Choosing a Dialysate Sodium Concentration for Hemodialysis Patients

Abstract

One of the key goals of hemodialysis is to control sodium balance and volume status. The traditional view is that inter-dialytic sodium gains can be adequately controlled by ultrafiltration with the convective removal of sodium. However, dialyzing all patients using the same dialysate sodium concentration may potentially lead to excessive losses on the one hand and sodium gains on the other depending on dietary sodium intake, resulting in increased intra-dialytic hypotension and cramps, or greater inter-dialytic weight gains and hypertension.

If a 75 kg patient had 2.0 L of ultrafiltrate removed each dialysis session, then around 810 mmol of sodium would be cleared weekly. Thus, by ingesting around 120 mmol of sodium/day, they would then be in balance. However, society has changed, particularly in North American and Western Europe. As in the 1970s and 1980s, most people cooked their own food, using fresh ingredients, whereas nowadays there is an increasing reliance on fast food, pre-prepared meals, and use of sauces and condiments. So, dietary sodium intakes have increased, which has led to increased interest in using lower dialysate sodium concentrations potentially promoting an additional diffusional loss of sodium. Observational and randomized studies reported lower dialysate sodium of 136–137 mmol/L, resulted in lower inter-dialytic weight gains compared to ≥140 mmol/L, but greater intra-dialytic hypotension [1,2,3,4,5,6,7]. However, meta-analysis failed to demonstrate the significant effect of dialysate sodium on blood pressure or weight gains [8]. Interestingly, many single-center studies did demonstrate an effect, whereas most multi-center studies did not [9,10]. Different dialysis machine manufacturers use different technology to adjust the mixing of dialysis water, bicarbonate, and acid solutions, and whether they have a positive feed-back loop to continually adjust the delivered dialysate sodium compared to that set for the dialysis prescription [10,11]. These variations coupled with differences in the sodium concentration in the acid and bicarbonate solutions from different suppliers may account for the differences in results reported between single-center and multi-center studies [10,12].

So, how should one choose a dialysate sodium concentration for a dialysis center?

Studies modelling the effects of lower dialysate sodium (136 mmol/L) demonstrated that blood pressure can be lowered for older patients, with a greater effect for women than men, but there would be no such effect on blood pressure when using a dialysate of 140 mmol/L [13]. Older patients generally eat less dietary sodium, and women less than men [14]. So, adding a diffusive loss of sodium in patients with a lower dietary sodium intake can lead to a reduction in blood pressure, but adding a diffusional loss of sodium is not so effective for those with a higher dietary sodium intake. As such, although we choose a lower dialysate sodium of 136–137 mmol/L for our dialysis centers, this is increased to a dialysate sodium to 139–140 mmol/L for older frail patients, typically those aged 75 years, as well as those with a clinical frailty score of 5 or more [15] and patients with reduced nutritional intakes.

Should dialysate sodium be used for patients with intra-dialytic symptoms?

Previous studies have ramped up the dialysate sodium concentration to compensate for the fall in plasma osmolality as urea concentration falls, so reducing intra-dialytic symptoms [16]. However, meta-analyses have failed to demonstrate any beneficial effects, and ramping up the sodium risks sodium retention with increased thirst and inter-dialytic weight gains [17]. This led to the concept of isonatric dialysate, matching the dialysate sodium to the patients’ serum sodium [18]. However, with the increasing number of diabetic patients, there are a number of confounders as laboratories typically estimate serum sodium activity, making this approach less reliable due to the effect of higher glucose concentrations on the assay [19]. An alternative is to use dialysis machine technology to estimate serum sodium activity during dialysis and then adjust the dialysate sodium accordingly [20,21]. I suspect that this will be a technological solution for patients with intra-dialytic symptoms in the future; however, in the meantime, careful re-assessment of target weight and review of the dialysate sodium prescription on a regular basis remain the mainstay of treatment.