This study has established that there is no association between dialysate sodium concentration and interdialytic weight gain. Furthermore, there was no association between dialysate sodium concentration and blood pressure. This was similar to what was found by Beduschi et al when comparing DNas of 135 and 138 meq/l where there was no significant difference in the interdialytic weight gain or the blood pressure (11). This was also the case in the study by Thein et al where there was no significant difference in the interdialytic weight gain between dialysate sodiums of 141 and 138 meq/l, however there was a significant reduction in blood pressure with the lower dialysate sodium used (12). As much as there was no statistical significance in the interdialytic weight gain and blood pressure between the two groups, there could still be clinical significance as both parameters were lower in the lower dialysate sodium group.
It is important to note that there was no exclusion of patients with residual renal function in this study, yet residual renal function (defined as urine output of greater than 200ml/day) theoretically has a physiologic role in sodium balance (13). Ipema et al established that patients with residual diuresis had significantly lower interdialytic weight gain (14). However in terms of outcomes, Hecking et al noted no difference in mortality in patients with or without residual renal function in spite of the dialysate sodium concentrations used (10). In the present study, there was no difference in the outcomes in patients with or without residual renal function.
It has also been established in some studies such as that by Titze et al that large amounts of sodium can be accumulated without water retention by the sodium ions binding to extracellular matrix components such as glycosaminoglycans (15). In addition, there are other sodium reservoirs in the body such as the bone, skin cartilage and connective tissue and as a result, lowering the dialysate sodium concentration could have caused loss of sodium without loss of water and had no impact on the interdialytic weight gain and blood pressure (11).
There is also theory that every individual has their own individual osmolar setpoint based on parameters such as dietary salt intake, urinary sodium excretion, tissue sodium stores as well as physiologic response of the body to sodium. For this reason, a change in the dialysate sodium concentration may not have that much of an impact on the interdialytic weight gain and blood pressures unless the sodium level is individualised. This was shown in a study conducted by Radhakrishnan et al where they compared a set dialysate sodium concentration of 140meq/l to an individualised dialysate sodium concentration. There was a significantly lower interdialytic weight gain and pre-dialysis systolic blood pressure in those patients who had an individualised dialysate sodium concentration in comparison to the standard dialysate sodium concentration of 140 meq/l (16).
This study showed no significant difference between the systolic or diastolic blood pressure in both dialysate sodium concentration groups. Charra et al described the concept of ‘lag time’, whereby it takes several months for the correction of the extracellular volume overload (in our case from the high sodium) to manifest as improvement in blood pressure (18). This study was 12 weeks long and therefore patients may not have completed this lag time.
An important finding on the study was the average interdialytic weight gain regardless of the dialysate sodium concentration used. The mean interdialytic weight gain was 2.14 kg and 2.35 kg for dialysate sodium 137meq/l and 140meq/l respectively. Comparing to other studies, for instance the PanThames renal audit done by Davenport et al undergoing dialysis three times a week showed an interdialytic weight gain range of 1.7 to 2.75 kg (9). Therefore, interdialytic weight gain in the this study is comparable to the interdialytic weight gains in the audit, despite the fact that our patient population was on twice weekly as opposed to thrice weekly dialysis. This is an interesting finding given that there are significant resource constraints in Kenya and therefore patients who are recommended to dialyse thrice weekly are dialysing twice weekly since the NHIF only covers dialysis twice a week. This raises the question as to whether twice a week dialysis is sufficient for our population considering the interdialytic weight gain is not drastically high, and in effect mortality and adverse cardiovascular outcomes may also not be so high. This also raises the question as to whether our patient population is adhering to fluid restriction and salt restriction practices more than patients in other parts of the world or whether diuretics in patients with residual renal function have an effect on this.
It is recommended that the same study be conducted using a larger sample size to assess the association between dialysate sodium concentration and interdialytic weight gain. It would also be beneficial to do a comparison study of the same outcomes of interdialytic weight gain and blood pressure control in patients on twice weekly versus thrice weekly dialysis to ascertain whether the different dialysate sodium concentrations are affected by the frequency of dialysis.