Sodium Modeling

Many modern dialysis machines offer options for ultrafiltration (UFP) and sodium profiling (or sodium variation system [SVS]). User interest in these options and the eventual response of industry was driven by an attempt to improve tolerance for ultrafiltration as dialysis session time became shorter and an associated increase in patient intolerance for the consequent higher UF rate was observed.

The concept behind UFP is simple: vary the UF rate in such a way as to favor improved vascular refilling. The principles behind SVS are more complex. One idea is that a high dialysate sodium at the beginning of dialysis, when solute removal rate is maximal, would deliver osmolal solutes to plasma water and combat any tendency for movement of extracellular water into the intracellular space. A second is that diffusion of sodium from dialysate into plasma water would exert an osmotic effect tending to increase plasma volume filling. Another basis for SVS is that even a small consequent rise in plasma water sodium concentration exerts an effect to increase peripheral vascular resistance.

In SVS, the sodium content of the dialysis fluid is varied during the course of the treatment to directly influence plasma sodium levels. The intent is to slow the rate at which sodium leaves the bloodstream into the dialysate.

There can be negative consequences to a high dialysate sodium concentration. Sodium can accumulate in the patient leading to an increased post-dialysis thirst, increased interdialytic weight gain and the development of hypertension. SVS was developed to achieve the benefits of high plasma sodium levels while at the same time avoiding unnecessary high intradialytic sodium uptake by the patient with the associated risk of sodium loading. SVS allows modification of the plasma osmolality during the dialysis session to minimize intradialytic side-effects while removing the amount of sodium necessary to avoid sodium overload.

In isonatremic dialysis, the sodium concentration in the dialysate should be higher (approximately 2-3%)  than in the plasma to avoid net diffusive loss and gain of sodium1.

When the difference in concentration exceeds this, the plasma sodium concentration increases linearly with increasing concentration gradient between dialysate and plasma, and at a rate which depends on the body weight and total body water. The minimum and maximum dialysate sodium concentrations are generally limited for physiological and technical reasons (i.e. 130-155 mmol/L in Fresenius Medical Care-NA machines). It is recommended not to exceed 4-5 mmol/L. SVS is only advantageous for the patient in the long-term if it results in a neutral sodium balance.

Interdialytic water and sodium loads must be corrected on an individual basis because they vary from patient to patient and from one session to the next. Therefore, a post-dialysis plasma sodium concentration target must be determined for each individual patient. To determine the optimal target, the clinical circumstances (absence of hypertension and fluid overload), the individual pre-dialysis sodium level and overall sodium balance of the patient must be taken into consideration. This is easiest for a stable, normonatremic patient as the sodium concentration post dialysis should generally be the same as that pre-dialysis.

A normal sodium value post dialysis can also be reached by SVS for hyper- or hyponatremic patients. However, monitoring of the patient’s clinical status will indicate if post dialysis normonatremia is indicated. For example, hypernatremic patients often stabilize at a sodium level that is higher than the post dialysis level and may suffer side effects if hyponatremic dialysis is conducted with the intention of lowering their sodium levels. Nevertheless, gradual normalization of plasma sodium levels is necessary in this patient group if hypertension due to sodium and fluid overload exists.

Hyponatremic patients often regain their lower plasma sodium concentrations during the interdialytic interval due to thirst and a subsequent increased water intake. This occurs from hypernatremic dialysis sessions that are conducted with the intention of normalizing post dialysis plasma concentrations. This could be especially dangerous for patients in which hyponatremia is secondary to water overload and/or is associated with heart or liver failure.

Types of Sodium Modeling Profiles

There are a number of sodium modeling profiles that have been proposed for different clinical situations.

