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2 : 1 shunt in Atrial septal defect means what ? Is it same as qp/qs ?

Posted Jun 20 2012 12:24pm

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 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.

Final message

I am still not fully convinced about the above reasoning . It 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 Eisenmneger syndrome is going beyond my heads !


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