More Cr and Lasix questions

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Hi all,

I have another question on Cr and Lasix.

I don't know the PMHx of this patient or much about her, as I had nothing to do with them but just was listening to a few parts. Her Cr was 3.0, in for heart failure. So her heart is inadequately perfusing her body with blood and kidneys, resulting in insufficiency causing her Cr to elevate. Resident gave the lady 40 mg of Lasix IV, and the attending told her that wasn't enough to get her to diurese, and suggested 80 or 120 mg IV instead.

- My question is why does a worse Cr need a higher dose of Lasix?

- Also, if this patient was hypotensive (she was), and we give her a lot of Lasix, wouldn't she just be getting rid of the fluid that's in her vessels, which would further exacerbate her hypotension?

- I guess I don't understand why the patient becomes hypovolemic when they are fluid overloaded.

Thank you

Originally Posted by LouisianaDoctor

Hi all,
- My question is why does a worse Cr need a higher dose of Lasix?

From Micromedex:

Excretion:
Renal: 60% to 90% unchanged, more excreted unchanged with IV formulation than oral formulation

No relationship exists between plasma levels and therapeutic effect. Response is related to the concentration of the drug in the urine rather than plasma.

Site of action is in the Loop of Henle, worse perfusion, worse renal function, higher dose needed to get to site of action.

Originally Posted by LouisianaDoctor

Hi all,

- I guess I don't understand why the patient becomes hypovolemic when they are fluid overloaded.

Did you mean to say "hypotensive" here?

Perhaps the patient is an end-stage heart failure patient and this is causing the hypotension.

Caveat - I'm a peds resident, I don't deal with heart failure on a regular basis like my adult colleagues do. However, it's clear that you are thinking about this wrong.

The problem you've presented is cardiac in nature. Really focus on that. The creatinine is just a marker of her overall issues. You said it in the beginning - her heart can't keep up with the demands of her body - in this particular instance, she's not pumping enough blood to the kidney's to generate sufficient GFR and her creatinine trended up.

While hypotension can be due to hypovolemia, in this case that's not her problem. Remember that blood pressure is the product of CO x SVR and CO=Stroke Volume x heart rate. If her heart function is poor, that will reduce your stroke volume. She's fluid overloaded in that her body has tried to improve her function by increasing her pre-load, but has done so well past the point that her heart can actually push that pre-load - go back to your Frank-Starling curves and note that as pre-load increases, there's a law of diminishing returns until you get so far out, that function actually decreases. Giving the lasix will reduce her volume status, bring down her pre-load to a point where you optimize her cardiac output. In other words, you get her to the point where you're in balance with what her heart is able to do, so that - hopefully - you avoid all the sequelae that comes from having heart failure (pulmonary/generalized edema, difficulty breathing, decreased functional status, kidney injury, etc). The problem and the reason why there are a growing number of cardiology fellowships offering extra training in heart failure, is that if her cardiac output is too low to adequately oxygenate her tissues, she'll have a whole host of other needs. It's a complex problem and like I said, not one that I experience routinely.

Thank you all for the reply. It really helped make this a lot more clear in my head. I believe that I was definitely mixing up hypovolemia and hypotensive. Thank you again.

Originally Posted by LouisianaDoctor

Hi all,

I have another question on Cr and Lasix.

I don't know the PMHx of this patient or much about her, as I had nothing to do with them but just was listening to a few parts. Her Cr was 3.0, in for heart failure. So her heart is inadequately perfusing her body with blood and kidneys, resulting in insufficiency causing her Cr to elevate. Resident gave the lady 40 mg of Lasix IV, and the attending told her that wasn't enough to get her to diurese, and suggested 80 or 120 mg IV instead.

- My question is why does a worse Cr need a higher dose of Lasix?

- Also, if this patient was hypotensive (she was), and we give her a lot of Lasix, wouldn't she just be getting rid of the fluid that's in her vessels, which would further exacerbate her hypotension?

- I guess I don't understand why the patient becomes hypovolemic when they are fluid overloaded.

Thank you

As BigRedBeta said, the lasix is given to reduce pre-load which has stretched the heart past the "happy" part of the frank-starling curve and you're bringing the heart back which will increase cardiac out-put, via increased stroke volume. As long as these patient's are not too hypotensive, we'll even give them vasodilators to also reduce the afterload. It's always fun to watch the nitroprusside drip actually bring the blood pressure UP.

Originally Posted by LouisianaDoctor

Hi all,

I have another question on Cr and Lasix.

I don't know the PMHx of this patient or much about her, as I had nothing to do with them but just was listening to a few parts. Her Cr was 3.0, in for heart failure. So her heart is inadequately perfusing her body with blood and kidneys, resulting in insufficiency causing her Cr to elevate. Resident gave the lady 40 mg of Lasix IV, and the attending told her that wasn't enough to get her to diurese, and suggested 80 or 120 mg IV instead.

- My question is why does a worse Cr need a higher dose of Lasix?

- Also, if this patient was hypotensive (she was), and we give her a lot of Lasix, wouldn't she just be getting rid of the fluid that's in her vessels, which would further exacerbate her hypotension?

- I guess I don't understand why the patient becomes hypovolemic when they are fluid overloaded.

Thank you

My undertanding is that the dose wasn't directly related to the higher Cr. I am willing to bet that this patient was on Lasix PO 40mg BID at home.

Classic way to dose Lasix for a patient admitted with acute decompensated heart failure is to give 2-2.5x the single home dose and give it IV.

She'll mobilize that interstitial fluid, which will replete her intravascular volume. I'm sure she had serious pitting edema and some crackles.

Originally Posted by jdh71

As BigRedBeta said, the lasix is given to reduce pre-load which has stretched the heart past the "happy" part of the frank-starling curve and you're bringing the heart back which will increase cardiac out-put, via increased stroke volume. As long as these patient's are not too hypotensive, we'll even give them vasodilators to also reduce the afterload. It's always fun to watch the nitroprusside drip actually bring the blood pressure UP.

Frank-Starling. FTW.

We didn't hear the whole story, because there was nothing that told us the lady's volume status, e.g. comparison of current weight to dry weight, physical exam findings, etc.

I am willing to bet that they held the Coreg and ACE.