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Discussion

Student Question: Preload

Sorry to bother you. I've been trying to figure this out all weekend. I know I should go to my prof, and she has personally never said this, but it would *kill* me if I heard, one more time, "Don't worry, it won't be on the test."

I understand Starlings Law via a rubber band analogy, that the more the heart is filled, the more forcefully it contracts.

And I think I understand that, in tachycardia, the heart does not have time to fill up, so it doesn't contract as forcefully, and we have decreased cardiac output.

Now I'm all confused about left ventricular end-diastolic volume. My textbook, Iggy (Ignatavicius & Workman, 2006, p.681) says that "Excessive LVED volume and pressure ... may result in decreased cardiac output."

Doesn't this contradict Starling's law? Iggy (p.681 again) says, "The more the heart is filled during diastole (within limits), the more forcefully it contracts." I'm hoping to find someone willing to explain that intriguing "within limits" to me. My rubber band analogy has broken down (and so, incidentally has my rubber band. *OUCH*!)

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Sounds like you have a good grasp of this!

This is talking about heart failure. People with congestive cardiac failure, cardiomyopathy etc have been so chronically volume overloaded that they have stretched the compliance out of their ventricle. Now it requires a massive amount of filling to get a decent cardiac output. Eventually, even that doesn't help, as the rubber band is stretched beyond it's limits and can't rebound.

This is when you end up with pulmonary edema, shortness of breath, etc and the ventricle is unable to eject the volume, which backs up in the pulmonary system.

Sounds like you have a good grasp of this!

This is talking about heart failure. People with congestive cardiac failure, cardiomyopathy etc have been so chronically volume overloaded that they have stretched the compliance out of their ventricle. Now it requires a massive amount of filling to get a decent cardiac output. Eventually, even that doesn't help, as the rubber band is stretched beyond it's limits and can't rebound.

This is when you end up with pulmonary edema, shortness of breath, etc and the ventricle is unable to eject the volume, which backs up in the pulmonary system.

:yeahthat:

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