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Left to Right Shunts

The flow through the systemic and pulmonary circulations is normally balanced and equal. As the two circulations are placed in series with each other; blood first makes its way through the systemic circulation, then the pulmonary circulation, then back to the systemic circulation, and so on.

Left to right shunts are characterized by a "back-leak" of blood from the systemic to the pulmonary circulation. As a consequence, the pulmonary circulation carries not only the blood that legitimately entered the right atrium and ventricle through the superior and inferior vena cava, but also the additional blood entering through an ASD, VSD, or a PDA. Blood volume and pressure in the pulmonary circulation become abnormally high. If the shunt is significant, there is progressive damage to the pulmonary vasculature and gradual development of irreversible pulmonary hypertension. The pressure in the pulmonary circuit may ultimately exceed the systemic pressure with reversal of blood flow from the right side of the circulation to the left (Eisenmenger syndrome).

Lesions resulting in left to right shunts include:

  • Ventricular septal defect (VSD)
  • Patent ductus arteriosus (PDA)
  • Atrial septal defect (ASD)
  • Atrioventricular defect (AVSD)

In VSD and PDA, the direction and magnitude of the shunt depends on the size of the communication and the relative resistance in the pulmonary and systemic circuit.

In ASD and AVSD, the magnitude of the shunt depends largely on relative ventricular compliance (elasticity).