why fixed splitting in ASD?

[MudPhud20XX]

FA says in case of ASD, regardless of breath, pulmonic closure is greatly delayed. But shouldn't you even get more delayed closure for P2 during inspiration due to increase in venous return? I don't understand how you would get fixed splitting in ASD.

Many thanks in advance.

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[masaraksh]

There is L->R shunting through the ASD, that makes the right heart consistently volume overloaded which delays P2.

 

[MudPhud20XX]

There is L->R shunting through the ASD, that makes the right heart consistently volume overloaded which delays P2.

I get that, but with inspiration, you are also still getting more venous return, thus shouldn't you still end up with wider splitting of S2 during inspiration in ASD?

 

[worldbeater]

This is how I remember it, I don't know if it's the actual physiological reason though, I just match ASD as an answer whenever I see fixed splitting S2.

On inspiration, pressure is more on the right side of the heart. On expiration, pressure is more on the left side of the heart. So the blood pushes through normally on inspiration on right side through the pulmonary valve, and then on expiration a portion of the blood shunts from the right atrium to the left atrium (the pressure gradient between the two isn't as strong as between the ventricles), and blood goes through the aortic valve, bypassing the lungs. So an amount of blood that has an equal difference is being contracted by both the aortic and pulmonary valves, causing an S2 that has a fixed splitting.