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Discussion

Cardiac question help, please!

I was re-reading the PPT from my cardiac class and there's a slide that just makes no sense to me. Test is on Monday, of course, so I can't ask the teacher then.

"Important to note: 90% of oxygen consumption by the heart occurs during the exertion it takes for the heart to overcome afterload, in order to pump blood.

Therefore, a pt's diastolic blood pressure reflects their afterload."

So if I'm reading this right, 90% of cardiac muscle O2 use occurs during systole. This makes sense to me. What does not make sense is how afterload is related to diastolic BP. Can someone please walk me through this? I'm sure I just need a poke in the right spot in my brain to put it all together, but I haven't found the spot yet.

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    When the heart is completely relaxed, the pressure inside the ventricles is roughly the same as it is outside the heart.

    Let's say that diastolic pressure is 70 mmHg.

    When the heart is in diastole, it is not opposing that 70 mmHg in the aorta and the rest of the vasculature. It's just chillin' and allows blood to flow in.

    If the heart is to accomplish its job and propel blood out, it has to meet that 70 mmHg and overcome it. The left ventricle has to go up to 80, 90, 100, 120 mmHg... or whatever to be able to direct the blood outward.

    So, in that way, diastolic pressure reflects afterload, the pressure the heart has to work against in order to pump blood.

    Does that help?

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    • Author

    YES. Thanks so much! I think it's how the first part is written, i.e. poorly. So just to be sure I have this right: increased DBP --> increased LV workload, right?

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    Yes, you got it.

    This is why controlling blood pressure is so important in general. The higher the diastolic, the more the heart has to work to keep blood flowing out.

    This is a critical concept in post-MI care where beta blockers are commonly given to ease the afterload an already sick heart has to deal with. Also a very huge consideration when using vasopressors. You want the diastolic blood pressure to be high enough to sustain life, but not so high that the heart will have a hard time counteracting it.

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    • Author

    Diastole is when atria fill, and once they reach a certain pressure/stretch, the pressure opens the valves and allows the blood to flow into the ventricles, right? So the pressure required to open the valves into the ventricles is the diastolic pressure, and the volume that it takes to overcome that pressure is how much volume must be pushed against SVR to move it into circulation. Is that right? And the larger the volume of blood in the ventricles, the more the heart has to push against SVR? Could I be having a lightbulb moment here??

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    • Expert

    cardiac diastole is the period of time when the heart relaxes (rests) after contraction in preparation for refilling with circulating blood. ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing. together they are known as complete cardiac diastole.

    during ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. when the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, and the left ventricle fills with blood that was accumulating in the left atrium. the isovolumic relaxation time (ivrt) is the interval from the aortic component of the second heart sound, that is, closure of the aortic valve, to onset of filling by opening of the mitral valve. likewise, when the pressure in the right ventricle drops below that in the right atrium, the tricuspid valve opens, and the right ventricle fills with blood that was accumulating in the right atrium. during diastole the pressure within the right ventricle is lower than that in aorta, allowing blood to circulate in the heart itself via the coronary arteries.

    a good picture of this

    http://www.brianmac.co.uk/bloodp.htm

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