map & dbp question

papawjohn · Apr 29, 2006

Hey Chaos!!!

Let me try to answer your question. Obviously I was not there with you last nite--so whatever I say is kinda theoretical. (And cannot be interpreted--because NEVER meant--as critical.)

So let me begin by like quoting you: the BP was 135/44 with the MAP in the low 50s. Now the MAP (if the BP was 135/44) was 74.

Two things: First--remember that the 'mean' is a CALCULATED number. And the calculation has a simple goal. It tries to figure out WHERE on a wave, is half the fluid above the 'mean' and half is below.

Now you can see that--from the standpoint of a nephron or neuron or whatever cell you imagine--one half the wave means roughly the 'constant' perfusion pressure.

I hope that's not confusing to you. If you have never calculated say, Intercanial Perfusion Pressure---or Coronary Perfusion Pressure, this might be a hard thing to imagine going on in the human body. If you are having a problem with it--let me take you across town (since I live in Florida) to the beach. The waves run up the beach, then back down. The sand is soaked as the waves run over it, then quickly 'dries' kinda as the wave recedes. OK? Well, imagine that our nephron (for example) is a little beach critter. It MUST live at some point on the beach where the waves go over it and then recede--but the sand must never dry completely out.

That might make the idea of the 'mean' of a wave kinda reasonable.

So--WHERE on a wave is the half-way point for the AMOUNT OF FLUID. Obviously, if you draw the line half-way up the wave, you've got the smallest part of the wave above and the way-biggest part below. So that is not the 'mean'.

It turns out that roughly 1/3d of the way up the wave, you split the amount of fluid in half.

So lets look at 'Mean Arterial Pressure': The wave is the PULSE PRESSURE, ok? Not the BLOOD PRESSURE!!! The PULSE pressure. Because we're talking about a WAVE. And the Diastolic is like the surface of the sea. The wave builds up onto the surface of the sea and the arteries have a 'constant' pressure on their walls from the Diastolic BP.

So the MAP is like this: Pulse pressure is 135 minus 44 equals 90. Mean is 1/3d of that, or 30. To get the true Mean Arterial Pressure add 30 (mean of the pressure wave) to 44 (the surface of the sea) and the pressure on the beach is 74.

You see?

OK, number 2. You're Pt is still not doing well. He's showing signs of poor perfusion (low Urine output). So how's that square with a MAP of 74?

Let's think about how we get our numbers. Either you used an Art Line or an NBP or you did like generations of us did in antiquity and used a cuff and stethoscope. (Frankly---I doubt any of us are still using cuff & steth.) So you were almost certainly LOOKING AT THE MONITOR.

Now, if you were using an ArtLine---the monitor was 'calculating' the MAP from the top and the bottom of the wave form. Which puts the responsiblity on you of ZEROing and positioning the transducer, yes, but really gives the 'Gold Standard' of measuring BP.

But if you were using an NBP---the monitor was 'sensing' the pressure wave of the Pt's artery against the cuff and up the tubing. It then 'felt' where on the series of pulses it experienced was the 'hardest' and determined that THAT was the mean. It thought it was doing you a favor by adding the highest (systolic) and lowest (diastolic) pressures on the screen. But the number on the screen of an NBP that you really want to trust is the MEAN.

Now I bet that your BPs were from an NBP, is that right Chaos?

So that means that the SBP and the DBP are both suspect. You see? Cause the MEAN BP explained your Pt's problems but the SBP/DBP didn't.

OK? (I've gone on way too long with this.) You've got a new valve with obvious problems of renal perfusion. I'd watch my "Central" numbers (PAP monitoring would be nice, CVP in a pinch, Urine output exactly on the hour--a 'uro-swan'. And I'd give fluids That's how to get the Diastolic up--in your Papaw's experience. I'd give (based on my experience with the surgeon) 250cc/hr until the Diastolic came up and Urine showed appreciation. If the SBP went above 140, I'd titrate the NTG or Nipride (prob'ly the Nipride last--because I don't like it and would have a goal of DCing it and going with NTG alone).

Hope that helps

Papaw John

chaosRN, ASN, RN · Apr 29, 2006

Thank you so much for your reply :thankya: I really do learn a lot from your posts.

I was looking at the artline which was zeroed and positioned correctly (I checked so many times!) My CVP was 18. (no PA line :sniff: ) I had given a total of 750cc of albumin (per md's order), not counting the blood products.

Your post also explains why we not only titrate meds for the sbp, but also the mean - pressure on the walls. You just explain things so well.

So in order to increase the MAP and DBP - use volume. Even though the CVP was 18, we still needed to give volume (low uop).

I do understand that the mean was explaining the pts problems (low uop) so I guess we do need volume.

