Cardiac and vascular function curves

[Fallot101]

Hi guys,

I can't seem to understand the cardiac and vascular function curves. I mean, I get how the cardiac curve explains the Frank-Starling relationship, and why RAP is inversely proportional to venous return in the vascular curve. What I don't get is: if ↑ venous return leads to ↑ cardiac output, how can an ↑ in RAP at the same time ↓ venous return and ↑ cardiac output?

Also, Costanzo says the flat portion of the vascular curve is due to negative RAP, which leads to vein collapse and decreased venous return. But why is VR > 0 and CO = 0 in this situation?

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[Jabbed]

Costanzo does a pretty poor job of explaining this. IMO the best resource to read for this is Guyton, as he was the one to develop the vascular return curves.

Highlights:

• Cardiovascular output is a steady-state between two separate systems: cardiac-vascular flow (modeled by Frank-Starling curves) and vascular-cardiac flow (modeled by vascular return curves).
• Vascular return curves represent the gradient achieved between systemic blood vessels and the right-side of the heart.
  
• Now if you think of venous return as a closed system with a given pressure gradient, it makes sense that: ↑ venous pressure → ↑gradient → ↑venous return. The flip side of this is that: ↓right atrial pressure → ↑ gradient → ↑venous return. In essence right atrial pressure is a barrier to venous return.
  
• Once you convince yourself that this makes sense, look at the factors that shift and rotate these curves (i.e., venous tone, fluids, and arterial constriction)

Here's a good article that expounds on these curves a bit more: http://emcrit.org/wp-content/uploads/2014/12/Venous-Return-In-Critical-Illness.pdf

 

[Fallot101]

Thanks.

I guess my problem was that I was trying to interpret the two curves at once, as if Cardiac Output and Venous Return were synonyms. I though Costanzo's explanation was pretty bad but then I read further and it does clarify things. I get now that CO = VR in the steady state. Apart from the steady state, the 2 curves should be interpreted independently. At least that's how I got it to make sense.

I also managed to understand the alterations due to contractility, blood volume and TPR changes. Now I hope UWorld does a goob job in testing my understanding of these concepts.

Will definitely check out the article.

Thanks again.

 

[BornDJnowDoc]

Costanzo does a pretty poor job of explaining this. IMO the best resource to read for this is Guyton, as he was the one to develop the vascular return curves.

Highlights:

• Cardiovascular output is a steady-state between two separate systems: cardiac-vascular flow (modeled by Frank-Starling curves) and vascular-cardiac flow (modeled by vascular return curves).
• Vascular return curves represent the gradient achieved between systemic blood vessels and the right-side of the heart.
  
• Now if you think of venous return as a closed system with a given pressure gradient, it makes sense that: ↑ venous pressure → ↑gradient → ↑venous return. The flip side of this is that: ↓right atrial pressure → ↑ gradient → ↑venous return. In essence right atrial pressure is a barrier to venous return.
  
• Once you convince yourself that this makes sense, look at the factors that shift and rotate these curves (i.e., venous tone, fluids, and arterial constriction)

Here's a good article that expounds on these curves a bit more: http://emcrit.org/wp-content/uploads/2014/12/Venous-Return-In-Critical-Illness.pdf

Brilliant dude thanks