Left to Right Shunts
The flow through the systemic and pulmonary circulations is normally balanced and equal in volume (Qp/Qs =1). The two circulations are placed in series with each other. The same volume of 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. This causes the pulmonary flow to be larger than the systemic flow (Qp/Qs >1). As a consequence, the pulmonary circulation carries not only the blood that entered the right atrium and right ventricle through the superior and inferior vena cava, but also the additional blood entering through a VSD, ASD, AVSD or PDA. Blood volume and/or 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 causing reversal of blood flow from the right side of the circulation to the left (Eisenmenger syndrome). This may take as short as 1-2 years in a large VSD, AVSD or PDA or as long as a few decades as in ASD.
Lesions resulting in left to right shunts include:
- Ventricular septal defect (VSD)
- Patent ductus arteriosus (PDA)
- Atrial septal defect (ASD)
- Atrioventricular defect (AVSD)
In small (restrictive) VSD and PDA, the direction and magnitude of the shunt depends on the pressure difference across the shunt. In large VSD and PDA, the direction and magnitude of the shunt depends on the relative resistance in the pulmonary and systemic circuits.
In ASD, the magnitude of the shunt depends largely on relative ventricular compliance (elasticity).
AVSD is a mix between ASD and VSD depending on the predominant lesion.
The defect size is determined by comparing its diameter with the aortic annulus. A small defect is less than 1/3 of the aortic annulus. A moderate sized defect is 1/3 to 2/3 and a large defect is >2/3 the size of the aortic valve annulus