements. Pulmonary arterial hypertension is a frequent complication of congenital heart disease, particularly in patients with systemic-to-pulmonary shunts. Persistent exposure o f the pulmonary vasculature to increased blood flow and pressure may result in vascular remodeling and dysfunction. This leads to increased pulmonary vascular resistance and, ultimately, to reversal of the shunt and development of Eisenmenger's syndrome. It may be more appropriate to define pulmonary hypertension according to the ratio of MPAP to mean systemic arterial pressure (MPAP/MAP) because children may have a low mean systemic blood pressure. MPAP/MAP ratio of < 0.25 is normal, a ratio of 0.33-0.5 indicates moderate pulmonary hypertension, and a ratio of > 0.5 is indicative of severe pulmonary hypertension