Stroke Volume, MAP, and Pressure-Volume Loop

[Wannabedoctah]

Hi all,

In a pressure-volume loop, if preload is increased stroke volume is increased, but it doesn't appear to increase systolic pressure at all. However, from the equation: MAP= SV x HR x TPR I would be inclined to think an increase in stroke volume would increase systolic volume. Where am i going wrong? Does an increase in stroke volume only increase MAP by increasing diastolic pressure?

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[whatTREEisTHAT?]

I think that in this example TPR would decrease to allow for increased cardiac output.

 

[.hematoma.]

Try to think about what causes an increase in that preload. Must be increasing blood coming to the heart right? Usually the reason why increased preload doesn't automatically increase systolic pressure is because a reduced TPR is the cause for the increased preload. We can see this if we were to take a look at the cardiac and vascular function curves (check first aid).

Another perspective: the normal heart muscle fibers have enough "give" in them in the left ventricle to hold enough pre-load without a compensatory effect on the heart's function. This is the frank-starling curve, so increased preload will just move you further along the right in that starling curve, which would increase CO. Make sense? The heart is built for variable pre-loads without messing the whole cardiovascular system up!

It's also important to remember that preload is not a surrogate for systolic pressure; it is afterload that is the surrogate for systolic pressure.

Hope I could help!

 

[Dr Tony T. Chopper]

Hi all,

In a pressure-volume loop, if preload is increased stroke volume is increased, but it doesn't appear to increase systolic pressure at all. However, from the equation: MAP= SV x HR x TPR I would be inclined to think an increase in stroke volume would increase systolic volume. Where am i going wrong? Does an increase in stroke volume only increase MAP by increasing diastolic pressure?

remember that MAP homeostatic mechanism is an inverse relationship between tpr and co. if map is kept constant, an increase in co is met with a drop in tpr. also don't forget that the map term has summed components of 1/3 pulse p and dias p. if map is unchanged, either none of the p changes, or the pulse p is unchanged via an equal increase/decrease of both dias and systol p.