Can someone tell me the formula used to calculate the theoretical TMP on an H machine? I used to know it but haven't used it in a while and lost it. Any help would be appreciated. Thanks!

quote:

Originally posted by GTSCSS:
Can someone tell me the formula used to calculate the theoretical TMP on an H machine? I used to know it but haven't used it in a while and lost it. Any help would be appreciated. Thanks!

I have 2 from my level 2 training several years ago. It does not refer to a theoretical TMP but see if this helps.
TMP=venous press-dialysate press.
or, TMP= UFR/KUF

Using that formula... I came up with a theoretical TMP of 3. (doesn't seem right) The insert for the dialyzers (if I'm reading it correctly) says the KUF is 83 (Gambro 21R's). The problem I'm having is the clinic was running a patient with a goal of .750 kilos for 3 hrs. Venous pressure was about 100mm/Hg and they were getting low TMP alarms and running in the +20 - (+40). I'm thinking it's just a matter of physics not allowing that kind of weight removal with that dialyzer at that treatment time without going into a low TMP situation. When the goal was changed, the TMP came up to about -60 to -80. I'll check the machine out to make sure, but I'm thinking they were just trying for too little in too much time to maintain the desired TMP.

I would think a 21R would be a rather large dialyzer for such a small amount of fluid removal. How about the high and low limits for your TMP? Does the lower limit need to set even lower. Of course if a smaller dialyzer created a higher TMP, changing the alarm limits would be a mute point.

I was thinking a 21R was too big for the goal or the goal was too small for the 21R (i.e., needs a smaller dialyzer or a higher goal). I just can't explain it to the Center Director without knowing the formula for determining the theoretical TMP given those circumstances. I know in my own head that it's gonna cause problems, I just can't remember the formula to show her. Formula be damned!! Lol. BTW, the machine checked out okay and right on the mark as far as the VP and TMP cals go.

Matt,

The formula for calculating required TMP is... Goal(in ml) / hours / KUF = TMP.

The "3" you came up with is most likely "correct" but it really dosn't mean anything. When you're using dialyzers with such a high KUF you can throw TMP calculations out the window because even with a perfect 0 TMP, you will still get fluid shift. I'm not certain of the physics behind this but rest assured that it happens BIG TIME which is why you see the +TMP's.

Using a smaller dialyzer probably isn't a very good option because I imagine that the patient needs that large dialyzer for the sake of clearances.

I would suggest giving the patient 300-400cc of NSS every hour and adjust the goal to allow for this. Doing this should bring your TMP back within alarm limits. If possible you can also use a larger gauge needle on the venous side to lower the venous pressure which will help.

If you think this is a problem with the H machines, it is MUCH worse with the Baxter/Althin's. On those machines we can't even use a F70NR without getting a TMP alarm every 2 minutes unless the patient needs to lose around 1.5L/hr. I can't tell you how many times I get machines pulled for "TMP problems" because of this.

Chuck

Simple TMP = Pb (blood pressure) - (-Pd) Dialysate pressure. Or TMP = Pb + Pd

As dialysis machines can only monitor pressures available for measurement external to the body, the TMP control on the machine cannot take into account the plasma colloid (oncotic) pressure (Po) or dialysate pressure exerted by glucose (Pg).

To get the true pressure difference over the membrane these pressures have to be considered, so that;

true pressure difference = Pb + Pd - Po + Pg

Add to that the membrane co-efficient and the pressure gradient within the dialyser itsself and you can see where the problems start to mount-up.

This is why machines control the UF volume, and TMP as such is irrelevant. Any modern machine should be able to cope with controlling to the UFR cited without the need to 'fiddle' the figures and use top-up infusions.

I like what tman says about the TMP being irrelevant. If this is true, why do we still have TMP monitoring on dialysis machines? I believe there are other ways to determine that the dialyzer is clotting. Does anyone record the tmp before or during the treatment? On some machines the TMP monitor has been used to determine the integrity of the dialysate circuit.

With newer and bigger dialyzers you rarely see a negative dialysate pressure anymore. If your UFR needs to be zero, then your dialysate pressure must/will match the blood pressure to prevent the loss of fluid from the blood. This doesn't mean there is no fluid passage because there always is fluid passing across the membrane. It�s just a matter of degree depending on the size and porosity of the membrane.

My friend John Sweeney has given an excellent talk that includes an illustration of the amount fluid that during a treatment actually crosses a membrane....in both directions.

Chuck,

Wouldn't using a larger gauge needle (reducing the venous pressure) cause a lower (more positive) TMP? Anyhow, I know what you mean about the TMP problems. Most of the people I've worked with have learned and accepted that there are TMP problems with a small goal using a high flux dialyzer and have learned to compensate. But... some haven't. Those people still insist that it's an equipment issue even though the person before ran fine, the person after ran fine and the equipment checks out okay everytime. And to top it off, when they do what you and I suggest (give saline, bump up the goal, thus UFR), the TMP comes up. You know how it is. Just wanted to find a way to demonstrate my point if I could. Guess I'll just go along with it. Lol.

Tman,

Only in the perfect world would we throw TMP out the window and forget about it.

Matt,

A lower venous pressure would bring the TMP closer to the negative side. TMP is actually the DIFFERENCE between venous pressure (which is always +) and dialysate pressure (which can be either - or +). If you have say a venous pressure of 100, and a dialysate pressure of 0...your TMP is 100 or if your venous pressure is 100 and your dialysate pressure is +100...your TMP is 0.

Your TMP display goes to the + side when the machine has to fight against the other factors that "tman" explained so if you are able to reduce the venous pressure, thats less that the machine has to fight against thus bringing the TMP closer to 0.

Larry,

I believe what John is referring to is backfiltration. Backfiltration occurs no matter what and there is nothing you can do about it especially with highflux membranes. Its one of those things that some people have been scared into worrying about by sales people trying to sell their product (diasafe) to make "ultrapure" dialysate.

Chuck

Chuck,

I see. I believe I'm just thinking about it too much and confusing myself. Lol. But I understand now. Thanks for explaining!

Drip a liter of saline during the run and add the liter to the goal. Problem solved.
Regardless of what the books say, there is a real risk of backfiltration with huge KUF's
and small goals.

Chuck:

Yes, that presentation was given at a time when they were fighting for market share without a high-flux dialyzer.

Although backfiltration has truly never been to me a large concern, there is enough paper out there regarding the presence of anti-endoxin antibodies in patients to make it an interesting and debateable subject.

Then there's chronic inflammatory response, beta 2,.....uh oh.....I may be changing my mind :-)

Larry,

Agreed but....its all a matter of what school of thought you believe in. This is a topic I love to debate when sales reps tell me their mebrane is the most bio-compatible.

Chuck

quote:

Originally posted by GTSCSS:
Can someone tell me the formula used to calculate the theoretical TMP on an H machine? I used to know it but haven't used it in a while and lost it. Any help would be appreciated. Thanks!

Try explaining it this way: Think of the TMP as the amount of suction needed to remove fluid through a straw. A low KUF (thin) straw needs a fair bit of suction (TMP) to remove the fluid. Now try sucking the same amount of fluid through a high KUF(big) straw - the amount of TMP needed is a LOT smaller.