When the right atrium contract, why doesnt blood go back into the IVC and SVC?

[Ven0m]

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Also why doesnt blood go from LA to the pulmonary veins? They never told us about this in med school

Is this concept clinically significant?

 

[Taco Time]

Yo Ven0m,

Generally, when blood flows from the RA --> RV, there isn't an appreciable amount coming back into the IVC & SVC. In a diseased state where there is R side heart failure, a physical exam finding is jugular venous distention (JVD)--the patient lies down on an exam table, which is lowered at 45 degrees, then looks to the left and the jugular veins on the neck are actually pulsating. The jugular veins have a pulsating motion because with each contraction of the RA, blood is flowing back into the SVC and IVC instead of flowing into the RV.

I think it's easier to start out with this exam finding and realize it's abnormal to begin with. Blood should be flowing ahead and not regressing in circulation.

So why does blood flow back into the IVC and SVC when the RA contracts? There is higher pressure in the RV than there is in the venous return to the heart. This is badness because the RV should be relaxed and ready to receive oxygen poor blood from the RA and send it to the lungs. If the RV has undergone cardiomyopathy (hypertrophy to handle increased pressure --> eventual dilation after being worn out), it cannot relax enough to create a low enough pressure state to allow for blood to flow in. Also, a PE can cause R heart failure and produce this back flow of blood. Technically a pericardial effusion can compress the R side of the heart and also lead to JVD as well. Fluids will find the path of least resistance.

I hope this explanation gets the concept across.

 

[Ven0m]

Yo Ven0m,

Generally, when blood flows from the RA --> RV, there isn't an appreciable amount coming back into the IVC & SVC. In a diseased state where there is R side heart failure, a physical exam finding is jugular venous distention (JVD)--the patient lies down on an exam table, which is lowered at 45 degrees, then looks to the left and the jugular veins on the neck are actually pulsating. The jugular veins have a pulsating motion because with each contraction of the RA, blood is flowing back into the SVC and IVC instead of flowing into the RV.

I think it's easier to start out with this exam finding and realize it's abnormal to begin with. Blood should be flowing ahead and not regressing in circulation.

So why does blood flow back into the IVC and SVC when the RA contracts? There is higher pressure in the RV than there is in the venous return to the heart. This is badness because the RV should be relaxed and ready to receive oxygen poor blood from the RA and send it to the lungs. If the RV has undergone cardiomyopathy (hypertrophy to handle increased pressure --> eventual dilation after being worn out), it cannot relax enough to create a low enough pressure state to allow for blood to flow in. Also, a PE can cause R heart failure and produce this back flow of blood. Technically a pericardial effusion can compress the R side of the heart and also lead to JVD as well. Fluids will find the path of least resistance.

I hope this explanation gets the concept across.

Yeah, but don't the right atrium and left atrium contract to some degree (atrial kick)? Are you saying that normally the pressure in the pulmonary veins (to LA) and SVC/IVC (to RA) is less than the right atrium even during atrial contraction?

Logically, wouldn't some blood go back into the IVC/SVC + pulmonary veins. Because I know when the ventricles contract, they pull the AV valves closed. So what is causing the increased pressure in the pulmonary veins / IVC / SVC? Venous valves?

 

[lymphocyte]

Yeah, but don't the right atrium and left atrium contract to some degree (atrial kick)? Are you saying that normally the pressure in the pulmonary veins (to LA) and SVC/IVC (to RA) is less than the right atrium even during atrial contraction?

Logically, wouldn't some blood go back into the IVC/SVC + pulmonary veins. Because I know when the ventricles contract, they pull the AV valves closed. So what is causing the increased pressure in the pulmonary veins / IVC / SVC? Venous valves?

I don't understand what you're asking. Blood has two choices. An empty cavity or back into more blood. Blood does flow backward, as demonstrated by the CVP waveform or Swan-Ganz tracing, but the majority flows into the empty cavity. Remember, passive filling accounts for about 70% of ventricular filling at resting HR.

The amount of blood that flows backwards is clinically significant in different disease states, like RH failure, constrictive pericarditis, various kinds of shock, etc. While CVP monitoring and Swan-Ganz tracing aren't that commonly used anymore, the concept is still extremely important to know about.

 

[Ven0m]

I don't understand what you're asking. Blood has two choices. An empty cavity or back into more blood. Blood does flow backward, as demonstrated by the CVP waveform or Swan-Ganz tracing, but the majority flows into the empty cavity. Remember, passive filling accounts for about 70% of ventricular filling at resting HR.

The amount of blood that flows backwards is clinically significant in different disease states, like RH failure, constrictive pericarditis, various kinds of shock, etc. While CVP monitoring and Swan-Ganz tracing aren't that commonly used anymore, the concept is still extremely important to know about.

Like if theres no valves in the superior vena cava, and the right atrium contracts, some blood will flow into the RV, while some will kinda push the blood in the SVC back, right? Isnt that why we get jugular vein distension when there is RHF?

 

[lymphocyte]

Like if theres no valves in the superior vena cava, and the right atrium contracts, some blood will flow into the RV, while some will kinda push the blood in the SVC back, right? Isnt that why we get jugular vein distension when there is RHF?

That's right, though your concepts might be a little blurry (and forgive me if they aren't, this might help others).

When you talk about right atrial contraction, you're describing the "a" wave of a CVP waveform. And predictably, when you have no atrial activity, like in atrial fibrillation, the "a" wave is absent, and ventricular filling is entirely passive. When you talk about RVF, you're describing the "v" wave of a CVP waveform, since the "v" wave represents ventricular contraction. Both events generate pressure in the SVC but for different reasons.

http://www.respiratoryupdate.com/members/CVP_Waveform_Variations.cfm

 

[Ven0m]

ah crap. in our school we merely were taught that RHF means failure of the right side of the heart to pump blood sufficiently. be it extremely hypertrophied, or a conduction block, or dilation with thinning of the walls (resulting in less contractile force) etc. well that's why i'm sticking to usmle books now. Thanks lymphocyte😉