I was titrating the nipride off and just use NTG (even though I don't think it works real well for post op b/p).

Thank you so much!!!!

Chaos

dorimar, BSN, RN · Apr 29, 2006

In most cases the DBP is so imortant because it is 2/3 of the MAP calculation, and as you mentioned the MAP should be at least 60 for kidney perfusion and in many cases 70 for adequate cerebral perfusion. However your MAP was in the 70's with bp of 135/44. When we worry about DBP & the MAP, we are ususally worried about a low MAP (his was not). Now for the SBP, in this patient's case it is very important because you have a fresh valve. You do not want to be working the heart too hard. In this case, with an adequate MAP in the 70's I would be more concerned about the SBP being too high. as far as the u/o goes, i really don't know. His CVP was 18 which would indicate more than adequate volume status, and his MAP was also adequate. Too bad you didn't have a swan. What was his baseline renal function? Did he have a hypotensive event in OR? As Papawjohn mentioned, was this a consistent BP or were there large swings involving a drop in the MAP? Was the CVP 18 only AFTER the colloid and crystallooid boluses? What was it prior?

LCRN · Apr 30, 2006

Also you need to think about patient's co-morbities that could've caused a "false" high cvp such as any right sided heartfailure or issues with valves. In some patients you have to use the trend of the cvp to measure volume status rather than necessarily the actual number. For example the normal cvp depending what source you look at is about 2-6 but since cvp is the reflected value of the Right atrium someone with Right heart failure might have a cvp of 18 and be on the "dry" side.

It's important to understand your hemodynamics inside and out. I hope I didn't confuse you by adding this tidbit.

papawjohn · Apr 30, 2006

Hey Y'all!!!

What a privilege to have such good conversation!! I was checking my EMail and the allnurses' site directed me to the reply from Chaos and as I was thinking of how to respond--whoops!! here's LCRN with just the thing.

And what a shame you didn't have a PA Catheter!!! BOOOH!!! To Surgeons and Cardiologists who give us these Pts and don't give us the proper tools to do it right.

But back to Chaos' question of high CVP and lots of volume expansion and a high Systolic BP.....BUT---low mean BP and low urine output. What gives?

Well, the question is (as LCRN says) what kinda pathology was going on that required the valve replacement and how much the replacement valve relieved that pressure. So I'll suppose that this Pt has a long history of 'myopathy' and 'hypertropy' and pulmonary hypertension. In this case you have a pretty sharp picture of 'Right-Side CHF'. In other words, the Right Ventricle is weak in relation to the Pulmonary Vascular Resistance--and pump as hard as it can, it still cannot pump enough blood thru the pulmonary circulation to supply the Left Ventricle with sufficient blood to supply the general arterial circulation. A classic and CHRONIC case of Right Side CHF is severe pretibial edema and Liver engorgement and swollen neck veins, very low BPs and clear lungs. Because whatever part of the heart fails--the symptoms occur in the compartment BEHIND the failing part.

So what's a nurse to do? Well, the general rule of thumb is: Give Fluids!!

Altho I would defer instantly to an experienced Cardiac Surgery ICU nurse who would weigh in here. And always remembering that if your post op Pt is not doing well--the person to confer with is the SURGEON. They are famously possessive of their Patients--for about 24 hours.

The way Papaw John thinks about any Post Op Pt is that the hormone-release related to shock (cortisol) turns our Pt's tissue into a sponge. Altho you gave 750ml of Albumin (which is in short supply!!) and blood products, you STILL could be barely keeping up with loss of circulating volume into tissues. If you think of the high Pulm Vasc Resistance as a 'choke-point' or 'bottle-neck' that only allows a limited amount of fluid to pass thru it, you have the picture.

So in the ICU--we're not gonna worry about Lower Extremity Edema, are we? If the alternative is bad renal perfusion and low urine output--no, we're going to pour fluids in with the thought that a HIGH CVP actually helps push blood thru the Pulm Circulation and into the Left Side and arterial circulation. So--to paraphrase Commodore Perry in the War Between the States--"Damn the CVP!!! Full speed ahead!!!

Papaw John

dorimar, BSN, RN · Apr 30, 2006

All this is very true. You guys are so smart. Most of the surgeons i have worked with (including CV surgeons) prefer a little extra volume post op. And as far as what choas said,the CVP may have not been a true indication of volulme status anyway. Again a swan would have been nice. I am used to the cv surgeons setting very tight guidlines on bp (ie: keep sbp>=90 but less than 120) post op, i think in hopes of keeping MAP up for adequeate perfusion but not working the heart too hard either. I am used to dumping albumin and blood products in and hanging nipride. Always helps to have a swan though.

hrtprncss · May 1, 2006

Quick question to further this discussion more....If we're thinking Right Sided Heart Failure, without knowing a full body assesment and we're only going with the following, sbp 130's and I believe this patient had nipride being titrated off? Just to further this since we don't know the assesments and there's no swan and we're only going on a few facts, meaning mean of 74, low u/o, cvp of 18, and some type of valve replacement, now we don't know if it's mitral or aortic valve replacement right? I would like to pose a question for discussion...

