Shunt assessing:

For calculations, the NET forward flow should be taken (Net FF = FF - BF), where FF is forward flow and BF is backward flow. 

• ASD/PAPVC
  

  Fig 3: Shunt calculation in ASD

ASD is pre-tricuspid shunt and results in L–R shunt because of the higher compliance of the RV compared with the LV and causes RV volume overload and pulmonary overcirculation.

RV SV > LV SV

RV SV = MPA flow

MPA flow > AAo flow

RV SV = LV SV + ASD flow

MPA NFF = AAo NFF + ASD flow

ASD flow = MPA NFF - AAo NFF

ASD flow = RV SV - AAo NFF

Mitral regurgitation has no effect on calculations in case of ASD.

Qp:Qs = MPA NFF / AAo NFF

• VSD
  

  Fig 4: Shunt calculation in VSD

VSD is post-tricuspid shunt and occurs mostly during systole with blood flow directly into the RVOT, causing LV volume overload with no volume overload in the RV  (Figure 3).

LV SV > RV SV

LV SV = RV SV + VSD flow

LV SV = MPA flow

MPA flow > AAo flow

MPA NFF = AAo NFF + VSD flow

VSD flow = MPA NFF - Ao NFF

or

VSD flow = LV SV - Ao NFF

In case of mitral regurgitation:

LV SV = AAo NFF + VSD flow+  MR

VSD flow = LV SV - AAo NFF - MR

Qp:Qs = MPA NFF / AAo NFF

• PDA
  

  Fig 5: Shunt calculation in PDA

PDA occurs mostly during systole and originally results in L–R shunt and LV volume overload  (Figure 4).

In PDA shunt occurs beyond the level of the Ao and proximal MPA where the QAo and QMPA are usually measured, that is why Ao flow can not be used for Qs and MPA flow can not be used for Qp.

In case of a PDA flow in the AAo is greater than Qs so that one way to calculate the shunt volume is to subtract the pulmonary flow from the systemic one.

LV SV = Ao flow

Ao flow > PA flow

Ao flow > Qs

PDA flow = AAo NFF - MPA NFF

Qp:Qs = (MPA NFF + PDA flow) / (SVC flow + IVC flow)

or

Qp:Qs = AAo NFF / (SVC flow + IVC flow)