Atrial Pressures

[seminoma]

What establishes the right and left atrial pressures? I understand that RAP has to be really low to allow flow from the IVC, but I can't wrap my head around how the pressures develop or are maintained when comparing RA vs LA. What contributes to LA pressure being higher than RA pressure?

I'm trying to understand this in the context of atrial shunting (L->R) as a life-saving mechanism in transposition.

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[Jabbed]

Not sure that I understand your question, but the passive filling pressures of the left and right atrium are determined by the right and left ventricle respectively. The increased diastolic pressures of the atria are determined by atrial contraction.

 

[seminoma]

Not sure that I understand your question, but the passive filling pressures of the left and right atrium are determined by the right and left ventricle respectively. The increased diastolic pressures of the atria are determined by atrial contraction.

So why is there a left to right shunt if you created an ASD in a newborn with transposition (as temporary treatment before surgical correction)? Right at birth the RV is larger than the LV because it was doing more of the work in utero. So if LV and RV were the determining factors for atrial filling pressures, then you would expect a right to left shunt, which would make maintaining or inducing an ASD useless as a temporary treatment. Likewise, since the RV is more hypertrophied than the LV at birth, the RA is going to reach higher pressures and blood will go R-->L through the induced-ASD.

 

[Jabbed]

So why is there a left to right shunt if you created an ASD in a newborn with transposition (as temporary treatment before surgical correction)? Right at birth the RV is larger than the LV because it was doing more of the work in utero. So if LV and RV were the determining factors for atrial filling pressures, then you would expect a right to left shunt, which would make maintaining or inducing an ASD useless as a temporary treatment. Likewise, since the RV is more hypertrophied than the LV at birth, the RA is going to reach higher pressures and blood will go R-->L through the induced-ASD.

Because the infant takes his first breath at birth. In utero the lung vasculature is high resistance, so right ventricular pressures are high but left atrial pressures are low (RV pressure - LA pressure = Q x R). Expansion of the lungs at birth massively decreases the pulmonary vasculature resistance, which seemingly paradoxically decreases right ventricular pressures and increases left atrial pressure (RV pressure - LA pressure = Q x R). This is a subtle point to understand, but the decreased resistance causes a decreased pressure gradient associated with decreased RV pressure and increased LA pressure.

Additionally, the decrease in RV pressure will decrease the RA pressure through a decrease in afterload.

Overall you have a drop in RA pressure and an increase in LA pressure, which causes the left to right shunt.

Hope that helped.

 

[seminoma]

Because the infant takes his first breath at birth. In utero the lung vasculature is high resistance, so right ventricular pressures are high but left atrial pressures are low (RV pressure - LA pressure = Q x R). Expansion of the lungs at birth massively decreases the pulmonary vasculature resistance, which seemingly paradoxically decreases right ventricular pressures and increases left atrial pressure (RV pressure - LA pressure = Q x R). This is a subtle point to understand, but the decreased resistance causes a decreased pressure gradient associated with decreased RV pressure and increased LA pressure.

Additionally, the decrease in RV pressure will decrease the RA pressure through a decrease in afterload.

Overall you have a drop in RA pressure and an increase in LA pressure, which causes the left to right shunt.

Hope that helped.

Thanks for typing that out. I understand all that as it pertains to normal anatomy, but does it hold even when the baby has transposition of the great vessels? Now that I think about how though I think it doesn't matter which ventricle is pumping into the pulmonary trunk... The LA is still getting the venous return from the lungs, which is what increased the pressure in the first place.

So even though the RV is the systemic ventricle, pressure in LA is still higher than RA for the reasons you explained.

How is a VSD beneficial in transposition? It would be right to left shunting (right?) and help oxygenate blood.. which I guess would then hopefully increase the LA pressure and help more O2blood to come through the ASD?

 

[Transposony]

How is a VSD beneficial in transposition?

In transposition, there has to be a connection like VSD/ASD/PDA (any permutation or combination) between the two circuits to be compatible with life.
Otherwise, they just two different circuits pumping deoxygenated and oxygenated blood back to body and lungs respectively.

Two separate circuits are formed — one that circulates oxygen-poor blood from the body back to the body, and another that recirculates oxygen-rich blood from the lungs back to the lungs.