Correct calibration of delivered sodium dialysate concentration

Edwards C, Singh S, Power A, Griffiths M, Cairns T, Taube D, Duncan, N
Imperial College Kidney & Transplant Institute, DuCane Road, London
Sodium balance is integral to intradialytic cardiovascular stability and good interdialysis blood
pressure control. Hypernatraemic dialysis risks positive sodium balance and increases thirst and
hypertension. Hyponatraemic dialysis may lead to negative sodium balance and distressing
symptoms on dialysis. There is a large inter-patient variability in pre-dialysis serum sodium
concentration, [Na]d , at our centre the range is 125-147mmol/l, mean 140 ± 2.9 mmol/L. 37%
(85/231) of our patients had a pre-dialysis serum sodium of <140mmol/l. Our standard [Na]d is
140mmol/l, but this may result in positive sodium balance in individual patients, with a negative
impact on blood pressure control. Dialysis machines measure dialysate Na using ionic
conductance between sensors
As part of a quality control procedure prior to a clinical trial we measured delivered [Na] d on
seventy-one Braun Dialog dialysis machines and assessed the effect of this on blood pressure
control in 231 patients at a single haemodialysis centre. The dialysis machines were set to
deliver [Na]d of 140mmol/L by conductivity and were then sampled cyclically over a 6 month
period. Sampling comprised of 3 aliquots of dialysate, these were analysed using flame
photometry and the mean dialysate sodium was derived.
There was a significant decrease in mean [Na]d from 143.5 +/- 2.1 pre calibration to 141.5 +/1.4 mmol/l post calibration (p<0.0001), figure 1. There was a significant decrease in mean
number of anti-hypertensive agents, from 1.86 pre calibration to 1.66 post calibration (p<0.05),
figure2. Although there was a trend towards lower pre dialysis systolic blood pressure,
142mmHg v 138mmHg this did not reach significance (p=0.07). There was no change in
diastolic blood pressure pre and post re-calibration.
Figure 1: Mean [Na]d
Figure 2: Use of antihypertensive agents
The discrepancy between delivered and prescribed [Na]d was ascribed to build up of precipitant
upon the electrodes, reducing their accuracy. The clinical impact of quality control of [Na]d in
this study was a significant decrease in the number anti-hypertensives prescribed whilst
maintaining similar blood pressure control.