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"To Two Pool, or Not to Two Pool-That is the Question"
by Mike Baumann
If you think urea reduction (URR) - or KT/V from a laboratory is the best thing since sliced bread, don't read this article. Instead, read the National Instititutes of Health (NIH) Consensus statement i , the Renal Physicians Association Clinical Practice Guidelineii , or see the DOQI Guideline #2 "Method of Measurement of Delivered Dose of Hemodialysis. All three conclude that URR or a nonKinetic KT/V can be off by as much as 40% and urea kinetics should be used. There's also Owen's classic Articleiii correlating mortality with nutrition and Lindsay's workiv showing that the Protein Catabolic Rate (PCR) from urea kinetics is the most sensitive indicator of malnutrition - up to three months "faster" than serum albumin changes.
The purpose of this article is to help explain what Two Pool Urea Kinetics is, when to use it, and how NOT to use it.
We know that the majority of toxins in the human system are in body water. There are many different pools of water in the body, the blood, and the different cells of the body. To simplify things, we'll assume that all the pools of water in the cells act the same. So, we're dealing with two pools of body water-in the blood and the cells.
The body water is constantly moving back and forth between the two pools. This rate of movement from the cells into the blood can be measured. We'll call it the "Flow Rate" (FR).
In most patients, the FR is the same and is greater than the rate that the dialyzer removes toxins from the blood. But this is not Always the case. For some unknown reason, some patients, particularly children, have a reduced FR, and some studies have shown that the FR can vary dramatically among patientsv. Also, it's possible with high flux dialysis to remove toxins at a rate higher than the FR. It's these patients who need to use Two Pool Urea Kinetics.
You're probably aware of the "rebound" concept. If you draw a Blood Urea Nitrogen (BUN) at the end of dialysis and compare it to the BUN drawn 30 minutes later, the last BUN drawn will be higher. Here the FR is trying to bring the blood pool into equilibration with the cell pool.
"So what? Why worry about Two Pool?" you might ask. The answer is that, using the normal urea kinetics of Gotch and Sargent, Two Pool is not taken into consideration, and the computed KT/V will be falsely high resulting in underdialysis of the patient. The same problem exists with a URR or a KT/V computed using a linear regression equation such as the Daugirdas II equation.
BEFORE you even worry about Two Pool, you had better make sure the Post BUN is drawn Correctly. According to DOQI:
* Draw it immediately after the dialysis is over
* Drop the blood pump to a low value (50-100)
* Wait 15 seconds
* Draw the BUN from the arterial sleeve
Some health care professionals wait 30 minutes after dialysis, then draw the Post BUN. They assume they have obtained the equilibrated BUN and have corrected for the Two Pool effect.
DON'T DO THIS!
While, in theory, this is the correct approach, there's a problem. It is not know when the patient reaches the equilibrium between the two pools. It's almost impossible to tell when the equilibrium between the two pools takes place. With some patients, it may be only 30 minutes, while with others it may take 1 hour. Therefore, waiting 30 minutes and taking the BUN may not only produce an incorrect KT/V, but it will also make it impossible to compare the results with any of the National Studies which have determined that 1.2 KT/V is the magical number.
Since the two pools need to equilibrate, all patients have a rebound after dialysis. The rebound in the patient who doesn't need Two Pool is lower than the one who needs it, but is still occurs. The Gotch methodology is designed to use the "unrebounded" Post BUN.
Previously, doing Two Pool Urea Kinetics was impractical and not recommendedvi. This was due to the problem of determining when equilibrium had taken place. The only option was to have the patient remain 1 hour after dialysis, and then to obtain a rebounded BUN. Of course this is unrealistic and relegated Two Pool for research only until Dr. Smye changed all this with his new modelvii. With Dr. Smye's method, one additional BUN was drawn at 70 minutes into the run. His equations would then compute the FR, and the rebounded BUN WITH OUT the patient having to wait until rebound was completed!!!
Since we now have a method to easily compute Two Pool Urea kinetics, the $64000 question is which patients need Two Pool? Dr. Smye's excellent work clearly shows this is a MUST for children. I strongly disagree with the DOQI conclusion that since there have been no published studies showing an increased longevity of children using Two Pool that they can not recommend it for children. By time such studies are completed, it will be too late for children who are now on dialysis! The real issue for pediatric dialysis is not whether to use Two Pool, rather what is a good goal Two Pool KT/V for children.
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