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r/step1•
MooseHorse123

Ejection Time

Can someone please help me understand this concept? Specifically in the context of nitrates and Beta-Blockers?

This thread on SDN said the following:

https://forums.studentdoctor.net/threads/ejection-time.1185096/

Nitrates: ↓venous return → ↓EDV → ↓SV → ↓ejection time

Beta blockers: ↑vascular resistance (ß2 vascular receptors) → ↑afterload → ↑ejection time

But I'm having trouble figuring out the logic here. Nitrates will vasodilate, so i understand that this will lead to decreased EDV since more venous pooling. This will lead to decreased SV because less blood is returning to the heart, less will be pumped. But why does this mean less Ejection time? How is this at all controlling the chronotropy of the heart?

Posted Mar 22, 2019, 8:35 PM UTC

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MooseHorse123OP•

Is it just baroreceptor?

I feel like that graph in first aid is a bit misleading because generally I thought we should be focusing on direct effects of things like drugs... Otherwise you can say that anything will always cause anything in the body due to some response.

Like Nitrates don't decrease ejection time directly. They vasodilate, then of course there is a reflex tachycardia... Maybe i'm just frustrated lol

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Kangarou_Penguin•

I don't think ejection time is referring to chronotropy.

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Ejection time is probably the amount of time that it takes from when the aortic valve opens to when it closes. It's a function of how much blood you have to eject & the after load that you have to pump it out against.

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If you have less blood and less after load, then the aortic valve open to close will be shorter.

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MooseHorse123OP•

I guess I'm a bit confused by this... Because isn't Aortic Valve opening time supposed to be the same as ventricular systole, which should be the same ventricular depolarization, which should be directly tied to chronotropy?

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Kangarou_Penguin•

The ejection time is a measure of the period of time that the left ventricle sustains pressures > than the afterload. By that logic, if you have less preload (nitrates), you generate less pressure (starling mechanism) and so you won't be able to sustain a high pressure for long and the valve will close quickly. That's why the nitrates decrease ejection time.

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On the other hand, I'm not sure your beta blocker mechanism is correct. I think they do increase ejection time but likely because they decrease chronotropy and allow for a greater EDV. The opposite of the nitrates.

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MooseHorse123OP•

Yes I agree with your Beta-Blocker logic. I don't think that SDN person is correct on that

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jcorpsmanMD•

Beta blockers decrease HR so they’re negative chronotropes where as nitrates don’t affect HR (at least not directly)

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