Low sodium haemodialysis reduces interdialytic fluid consumption but paradoxically increases post-dialysis thirst

Alister Oliver1, Mark Wright2, Andrew Matson1, Graham Woodrow2, Neil King3 and Louise Dye3

1 Leeds University School of Medicine, 2 Renal Unit, Leeds General Infirmary and 3 Department of Psychology, Leeds University, Leeds, UK

Correspondence and offprint requests to: Mark Wright, Renal Unit, Leeds General Infirmary, Great George Street, Leeds LS1 3EX, UK. Email: mark.wright{at}leedsth.nhs.uk



   Abstract
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. Interdialytic weight gain (IDWG) can be reduced by lowering the dialysate sodium concentration ([Na]) in haemodialysis patients. It has been assumed that this is because thirst is reduced, although this has been difficult to prove. We compared thirst patterns in stable haemodialysis patients with high and low IDWG using a novel technique and compared the effect of low sodium dialysis (LSD) with normal sodium dialysis (NSD).

Methods. Eight patients with initial high IDWG and seven with low IDWG completed hourly visual analogue ratings of thirst using a modified palmtop computer during the dialysis day and the interdialytic day. The dialysate [Na] was progressively reduced by up to 5 mmol/l over five treatments. Dialysis continued at the lowest attained [Na] for 2 weeks and the measurements were repeated. The dialysate [Na] then returned to baseline and the process was repeated.

Results. Baseline interdialytic day mean thirst was higher than the dialysis day mean for the high IDWG group (49.9±14.0 vs 36.2±16.6) and higher than the low weight gain group (49.9±14.0 vs 34.1±14.6). This trend persisted on LSD, but there was a pronounced increase in post-dialysis thirst scores for both groups (high IDWG: 46±13 vs 30±21; low IDWG: 48±24 vs 33±18). The high IDWG group demonstrated lower IDWG during LSD than NSD (2.23±0.98 vs 2.86±0.38 kg; P<0.05).

Conclusions. Our results indicate that patients with high IDWG experience more intense feelings of thirst on the interdialytic day. LSD reduces their IDWG, but paradoxically increases thirst in the immediate post-dialysis period.

Keywords: thirst; weight gain; low sodium haemodialysis



   Introduction
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Reducing the dialysate sodium concentration ([Na]) removes excess sodium from haemodialysis patients and has produced reductions in crude thirst ratings and interdialytic weight gain (IDWG) [1]. We have previously used a palmtop computer to record serial visual analogue scores for thirst. This allowed us to produce thirst profiles and derive summary statistics to compare groups [2]. In this study, we assessed the effect of low sodium dialysis (LSD) on two groups of haemodialysis patients. One had high baseline IDWG, the other low. We sought to compare the thirst patterns of the two groups at baseline and test the hypothesis that LSD would reduce thirst and, therefore, fluid consumption.



   Subjects and methods
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Fifteen stable patients were recruited. Eight had initial weight gains >1 kg/day (mean: 1.55±0.25 kg/day) (HiWG) and seven had initial weight gains <1 kg/day (mean: 0.48±0.23 kg/day) (LoWG). Patients taking angiotensin-converting enzyme inhibitors or angiotensin receptor blockers were excluded. None were diabetic. Most patients dialysed against a sodium concentration of 137 mmol/l, but three used a ‘programmed sodium’ regime with a stepped reduction in sodium concentration halfway through the treatment session (e.g. 142 mmol/l for the first 2 hours then 140 mmol/l for the second 2 hours). These individuals had previously experienced intradialytic symptoms such as hypotension, headache or cramp, which had been attributed to sodium removal during treatment. Such patients were not excluded from the study because they had high weight gains and it was difficult to determine whether their symptoms were due to water removal or sodium removal.

During their dialysis session, the patients were instructed in the use of the palmtop computer. This was programmed to alarm at hourly intervals until 20.00 on the dialysis day and between 08.00 and 20.00 on the interdialytic day. Each time the alarm rang, the patients would open the palmtop and the question ‘How thirsty do you feel?’ would appear on the screen. A line accompanied this with a cursor in the middle. The ends of the line were labelled as ‘not at all’ and ‘extremely’. The patient would then use one of two keys to move the cursor along the line, thus, providing a visual analogue rating of their thirst at that moment. The palmtop stored this data so that the complete recordings could be downloaded to a computer for analysis. The mean of each individual's daily scores was used to generate a group mean score. The mean of the recordings from the 2 h immediately following the end of dialysis is shown as ‘post-dialysis’. For a more detailed description of the technique, see Wright et al. [2,3].

The patients performed an interdialytic urine collection to quantify residual urine output. The IDWG results represent the mean weight gains from the last week of each phase of the study. The sum of the mean IDWG and urine output at each sampling point was taken to represent the interdialytic fluid consumption.

