Haemodynamics

During systole, blood flows through the defect from the left to the right ventricle, through the pulmonary valve into the low resistance pulmonary vascular bed. The blood returns to the left atrium and then to the left ventricle. Some blood then recirculates to the lungs via the ventricular shunt. Recirculation of the blood results in volume overload of both ventricles and the left atrium.

The magnitude of shunt depends on:

• the size of the defect - the larger the defect, the less the resistance to blood flow.
• pulmonary vascular resistance which is dependent on the length of time that there has been a high pulmonary arterial pressure and high blood flow. In infancy the pulmonary vascular resistance is low relative to the systemic resistance and the pulmonary blood flow high. With time there is progressive medial hypertrophy and intimal thickening of the pulmonary arteries which leads to an increased pulmonary vascular resistance. Eisenmenger's syndrome results when the resistance of the pulmonary and systemic circulations become equal.