There was a big debate in one of my classes with cardiology fellows regarding the shunt quantification of ASD . We were talking about the significance of ASD shunting . We suddenly realised 2:1 left to right shunt is not a simple equation to comprehend . I was thinking 2:1 shunt would mean pulmonary flow would be twice the systemic flow . It was not to be !
Is the ratio of shunting and Qp/Qs convey the same thing ?
No . Qp /Qs is the ratio of pulmonary to systemic blood flow flow . When we want to quantify shunt we express it in two different ways .
1. The amount of blood shunted form left side to right side of the heart .
2. The amount of pulmonary blood flow to systemic blood flow in absolute terms .
Though both are closely linked entities they do not denote the same meaning . When we say 2: 1 shunt we refer to the shunted blood across the defect but when we calculate pulmonary blood flow we take into account venous blood which does not take part in the shunting .
The confusion arises because we use both terms interchangeably.The following illustration will try to prove A 2: 1 shunt would actually correspond to a qp/qs of three (Pulmonary flow is 3 times the systemic flow !)
Let us begin with a hypothetical ASD patient who has systemic cardiac output of 4 liters.
He shunts 2 : 1 from left to right . ie he shunts 2 /3 of three parts into RA (66% ) .
A patient who delivers 4 liters from LA in the presence of 2;1 ASD shunt would mean he would receive 12 liters from the lung as pulmonary blood flow.
I am still not fully convinced about the above reasoning . I guess it is correct. I argue the fellows to give further insight into this equation. The complexities in bi- directional shunt and effective pulmonary blood flow in Eisenmenger syndrome is going beyond my heads !