Initially in the presence of a ventricular septal defect, blood is shunted from the left ventricle to the right ventricle, resulting in increased pulmonary blood flow.
Pulmonary hypertension, caused by a ventricular septal defect, causes hyperplasia of the tunica intima and hypertrophy of the tunica media leading to narrowing of the pulmonary artery.
Longstanding ventricular septal defects, may produce an Eisenmenger complex, the **reversal of the shunt from the normal left to right blood flow to right to left due to increased pulmonary resistance compared to systemic peripheral vascular resistance, **leading to cyanosis from inadequate oxygenation.