  • Decreasing profiles. These profiles are the most widely used because you can take advantage of the positive effects of high dialysate sodium concentrations on plasma osmolality and plasma refilling at the beginning of the treatment, when ultrafiltration is best tolerated. Sodium accumulation at the end of the treatment may be prevented by the then lower dialysate concentration. The effects of linearstep and exponential profile forms have been examined in some studies. Post dialysis hypotension5 and early intradialysis hypotension6 were best attenuated by decreasing, stepped profiles, while linear profiles sometimes reduced muscle cramps5 and late intradialytic hypotension6. However, these positive effects were not seen in other studies7,8.
  • Alternating profiles.  These profiles were introduced in the early eighties with the aim of inducing an alternating fluid shift across the cellular membrane to improve the transport of uremic toxins out of the cell by solvent drag. The incidence of disequilibrium syndrome was found to be reduced with the use of these profile types1,2.
  • Increasing profiles.  These profiles are less commonly used; they can preserve the plasma volume during the last period of dialysis in cases where the ultrafiltration is still high and have been reported to be especially beneficial in reducing the incidence of muscle cramps3,4, probably because of reduced sodium removal1. Thus, caution should be exercised when using this profile. Increasing and alternating profiles are not provided by Fresenius Medical Care – NA machines.

Zhou et al. recently evaluated the effects of sodium balance-neutral sodium profiling, UF profiling and their combination on preservation of blood volume, cardiac function and occurrence of hypotensive episodes using a Fresenius 4008S machine9. They observed that with similar intradialytic sodium removal with the use of a sodium balance-neutral linearly decreasing sodium profile combined with linearly decreasing UF profile, greater intradialytic stability of the blood volume, blood pressure and cardiac function could be obtained and hypotensive episodes were significantly reduced.

Accurate SVS requires the ability to predict the final plasma sodium concentration that would be achieved by each profile type and techniques to accurately follow the changes in plasma sodium levels during the session. Mathematical models are available for the prediction of post dialysis plasma sodium levels that result from the profiles10,11.

In summary, UFP is the optimal choice to maintain cardiovascular stability during fluid removal as the UF rate can be varied to improve vascular refilling. Although there are many controversies surrounding it, SVS  may support fluid removal when used appropriately in carefully selected patients or in combination with UFP.  SVS should always be balance neutral and may not be necessary with longer or more frequent treatments.


  1. Van Stone JC. Individualization of the dialysate prescription in chronic hemodialysis. Dial Transplant 23:624-635, 1994
  2. Kouw PM, Olthof CG, Gruteke P, De Vries PMJM, Meijer JH, Oe PL, Schneider H, Donker AJM. Influence of high and low sodium dialysis on blood volume preservation. Nephrol Dial Transplant 6:876-880, 1991
  3. De Vries PMJM. Fluid balance during hemodialysis and hemofiltration: The effect of dialysate sodium. Nephrol Dial Transplant 5(Suppl 1):158-161, 1990
  4. De Vries PMJM, Olthof CG, Solf A, Schuenemann B, Oe PL, Quellhorst E, Schneider H, Donker AJM. Fluid balance during haemodialysis and haemofiltration: The effect of dialysate sodium and a variable ultrafiltration rate. Nephrol Dial Transplant 6:257-263, 1991
  5. Sadowsky RH, Allred EN, Jabs K. Sodium modeling ameliorates intradialytic symptoms in young dialysis patients. J Am Soc Nephrol 4:1192-1198, 1993
  6. Raja RM. Sodium profiling in elderly hemodialysis patients. Nephrol Dial Transplant 11(Suppl 8):42-45, 1996
  7. Sang GL, Kovithavongs C, Ulan R, Kjellstrand CM. Sodium ramping in hemodialysis: A study of beneficial and adverse effects. Am J Kidney Dis 29: 669-677, 1997
  8. Acchiardo SR, Hayden AJ. Is Na+ modeling necessary in high flux dialysis? Trans Am Soc Artif Intern Organs 37:M135-M137, 1991
  9. Zhou YL, Liu HL, Duan XF, Yao Y, Sun Y, Liu Q. Impact of sodium and ultrafiltration profiling on haemodialysis-related hypotension. Nephrol Dial Transplant 21:3231-3237, 2006
  10. Petitclerc T, Trombert JC, Coevoet V, Jacobs C. Electrolyte modeling : Sodium. Is dialysate sodium profiling actually useful ?  Nephrol Dial Transplant 11(Suppl 2):35-38, 1996
  11. Di Filippo S, Corti M, Andrulli S, Manzoni C, Locatelli F.  Determining the adequacy of sodium balance using a kinetic model.  Blood Purif 14:431-436, 1996