CO=SV x HR ??

[ktenor]

Hi-- I have really appreciated this site. For one I just moved to a new state and am lacking in my nursing peers.. SO when I have a question this is a great site. Thanks again!!

Question:

A nurse is working with a new admission who has a baseline blood pressure (BP) of 120/80 mm Hg with a pulse of 66 BPM. This client experiences a drop in BP to 100/40. Which of the following pulse rates would serve to maintain the client's baseline cardiac output (CO)? (Select all that apply)

1. 62 BPM

2. 68 BPM

3. 64 BPM

4. 70 BPM

2 and 4 are correct--I just need help in understanding this.. I get that you want the pt's HR to increase to maintain their CO- though I'm not sure how you get these answers exactly. Im doing the math. Its just not making sense to me.

Thanks!

[Nolli]

The problem isn't really about math but concepts. SV = EDV- ESV. EDV and ESV are about volume and thus SV can not be calculated using the above information. However we can still infer what happens to SV when BP drops. SV is affected by preload, afterload, and contractility. Here is a link to SV and the main factors involved that can explain all three in more depth than I can: CV Physiology: Regulation of Stroke Volume

Ask yourself if BP drops how is preload and afterload affected? How does that affect SV? If you can answer these questions you will understand why increasing HR is necessary to maintain CO in this scenario.

[Esme12, ASN, BSN, RN]

If the B/p falls the heart rate must increase.

this may help.......Controlling BP

[nurseprnRN, BSN, RN]

You can have huge cardiac output (normal, decreased, or even increased SV), and still have the BP in the toilet due to massive vasodilation (very low afterload/vascular resistance).