By Peter Laird, MD
I recently attended the Annual California Dialysis Council conference in Palm Springs, CA which was my first visit to that city. As Lori Hartwell said, the weather was great and the conference had many interesting presentations. I was able to catch the last presentation which featured three chief medical officers from FMC, Davita and Innovative Dialysis Systems. Unfortunately, Dr. Nissenson was not able to join Dr. Lazarus and Dr. Vincent Dennis. The most notable question dealt with ultrafiltration rates and recent studies showing UF rates higher than 13 ml/kg/hour are associated with several adverse reactions.
Dr. Lazarus noted first of all that we cannot escape the central fact that longer duration and more frequent dialysis is the answer to many of these issues that other surrogate targets will never equal in outcomes. However, he couched that answer by stating we don’t have a target like Kt/V to guide therapy. Dr. Dennis echoed the sentiments of Dr. Lazarus that the evidence is clear we can no longer ignore that longer and more frequent dialysis has improved outcomes. The response from the head of Davita in my opinion missed the mark all together talking about how we don’t to date truly understand the etiology of interdialytic weight gain and focused his comments on the many different conditions that lead to elevated weight gains.
Yet, is it really true that we do not have a target so to speak for ultrafiltation rates when it is known from more than one study that keeping UF rates at or below 10 ml/kg/hour reduces complication significantly and improves survival. In addition, granted that interdialytic weight gain is caused by several disparate factors, but the simple fact is that what ever the cause of the fluid gain, we are dealing with a finite 5 liter vascular compartment that must refill during dialysis from the intracellular and extracellular compartments to maintain adequate perfusion pressures and blood pressure. The rate limiting factor is the refill rates from these other tissues. The studies show in simplicity that when the ultrafiltration rates exceed 10 ml/kg/hour, that rate of fluid removal will in the setting of a 3-4 hour dialysis session exceed the refill rates from the other compartments and the patient will become symptomatic. Dr. John Agar in an article on this issue at Home Dialysis Central describes well the factors involved:
Time-limited fluid shifts
How fast can the blood volume be replaced from the interstitium during a treatment? Well, it depends. Lots of factors make a difference:
Your blood protein (albumin) level
How healthy your heart is
How big or small you are
How “leaky” your smallest blood vessels (capillaries) are to fluid
If you assume a maximum rate of fluid removal (ultrafiltration rate or UFR) of 350 to 400ml/hour for an average sized person, you won’t be far off:
If your UFR is less than 400ml/hr, interstitial fluid can refill your blood as fast as dialysis removes it. So, your blood volume won’t drop and your blood pressure will be stable.
If your UFR is higher than 400 ml/hr, your interstitium can’t keep up. Your blood volume must fall and your blood pressure will drop, too. (You’ll feel awful.)
The more your UFR exceeds 400ml/hr, the greater the gap between fluid loss and refill. The greater the gap, the higher the risk of affecting your blood pressure.
Thus, instead of focussing on the many ill defined variables that contribute to interdialytic weight gain alone, the focus needs to be on the simple physiology of fluid removal from the vascular compartment and the refill rates from the intracellular and extracellular compartments. Understanding the sophistication of most nephrologists on minute changes of the differing elements of renal physiology and the terror I felt as a medical student and as a resident in training presenting to these very precision based specialists, I am appalled that one of the easiest to understand principles of fluid diffusion is ignored in its application to dialysis and ultrafiltration rates. The optimal ultrafiltration rate is thus defined by this innate physiology of fluid mechanics in the human body. Perhaps it is time to simply go back to the basics for this complex question. Just as phosphorus and middle molecules are time dependent variables of dialysis, so likewise are ultrafiltration rates.