5.5.1 Haemodynamic effects
The haemodynamic effect of a VSD depends on its size and the amount of blood shunting across it. The larger the defect, the greater the amount of blood that can flow from the high-pressure left ventricle (LV) to the low-pressure RV. If a subproportion of the LV stroke volume flows into the RV, there will be volume overload of the RV, pulmonary circulation, LA and LV. If the volume overload is large, this may also result in an increased pressure in the pulmonary circulation and a rise in RV pressure. Flow across the defect depends on the pressure gradient between the LV and RV. An increase in RV pressure will decrease the flow across the VSD because it reduces the pressure gradient. Some right to left (reverse) shunting may occur at some time during the cardiac cycle in animals with large VSDs, but very seldom does RV pressure increase to the point that a right to left shunt predominates (Eisenmenger's syndrome).
Pulmonary hypertension and volume overload are likely to result in systemic signs such as jugular distension and peripheral oedema. Small defects limit the amount of blood that can flow from the LV to the RV and are termed restrictive VSDs. They are associated with high velocity jets in just the same way that a high velocity jet is created by almost complete occlusion of the end of a hose by one's thumb.