Venodilators, TPR, EDV, and Venous Return

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SKaminski

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So, i'm a little bit confused on venodilators and their effects on TPR, EDV, and Venous return.

On page 267 of FA 2016 it states that "Preload: Preload approximated by ventricular EDV. Vendoilators decrease preload."

Then on page 269 of first aid, graph C shows that a decrease in TPR will actually increase venous return.

These two statements seem to be at odds with each other? Isn't venous pressure part of TPR? Why does it say that venodilators decrease preload, while decreasing TPR increases preload?

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So the cardiac functioning curve is a different situation (the one that shows the venous return increasing with decrease in TPR that you mentioned). In the case of that curve you are assuming that everything in the system stays the same and you are looking specifically at what happens when you dilate or constrict the arterioles of the system. They are also not really designed to be thought of in steady state (the Guyton experiments were done quickly and represented "snapshots" of the system. Anyway, so in the cardiac function curves by dilating the arterioles you "allow" more blood to go to the venous side and assuming a static venous state this increases the venous return. Conversely, if you constrict the arterioles you will decrease the blood getting past that point of resistance and decrease preload. Basically those graphs are used to determine the theoretical range of CO and venous return given different RAP's... there is more on this in a different thread (sorry I would link it but I'm in a bit of a hurry at the moment, it had TPR in the title if that helps). Now, if you give something like a lower dose nitrate then you are going to preferentially dilating the veins (rather than arterioles the main drivers of TPR). So with this you are going to increase the capacity of the venous side of the circulation which will create a "pooling" effect, it essentially pools more of the circulation into the veins and lessens the preload going to the heart. Hope that helps.
 
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