A child with large perimembranous VSD has congestive heart failure. What may be the cause of improvement of cardiac failure in the patient –

A patient with large VSD develops CHF because of the large left to right shunt. Large amount of blood at high systolic pressure is entering into the pulmonary circulation and with this continued exposure of the pulmonary vascular bed to high systolic pressure and high flow, pulmonary vascular obstructive disease develops The pulmonary vascular resistance gradually rises and becomes equal to the systemic resistance. When this happens the shunt becomes bi-directional and the signs of heart failure diminish but the patient becomes cyanotic (Eisenmenenger physiology).