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  #1  
Old 03-02-2012
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Default AV fistula UWORLD question

ID 1625.

I have no idea what Acute vs. Chronic AV fistula even means??

The UWORLD explanation says for a CAV-fist "over time the sympathetic nervous system and kidneys will begin to compensate for a chronic fistula by increasing contractility, vascular tone, and circulating blood volume."

This explanation does not make sense to me..
What is the mechanism for this??
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  #2  
Old 03-02-2012
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Originally Posted by No_BS View Post
ID 1625.

I have no idea what Acute vs. Chronic AV fistula even means??

The UWORLD explanation says for a CAV-fist "over time the sympathetic nervous system and kidneys will begin to compensate for a chronic fistula by increasing contractility, vascular tone, and circulating blood volume."

This explanation does not make sense to me..
What is the mechanism for this??
Maybe this can help you... It helped me

Cardiac Output - Venous Return Curves

Cardiac Output and TPR relationship?
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"Disease is very old, and nothing about it has changed. It is we who change as we learn to recognize what was formerly imperceptible." JMC
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Old 03-03-2012
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Yea that didn't help.
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Old 03-04-2012
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....bump
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Old 06-07-2012
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Perhaps this might help you understand?
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Old 06-07-2012
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....bump

Got it??? Are you still confused???
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Old 09-26-2014
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most probably after a acute fistula ,body got more venous return and more cardiac output..so more blood is accumulating in the body in normal venous pull.that explains the increase in blood volume.to pump out more blood sympathetic system increase contractility and vessle tone.so more blood is pump out of the vein.
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Old 09-26-2014
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Quote:
Originally Posted by No_BS View Post
ID 1625.

I have no idea what Acute vs. Chronic AV fistula even means??

The UWORLD explanation says for a CAV-fist "over time the sympathetic nervous system and kidneys will begin to compensate for a chronic fistula by increasing contractility, vascular tone, and circulating blood volume."

This explanation does not make sense to me..
What is the mechanism for this??
Under normal conditions, Blood will flow from arteries -->arterioles--->capillaries ---> venules---> veins.

Arterioles are resistance vessels. They provide resistance.

In AV fistula, there is a fistula (direct connection between arteries and veins), that means blood flows directly from arteries into veins bypassing resistance.So blood will flow easily.

Blood is under high pressure in arteries as compared to that in veins. So blood will flow from artery to vein.

Now we have more blood in veins (veins store blood) --> Increase in venous return but cardiac contractility isn't increased yet (Its acute). So we are not pumping out all the blood out of the heart. After a while, this problem will become chronic.

There will be chronic decrease in perfusion to organs, specially kidneys. So, Kidneys will activate Renin angio. aldo. system. This will increase Blood volume. Angio II will increase vascular tone.

Also, decreased blood flow will decrease firing of baroreceptors which will increase sympathetic tone. This will increase contractility.

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Old 09-27-2014
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Originally Posted by DrNewB View Post
Under normal conditions, Blood will flow from arteries -->arterioles--->capillaries ---> venules---> veins.

Arterioles are resistance vessels. They provide resistance.

In AV fistula, there is a fistula (direct connection between arteries and veins), that means blood flows directly from arteries into veins bypassing resistance.So blood will flow easily.

Blood is under high pressure in arteries as compared to that in veins. So blood will flow from artery to vein.

Now we have more blood in veins (veins store blood) --> Increase in venous return but cardiac contractility isn't increased yet (Its acute). So we are not pumping out all the blood out of the heart. After a while, this problem will become chronic.

There will be chronic decrease in perfusion to organs, specially kidneys. So, Kidneys will activate Renin angio. aldo. system. This will increase Blood volume. Angio II will increase vascular tone.

Also, decreased blood flow will decrease firing of baroreceptors which will increase sympathetic tone. This will increase contractility.

nice explanation.Thanks.
so first there is a systolic failure of the heart.so in response to that kidney and sympathetic sysmtem compensate.it is nothing but a systolic heart failure situation??
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Old 09-27-2014
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Quote:
Originally Posted by cuanto117 View Post
nice explanation.Thanks.
so first there is a systolic failure of the heart.so in response to that kidney and sympathetic sysmtem compensate.it is nothing but a systolic heart failure situation??
Yes. Basically, acute AV fistula is a high-output type of heart failure. A big proportion of the cardiac output instead of going to the tissues is getting shunted directly into venous system and being added to the venous return. This leads to volume overload and cardiac failure. Also, since the blood is not passing through any capillaries, there is no delivery of oxygen and nutrients to tissues. When this tissue hypoxia is perceived by the kidneys, they respond by their natural response to ischemia/hypoxia - Activating the RAA system.
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  #11  
Old 09-28-2014
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Quote:
Originally Posted by cuanto117 View Post
nice explanation.Thanks.
so first there is a systolic failure of the heart.so in response to that kidney and sympathetic sysmtem compensate.it is nothing but a systolic heart failure situation??
Yup and as shaarang explained.
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