Cardiac Vascular Function Curve

[aspiringmd1015]

i know theres numerous posts about this, but one thing I don't understand is with increasing blood volume, , it increases the mean systemic pressure, with increased Venous Return right? so this increased venous return should lead to an increase in preload, and this increased preload however does not effect the cardiac function curve, but i thought the force of contraction is proportional to the preload i.e. increased preload, increases force of contraction?
also, with decreased TPR, you increase contractility as more venous return is occurring, but in the case with actual increasing blood volume, the contractility isn't increasing. Someone help please! Pholston the savior?

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[seminoma]

i know theres numerous posts about this, but one thing I don't understand is with increasing blood volume, , it increases the mean systemic pressure, with increased Venous Return right? so this increased venous return should lead to an increase in preload, and this increased preload however does not effect the cardiac function curve, but i thought the force of contraction is proportional to the preload i.e. increased preload, increases force of contraction?
also, with decreased TPR, you increase contractility as more venous return is occurring, but in the case with actual increasing blood volume, the contractility isn't increasing. Someone help please! Pholston the savior?

Force of contraction and contractility are not the same thing. Preload does not affect contractility. If changing blood volume changed the CO curve, it would mean that volume determines cardiac contracility.

Contractility is determined by the amount of calcium available in the myocytes.

 

[aspiringmd1015]

many books I've seen it say increased force of contraction(isotropy) via b1 stimulation. This increase force of contraction through b1 stimulaiton is by phosphorylating the ca+2 channels via PKA and allowing calcium influx etc. Seem the same thing to me?

 

[Jabbed]

Increased preload will move you along the Starling curve but won't shift the curve itself. This is pure biomechanics and isn't mediated by a beta-adrenergic mechanism.

 

[seminoma]

many books I've seen it say increased force of contraction(isotropy) via b1 stimulation. This increase force of contraction through b1 stimulaiton is by phosphorylating the ca+2 channels via PKA and allowing calcium influx etc. Seem the same thing to me?

Preload does not increase contractility. Any book that says it does is wrong, or you're misreading.

 

[aspiringmd1015]

right, contractility, and preload both are indexes of force of contraction.

 

[aspiringmd1015]

My questions till remains, as to why does increasing blood volume not shift your contractility curve, but decreasing TPR shift your curve?

 

[dfib slim]

My questions till remains, as to why does increasing blood volume not shift your contractility curve, but decreasing TPR shift your curve?

It's not a contractility curve, it's a cardiac output curve.

 

[aspiringmd1015]

okay, the cardiac output curve, sorry about that.

 

[dfib slim]

okay, the cardiac output curve, sorry about that.

Increasing blood volume causes increased CO through increased preload. 2nd graph in FA, you move up and to the right because you increased these 2 parameters.

Decreasing TPR will cause increased CO from a reduction in afterload. 3rd graph in FA, RA pressure will remain the same because of a concomitant increase in CO and increase in venous return. The reason you "shift curves" up is because both CO and venous return are increased while RA pressure remains the same. It has nothing to do with contractility.

 

[aspiringmd1015]

right, so shouldnt an increase in venous return increase your EDV?

 

[dfib slim]

right, so shouldnt an increase in venous return increase your EDV?

It does but the graph displays the data in terms of RA pressure, not EDV. Labeling the x axis as "RA pressure or EDV" doesn't make sense when RA pressure remains the same and EDV increases.

They are showing you that changes in preload and afterload affect cardiac output and not RA pressure in a simplified manner.

 

[aspiringmd1015]

i see, that all makes sense. I just dont understand the labeling of the graph as either RAP or EDV and CO/Venous return. Help someone!!

 

[aspiringmd1015]

what does the mean systemic filling pressure signify though? i underrtand its the pressure in the vessels when everything is stopped, but what is its significance? just the pressure returning to the heart?

 

[aspiringmd1015]

One thing i still dont understand, is that, why with a decreased TPR, does your cardiac output curve shift upwards, but with an increase in venous volume, does your cardiac output curve not shift upwards? Is it because with decreased TPR you have a decreased afterload?

 

[aspiringmd1015]

seminoma or someone for the love of god help.

 

[seminoma]

One thing i still dont understand, is that, why with a decreased TPR, does your cardiac output curve shift upwards, but with an increase in venous volume, does your cardiac output curve not shift upwards? Is it because with decreased TPR you have a decreased afterload?

You should think of it as the relationship of cardiac output per unit of EDV. Increasing volume DOES increase cardiac output, but it doesn't shift the cardiac output curve because EDV (preload) has no effect on contractility.

 

[Jabbed]

One thing i still dont understand, is that, why with a decreased TPR, does your cardiac output curve shift upwards, but with an increase in venous volume, does your cardiac output curve not shift upwards? Is it because with decreased TPR you have a decreased afterload?

Your cardiac output curve is just a Frank-Starling curve. The only things that shift it are inotropy/afterload. Changes in volume are not shifts, they are movements along the curve.

 

[aspiringmd1015]

okay thats clear now, so the contractility is increasing with a decreased TPR b/c of a decrease in afterload?

 

[seminoma]

okay thats clear now, so the contractility is increasing with a decreased TPR b/c of a decrease in afterload?

Yea because afterload and tpr are essentially the same thing.