Our unit uses low flux dialysers. We are currently looking at the value of increasing dialysate flow from 500 - 800mls per minute in a few patients who do not clear their chemistry very well. Any ideas, comments, suggestions or anecdotes.

Donna, What type of dialyzers are you using? Any reason why you are not using high flux dialyzers with a bigger surface area? Your clearances will improve with increased dialysate flow. What type of blood pump speeds are you running? One theory is that your dialysate flow rare should be twice that of your blood pump speed. ex. Qb 400/Qd 800. I forget the figures, but I've also heard the theory that after a certain point, increasing your blood pump speed will improve your clearances more than increasing just your flow rate. Of course, it will cost more money in dialysate to run at a higher flow rate and costs nothing to increase blood pump speed. We run about 85% of our patients and a 800 dialysate flow rate and achieve pump speeds of between 400 - 450. What is your average treatment length. That will affect clearances as well. Hope this helps. Ryan

Thanks for your reply Ryan. I do not know why we do not use high flux dialysers. Our blood flow rates vary between 250 and 325 mls/min. Our average treatment time is 4-5 hours.
Donna

Donna,
I would suggest trying to increase you blood pump speeds first before increasing flow rates and see how that impacts your clearances. By doing this first you don't incur any additional expense. What type of machines are you using? What type of dilyzers? The only thing that may hinder you increasing pump speeds is the type of dialyzers your using. Since you are using low flux dialyzers your venous and arterial pressure readings are going to be higher at a pump speed of 250 than they would be at a pump speed of 400 with a high flux dialyzer. Does your unit have a guideline set for what acceptable pressure readings are allowed during treatment? Got me interested now in what your outcome is. Ryan

Ryan, I agree with you about trying to increase the blood pump speed first - no additional expense, as you said.

We haven't been using high flux dialyzers, so I can't say I'm an expert, but I think the size of the needles or the catheter and the quality of the vascular access are very important factors affecting the highest possible blood pump speed.
I don't quite understand this, so could you explain to me how does the use of low flux dialyzer increase the arterial or venous pressure compared to a high flux dialyzer on a given blood pump speed (as I said I have no experience in using high flux). Where in the blood lines do you measure the pressures? When I speak about arterial pressure I mean the pressure between the arterial needle and the blood pump and to me the venous pressure is the pressure between the dialyzer and the venous needle.
And when you talk about a high arterial pressure, do you mean a really higher pressure or a bigger reading of negative pressure?

Arnold

Arnold,
When I mentioned arterial pressure, I was referring to a more negative pressure reading on the machine (pre-pump arterial tubing). I agree with you that needle size and access quality are definitely factors in improving treatment outcome. A low flux dialyzer will generate higher pressure readings than a high flux dialyzer because of surface area of the dialyzer. Think of T.M.P. If one dialyzer has a surface area of 1.0 and another dialyzer has a surface area of 1.8, the blood is going to flow with less resistance through the dialyzer with the bigger surface area therefore creating a lower pressure reading. Plus, a high flux dialyzer will give you better clearances without changing anything else except the dialyzer. Here are some facts of a sheet from Fresenius comparing there dialyzers to each other.
F4 F-80

Surface Area 0.7 1.8

Prime Volume 42 110

UFR 2.8 55

Urea 183 246

Creatine 145 220

Phosphate 88 216

Vitamin B12 34 150


The Urea, Creatine, Phosphate and Vitamin B12 are a taken at a blood pump speed of 300 and dialysate flow rate of 500. As you can see, just changing a dialyzer can make a difference. Does this clarify things at all? Where abouts are you guys located? Ryan

Arnold, My chart posted differently than I had made it while I was replying. Hope you can understand it. The first column of numbers is for the F4 dialyzer and the 2nd column is for the F80 dialyzer. Sorry for that. Let me know if it doesn't make sense. Ryan

Ryan, Im working in a unit in London, England. I have passed on the information you have given me to the unit sister. She is doing a two week trial, so I will keep you informed.

Donna,
I look forward to seeing what your units outcomes are. It's always interesting to see what other people are doing especially in other countries. Ryan

The difference between low flux and high flux dialyzers is the molecular weight (MW) solutes they clear. Low flux removes solutes with MW of 5000 daltons or less (Vit B12, Inulin). High Flux membranes are able to remove solutes in the Middle Molecular Weight range, for example Beta 2 Microglobulin (approx. 12,000 daltons). The surface area is not a determining factor between low flux and high flux. For example : Fresenius has the F4 which is low flux and the F40 which is high flux. They both have the same 0.7 M2 surface area, the permeability of the membranes is what makes the difference. Likewise for the F5 and F50, the F6 and the F60, and the F8 and F80.
Surface area is usually selected by the physician based on patient size and urea distributon volume. Surface area effects small molecular weight clearance. Permeability (KuF) of the membrane is what differentiates low flux from high flux.

I agree with increasing the blood flow until you've maxed it out before increasing dialysate flow. In other words use the largest needle the access will tolerate if it's an AVF or AVG. Set the blood flow rate so that the pre-pump negative pressure does not exceed -200 mmHg (DOQI recommendation). Excessive negative pressure ( > -200 mmHg.) compromises the actual delivered blood flow. If you are not currently monitoring pre-pump arterial pressure you have no way of knowing if the blood flow rate you have set for the patient is what is actually being delivered. (Unless of course you are monitoring blood flow with a Transonics...) This can adversely effect your outcomes. Check out an article in Advances in Renal Replacement Therapy, Vol. 6, No.1, 1999 by Richard A. Ward PhD. titled "Blood Flow Rate: An Important Determinant of Urea Clearance and Delivered Kt/V".

Another parameter that should be evaluated to help maximize the delivered dialysis dose is the heparin. What method of administration is being used ? Systemic, bolus, infusion ?
When was the last time the patient's heparin requirements were evaluated ? Has there been a change in the treatment time ? Change in dialyzer size ? Membrane ? Any changes in medications ? Hematocrit ? Is there an infection present ? A chronic inflammatory process ? All of these and many more factors can play a role in a patient's heparin requirements. They change frequently and should be re-evaluated as indicated.

Increasing the dialysate flow can help to increase dialyzer clearance but should be used judiciously. First the physician needs to prescribe any increase in dialysate flow rate. Second, make sure the dialyzer you are using is capable of handling the higher dialysate flow rate. Check with your dialyzer manufacturer. Keep in mind there is an increase in dialysate consumption when high dialysate flows are used. Plan accordingly. Avoid running out of concentrate during the run and losing dialysis time to conductivity alarms.

Good luck !



[This message has been edited by Patt Peterson (edited 05-04-99).]

Thank you all for the info. Following the mini trial the few patients who had all the data and blood tests recorded showed an improvement in their URR by approximately 5 %. A nurse new to our unit says this was practised in her previous unit and the patients used to fight over who would have the machine that could use higher dialysate flows as they felt so much better. I asked one of the patients if this was the case with him but he said he never feels unwell.

Type of membrane is also important as running higher Qb can do more harm if you use cuprophan than say cellulose triacetate or polysulfone.

Kevin,
Can you elaborate a bit? What do you mean when you say cuprophan causes more harm than CT or PS at high blood flows ?
Thanks.

Patt