Are we assuming that it's right heart failure, if we are...and let's say we'll assume that the right side heart failure is caused by left sided heart failure from a mitral stenosis that's why the patient needed a valve replacement. IF the cause of the right heart failure was left heart failure, giving fluids with a cvp of 18 might not be feasible. If you're thinking it is because of right ventricular INFARCT then I agree that fluid is the way to go, fluid fluid fluid, but again let's not forget the sbp is 130 and mean is 74 therefore the patient is not hypotensive. But if this patient is having complications from the valve replacement then giving this patient fluids could be detrimental because of the long standing history of the left side. Personally with just the mean of 74, cvp of 18, and low u/o, I would lean more towards giving this patient some lasix, the same reason why you want to give fluid, meaning I can argue from the other side because there's not PA line to give us a definitive hemodynamic profile therefore none of us could be 100 percent right, and thus make it open for discussion. I agree that a swan or a complete assesment is in order to get a definitive intervention. I'm just playing devil's advocate to further this discussion because I have been following it the last two days to see where it's going to lead...FLUID Challenge OR LASIX...Sorry guys to make this more confusing I just want to further this along;)

papawjohn · May 1, 2006

Hey Everybody!!!

Like I said above---when the REAL CVICU nurses show up, I give up. (Or words to that effect. ;>)

Dorimar---I'll return a complement: You could be MY nurse, PLEASE!!!---if I'm ever that sick.

And HeartPrincess (do I have that right?)---I only had to shift focus just a little bit to realize that you were as correct as I was. And only the lack of a SwanGanz made it a toss-up. Oh well.....

Speaking of which---(and completely off-topic)---a pulmonologist was in a chatty mood the other day and I asked how come so few PACatheters now-a-days. You know what he said? That doing 'procedures' like floating SGCatheters raises a Doctors malpractice insurance so much that most MDs don't do 'em anymore.

Wow---never would have thought of that.

Now--return to all of us here wishing we had a Swan in that Pt.

And the Conservatives can nod sagely and feel the Lawyers are to blame and the Liberals can nod sagely and feel that Universal Coverage will solve all problems and probably cure cancer and the common cold.

But at the bedside, the nurse is looking at THIS Pt and knows she could do a better job if only she had better information---but the information isn't there.

Gosh---and I promised never to get into politics on the allnurses!! Slap hand---BAD JOHN---smack!!!

Papaw John

hrtprncss · May 1, 2006

papawjohn - I do miss swans, i don't see them as often anymore either...wanted to say i do agree with your answer, fluid challenge first. Then if it doesn't work chase it with lasix, it's just to make this thread a little interesting, just for conversation sake....and just wanted to say that i learn so much from your posts, you make reading them so enjoyable and informative...

ZASHAGALKA, RN · May 2, 2006

What was the bun/cr before surgery? If your art line sys/dia is OK and the mean is suspect (but your wavefrom is ok and you are properly zero/balanced), with a suspected high CVP (presumed decent vascular volume) - and the only s/s you are seeing is low uop - is that really a problem?

All that extra fluid, not just the liter plus YOU gave, but the 4-6 liters given in the OR tend to cause nice (um, not so) little fluid shifts that can have a tendency to decrease UOP in the immediate hours post up while his/her body is trying to figure out what the h^%# to do with all this fluid.

Our CV surgeons don't even want to know UOP over night POD#1: "We'll fix it later with diuretics."

Normal renal fuction, as measured by previous bun/cr, in my experience, requires little intervention in the first 12 hrs post op Open Heart - unless there are other s/s as well.

I understand the suspicion that low MAP = low UOP - but in fresh Open Hearts, this is not automatically the case. Or at least, to the extent that it IS, keeping those freshly minted needle jobs safe and happy at a lower BP than those renals are used to: well that take a much higher priority.

I would be more concern w/ mental status: if that checks out, I'd likely just keep watching - or if you don't have docs that specifically write not to bother them about UOP the first night: then I'd go ahead and report the

~faith,

Timothy.

steelcityrn, RN · May 2, 2006

I seen a turn years ago in the use of the swan. I believe the benefit does not outway the risk. How many patients were damaged by these with wedging and infection. I believe there are enough tools to use and signs and symptoms to understand why a patient would be in decline.