After the baseline recordings, the dialysate sodium concentration was reduced by 1 mmol/l each treatment for five sessions, so, for most, LSD represented dialysis against a [Na] of 132 mmol/l. Patients using ‘programmed sodium’ regimes reduced at a similar rate, so a starting regime of 142 mmol/l for 2 h followed by 140 mmol/l for 2 h would end as 137 mmol/l for 2 h and 135 mmol/l for 2 h. If patients experienced intradialytic symptoms, the dialysate sodium was increased by 1 mmol/l per treatment until the symptoms were abolished. Dialysis continued at the lowest attained sodium concentration for 2 weeks before the thirst profiles and urine collections were repeated. All dialysate sodium concentrations returned to baseline for a month before the process was repeated (normal sodium dialysis, NSD).



   Results
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
HiWG vs LoWG
The HiWG group was younger (mean: 42.6±12.8 vs 68.6±5.1 years; P<0.001) than the LoWG group and had more anuric individuals (five vs one). None of the HiWG group passed > 500 ml of urine per day. The median baseline urine output for the LoWG group was 388 ml per day (range: 0–1000 ml). The HiWG group had been on dialysis for longer than their LoWG counterparts (median: 54 vs 30 months). Table 1 indicates that baseline pre-dialysis serum osmolality was non-significantly higher for the HiWG and displays serum sodium and urea values as well as weight gain data.


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Table 1. The effect of LSD on fluid consumption and serum osmolality

 
When the thirst data were compared, discrepancies were evident (Figure 1, Table 2). At baseline, the HiWG group generated higher thirst scores on the interdialytic day than the dialysis day, whereas the LoWG group produced similar scores on both days.



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Fig. 1. Baseline thirst profiles for the high and low weight gain groups. Each point represents the mean score at that time point, with the error bars showing SEM. Scores were similar during the dialysis day, but the group with the highest weight gain was consistently more thirsty on the interdialytic day.

 

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Table 2. Derived thirst scores

 
LSD vs NSD
The HiWG group had their lowest IDWG and fluid consumption on LSD (Table 1). The LoWG group demonstrated little change in these parameters. The pre-dialysis serum osmolality was non-significantly lower during the low sodium phase for both groups. The HiWG group consistently returned higher interdialytic day scores than the LoWG group throughout the study. Contrary to our expectations, both groups’ dialysis day and post-dialysis scores were highest during the LSD phase.



   Discussion
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
The different baseline thirst profiles of the two groups imply that there may be physiological differences between them. This may simply reflect the age discrepancy between the groups. Fluid consumption decreases with age [4] and the LoWG group were older than their HiWG counterparts. There were other differences, however. The HiWG group had more anuric individuals and a higher serum osmolality (although this was not statistically significant). It might be that residual kidney function metabolizes osmotically active molecules or provides clearance of a small but crucial amount of sodium.

There was a reduced fluid intake on LSD with little evidence of reduced interdialytic day thirst scores. Thus, it appears that thirst was maintained at its usual level by consumption of less fluid, but it was not reduced as claimed previously [1]. Indeed, there was a notable increase in post-dialysis scores in both groups, suggesting that a different drive to thirst perception might have been activated. LSD might have reduced the vascular refilling rate, such that intravascular volume depletion was greater then during normal dialysis. This might have stimulated thirst via central baroreceptor mechanisms or by activating the renin–angiotensin system.

In summary, it appears that LSD reduces fluid consumption for patients with high IDWG without causing a noticeable reduction in thirst. Indeed, LSD caused a marked increase in post-dialysis thirst for all participants.



   Acknowledgments
 
We would like to thank the patients and staff of the dialysis unit at Leeds General Infirmary for their help with this study. The support of John Blundell, Aleck Brownjohn and John Turney was greatly appreciated.

Conflict of interest statement. None declared.



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

  1. Raj Dominic SC, Ramachandran S, Somiah S, Mani K, Dominic SS. Quenching the thirst in dialysis patients. Nephron 1996; 73: 597–600[ISI][Medline]
  2. Wright MJ, Woodrow G, O’Brien S et al. Polydipsia: a feature of peritoneal dialysis. Nephrol Dial Transplant 2004; 19: 1581–1586[Abstract/Free Full Text]
  3. Wright MJ, Woodrow G, O’Brien S et al. Disturbed appetite patterns and nutrient intake in peritoneal dialysis patients. Perit Dial Int 2003; 23: 550–556[ISI][Medline]
  4. Stachenfeld NS, DiPietro L, Nadel ER, Mack GW. Mechanism of attenuated thirst in ageing: role of central volume receptors. Am J Physiol 1997; 272: R148–R157[ISI][Medline]
Received for publication: 17. 3.04
Accepted in revised form: 24. 6.04





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