Contractility change in AV fistula

[gk12321]

Uworld showed this

which hints at a decrease in contractility with an AV fistula...

whereas in First Aid 2016 we have this :

where number 6 is for the AV fistula.



should contractility increase or decrease in the presence of an AV fistula?

My understanding is that with more blood coming the myocyte length will be increased hence the contractility also should increase.

Can someone please help on that?

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[ASM student]

No decrease in contractility. The decrease is in the peripheral resistance and subsequently the systolic ventricular pressure, as an amount of blood will escape high pressure arterial system and instead will shunt to the low pressure venous system through the A-V fistula which will increase the venous return (right shift on X axis) and if you considered the fistula as a low pressure circuit added in parallel to the circulation the result will be a decrease in the total peripheral resistance ( lower volume pressure curve).


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[Kendragon]

The uworld graph does not hint at a decrease in contractability. Rather, it shows an increase in EDV (the bottom right point of the loop). An increase in EDV will cause an increase in contractability (as you correctly surmised) .

You might be mistakening the decrease in the height of the graph as a decrease in contractability. However, it's the horizontal axis that reflects contractability and stroke volume, not the vertical axis.

 

[gk12321]

it isnt the height of the graph that i considered but the gradient of the line from the left to the (0,0) as shown in another graph in uworld.