While there is no controversy about ASD, The chambers that enlarge in VSD, is by and large poorly (or rather wrongly) understood. Whenever we diagnose VSD, our brain is tuned to think this way: Blood will be shunted form LV to RV. RV would handle more blood and it should enlarge. In reality it does not happen.
The VSD shunts the blood from LV to RV outflow* or even directly into pulmonary artery. Hence, VSD even if it is large, does not dilate the RV until the onset of pulmonary hypertension and RV dysfunction sets in. It is surprising to note, even the RVOT (The entry point of most VSD jets) does not significantly enlarge.
It is also important to recognise, the VSD shunt predominately occur in systole (90% ?) In systole, the RV is also contracting along with LV, so it’s size is diminutive and hence RV can not be volume overloaded in most of the VSDs however large it may be. Instead in ASD, there is diastolic overloading involving all regions (Inflow, Body, Outflow) of RV. This enlarges the RV in a classical fashion.
