By Peter Laird, MD
Dr. Prince and I seem to be talking past each other with our comments when in many ways we do not actually disagree. I placed dietary salt compliance as the number one issue with intradialytic hypotension in my post as an area that the RFN post did not address but should have. Dietary noncompliance is a difficult issue to deal with of which I do not in the least disagree with Dr. Prince's comments that they are a contributing factor to this problem. Likewise, salt and fluid control by the patient was one of Dr. Scribner's highest ranking issues which Bill can personally attest; Anna added a post on salt restriction today to DSEN.
However, quotidian dialysis regimens, especially daily nocturnal, has shown that dietary restrictions can be nearly completely eliminated including relaxing the sodium limits. High dose dialysis allows lower UFRs due to longer runs and/or more frequent runs and less time between treatments. To the initial point I believe it is these high ultrafiltration rates that are the most important factor causing intradialytic hypotension.
Granted, some patients simple will not adhere to dietary sodium and fluid restrictions. And these may also be those patients that do not want to dialyze another moment. Nevertheless, even these patients (or especially these patients) could benefit from adding two dialysis treatments a month and those treatments should be offered. The implementation of an every other day dialysis schedule would eliminate the worst day of the conventional dialysis schedule, the day after the dialysis weekend when we see the highest level of fluid overload and we have our highest rate of cardiac deaths. (here and here)
So, while I agree that salt control is part of the solution, I would hope, actually plead, that we all take a look at the conventional American dialysis protocols. These protocols are perhaps an even larger part of the problem than patients that won't comply with salt restriction and fluid requirements. Simply put, short dialysis sessions which cause the sudden, violent depletion of the vascular compartment are the primary causation of intradialytic hypotension. We all should keep in mind and review Dr. John Agar's information on fluid diffusion rates (pdf link) to understand that the ultrafiltration rates American dialysis centers routinely subject patients to are higher than the physiologic fluid compartment diffusion rate can accommodate. From Agar:
This “waterfall,” of fluid moving from one compartment to another, is rate-limited to a maximum of 350 to 400ml/hr. It takes time for fluid to shift in your body so all is in balance. If fluid is pulled out of the blood too quickly during a standard in-center dialysis treatment, the rest of the chain reaction just can’t keep up.
Yes, if some one eats at McDonalds all week, all of that salt and extra fluid is going to be sitting in the interstitial spaces throughout their body. Any dialysis regimen that exceeds the basic diffusion rates of interstitial fluid back into the vascular compartment will cause intradialytic hypotension which must be considered even and perhaps most especially with noncompliant patients.
It is time, it is more than time, to come back to the basics of human physiology and recognize that short, rapid fluid removal is dangerous for anyone even if they are not adhering to good advice on salt restriction. It is time to recognize that there is a rate limiting diffusion process involved in ultrafiltration of fluid overloaded patients that should never be exceeded during fluid removal in usual chronic hemodialysis care. It is my opinion that it is not only harmful to continue this practice but also cruel to continually subject patients to violent fluid removal leading to the "usual" post dialysis symptoms when they can be avoided by simply acknowledging the basic interstitial to intravascular diffusion rates and then incorporate this into dialysis practice.