Haemodynamics

In a left to right shunt oxygenated blood flows across a defect into the lower pressure right heart. It is then directed to the pulmonary bed and is returned to the left atrium. Because of the increased volume load all chambers and vessels that are overloaded by the shunt dilate and hypertrophy. The pattern of vessels and chambers affected by a shunt is specific to particular types of shunt eg an atrial septal defect overloads the right atrium and right ventricle, a persistent ductus arteriosus overloads the left atrium and left ventricle, and a ventriculoseptal defect overloads both ventricles.

The magnitude of the left to right shunt is determined by a variety of factors:

• the size of the defect
• the pressure gradient across the defect
• the relative resistances in the systemic and pulmonary vascular beds