Stroke volume variance

aCRNAhopeful · Dec 9, 2009

Can't give you a firm answer but the same concept seems to occur with blood pressure. When the filling pressures are low, pressures are labile. Which makes sense with what you say, when volume is low, stroke volume is variable. As for a why, I think it's a matter of getting varying degrees of chamber filling from beat to beat and therefore a different stroke volume via the frank starling law, as opposed to when volume is adequate, the amount of chamber filling stays more consistent and you have a more stable stroke volume

WindwardOahuRN, RN · Dec 9, 2009

Hi, there,
New in the MICU, coming off a telemetry floor and trying to learn some of the hemodynamic parameters that we use. I'm okay with CO and such, but I am having trouble with stroke volume variance. We use presep catheters -- not swans in this icu -- and then are in a subclavian vein like a central line.
The problem I am having is this: I know now that SVV is higher when the patient is drier. Okay, fine. I can roll with that, but I was wondering the whys and hows of this -- the pathopysiology so to speak. I can get that stroke volume would be less with an "empty tank", but why would the stroke volume vary from beat to beat . . . or is it from some "normalized" stroke volume . . . I don't know. I know how to read the number, but I like to know why. Anybody out there have some helpful explanations -- or point me in the right direction.
Ionafey

First off, I'm confused. PreSep caths don't measure SVV---they measure ScVO2.

Did you mean Flo-trac/Vigileo systems? They're placed in arteries.

For really good educational material go directly to the Edwards site. You can even print out pocket cards full of info for quick reference.

BTW, the Vigileo SVV number is only reliable when used in a patient who is completely 100% mechanically ventilated.

Here is the Edwards website---go have fun:

http://www.edwards.com/education/cceducationmap.htm

Lynda Lampert, RN · Dec 9, 2009

Maybe I am confused. I would not doubt it, lol! I have been to the Edwards site and d/l some of the stuff they have there. I love learning, so I am not having a problem with this: I think being confused is the first step to understanding. And, I was speaking of a vented patient -- volume controlled, I think. I could have sworn the SVV was on the same screen as the ScVO2 along with CO, CI, SV, etc. It does beg the question, though, how would a venous cath know arterial stuff. Must ask preceptor. I'm likely to get another article to read. :)

Ionafey

WindwardOahuRN, RN · Dec 9, 2009

Maybe I am confused. I would not doubt it, lol! I have been to the Edwards site and d/l some of the stuff they have there. I love learning, so I am not having a problem with this: I think being confused is the first step to understanding. And, I was speaking of a vented patient -- volume controlled, I think. I could have sworn the SVV was on the same screen as the ScVO2 along with CO, CI, SV, etc. It does beg the question, though, how would a venous cath know arterial stuff. Must ask preceptor. I'm likely to get another article to read. :)
Ionafey

Okay---clearer now. The patient obviously had both a FloTrac and PreSep cath. Both are cabled into the Vigileo monitor and the values for both show up on the same screen. You can use the Vigileo for one or the other or both.

Now go have some more fun with that site!

danamobile · Dec 10, 2009

Stroke volume is like tidal volume, which will always vary because the lungs will expand different with each breath-- and the heart will not contract exactly the same with each beat, and the CO will not be the exact same with each contraction...

maybe I need a clue stick too.. :) good luck!!

detroitdano · Dec 10, 2009

Stroke volume is like tidal volume, which will always vary because the lungs will expand different with each breath-- and the heart will not contract exactly the same with each beat, and the CO will not be the exact same with each contraction...
maybe I need a clue stick too.. :) good luck!!

This is what I was going to say basically. If it doesn't make sense for you, graph the SVV on your monitor and look at it with the patient's inspiration/expiration and notice the difference in the height of the SV waves. The monitor does its fancy calculations and comes up with an average variation between those waves and you get an SVV.

This is the reason patient's must be on controlled mechanical ventilation for it to be accurate. If you have someone breathing on their own (CPAP for example) their tidal volumes can vary wildly (think about your EtOH'ers who breathe shallow at 40 breaths one minute, get exhausted and do 15 deep breathes the next), and the SVV is a completely useless number.

We use Vigileo's in my hospital, here's a very informative FAQ sheet about them and SVV in general:

http://ht.edwards.com/resourcegallery/products/mininvasive/pdfs/ar01969.pdf

Lynda Lampert, RN · Dec 11, 2009

Thanks for the replies, all. I think I am getting a better understanding of it. Still feel so lost in the unit right now, but it has only been two weeks. I'll likely be back with any other questions that frustrate my preceptors. :)

Ionafey

seanrowley1 · Dec 19, 2009

Hi Guys,

I will do my best to explain Stroke Volume Variance (SVV). It is easier to discuss verbally but I will give it a try here. SVV is a value given in % and it measures the ave variation of the beat to beat stroke volumes through out the respiratory cycle. If a patient is fluid responsive, i.e. more volume will improve stroke volume and thus improve cardiac output then you will see a high variation. This happens because the patients great vessels cant withstand the intra thoracic pressure changes caused by the vent. If the patient is adequetely volume recusitated then the vessals will be be more full and then able to withstand those pressure changes better.

Think of a firehose.........if you have the water running through a firehose at full blast then it is really tough to squeeze the hose and slow down the flow. However, if you cut the amount of water going through the hose way down then you will find it's easier to squeeze and then you can slow down and speed up flow depending on how hard you squeeze it.

Now apply this idea to what is happening intra thoracicly (sp?) If your patient is not adequetely volume resucitated then as the pressure increases from the vent pushing a breath of air in it will put pressure on your great vessels which will impede venous return back to the heart. If venous return is decreased then your stroke volume will decrease. Then when the intra thoracic pressure drops as the vent backs off the vessels open up and venous return is increased which now causes an increase in stroke volume. SVV is measuring the difference between the largest stroke volume and the smallest stroke volume over that respiratory cycle.

If the patient is adequetely volume resucitated then their vessels will able to withstand those changes in intra thoracic pressure much better and your variation will be lower.

I hope this helps and doesn't confuse things more. Have a great weekend.

danamobile · Dec 19, 2009

GREAT answer!!! Not confusing to me!!! I need to keep you in my back pocket for when I get tough questions

Lynda Lampert, RN · Dec 19, 2009

I agree -- Great answer! I will need to print this out to keep with my research stuff.

Iona

Bubbles_RN · Dec 22, 2009

That is an awesome answer!!! We are just starting to use SVV in our ICU and I've been a bit lost with it, as our educator got me going round in circles with what all the numbers mean.