Hypoplastic left heart syndrome is an underdevelopment of the left ventricle and the left ventricular outflow tract, including valves. This mixing lesion is duct-dependent for systemic blood flow and can rapidly deteriorate to low cardiac output syndrome and congestive heart failure with metabolic acidosis. Decreasing pulmonary vascular resistance PVR results in pulmonary overcirculation and decreased systemic blood flow. Anaesthetic management of the R → L shunt at ductal level means avoiding a decrease in PVR and avoiding an increase in systemic blood pressure.