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Department of Nephrology, North Staffordshire Hospitals Trust, Stoke-on-Trent, United Kingdom.
Correspondence to Dr. Simon J. Davies, Department of Nephrology, North Staffordshire Hospitals Trust, Princes Road, Hartshill, Stoke-on-Trent, ST4 7LN, UK. Phone: 01782-554164; Fax: 01782-620759; E-mail: SimonDavies1{at}compuserve.com
| Abstract |
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| Introduction |
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Exposure to hypertonic glucose dialysis solution, which also contains glucose degradation products that enhance the formation of advanced glycosylation end products (AGE), has long been suspected as a mechanism of peritoneal membrane injury. There is a growing body of circumstantial evidence to support this, including AGE deposition within the membrane (7,8,9), diabetiform changes in peritoneal blood vessels (10,11), and the finding that sclerosing peritonitis is associated with the use of more hypertonic exchanges (12). In the study by Hendriks et al. (12), the increased use of hypertonic glucose was evident in the first year of treatment. Recent observations in animal models demonstrated neovascular changes typical of diabetes (11), and early reports from the Peritoneal Biopsy Registry have indicated vascular injury indistinguishable from diabetes that worsens with prolonged periods on dialysis (13). It is likely that there is a link between these morphologic findings and the functional changes with time on treatment. The problem in defining a causative role for hypertonic glucose and the acquisition of functional changes in the membrane is complicated by the need to use more hypertonic exchanges as ultrafiltration failure develops. To clarify which comes first, it is desirable to demonstrate that the use of hypertonic glucose exchanges precedes the changes in peritoneal membrane transport characteristics. The present study reports the detailed evolution of peritoneal kinetics, RRF, peritonitis history, and exposure to glucose, gathered prospectively in a cohort subset of long-term PD patients.
| Materials and Methods |
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Longitudinal changes in peritoneal solute transport were used to categorize this subset into patients with stable solute transport within 0.1 (group 1) and those with increasing solute transport consistently >0.1 of that on commencing treatment (group 2). Patients from these two groups were then compared for a number of dialysis-related factors, including RRF, glucose exposure, peritonitis rate and severity, achieved ultrafiltration and dialysis dose, and demographic factors. Throughout the study period, the dialysate buffer used was lactate, either 35 or 40 mmol/L.
Peritoneal Equilibration Test
The peritoneal equilibration test, performed as described previously
(14,15),
was used to estimate solute transport. Patients were tested at least once a
year, and usually every 6 mo. When more than one test was performed in a year,
the mean value was used for analysis. Briefly, a standard 4-h dwell period was
used (first exchange of the day), using a 2.27% glucose concentration 2-L
volume exchange. The patient used his or her usual overnight dialysis regime,
and both the overnight and test drainage volumes were measured. The
D/Pcreat at the completion of the 4-h dwell period is an estimate
of low molecular weight solute transport. D/Pcreat correlates well
with the mass transfer area coefficient for creatinine but is not identical,
because of the variable influence of convection
(16). As glucose interferes
with the assay for creatinine in a linear fashion, concentrations for both of
these solutes are measured at 4 h and the true value for creatinine obtained
by subtracting the glucose concentration is multiplied by a correction factor
derived locally by our laboratory (0.47). With the use of this method, the 4-h
D/Pcreat is a highly reproducible measure of low molecular weight
solute transport across a wide range of values (0.45 to 0.9), in the short
term (3 mo or less, provided that there has been no clinical event such as
peritonitis or surgery), with a coefficient of variation of 4.8%.
Glucose Exposure
Glucose exposure was expressed in two ways. First, the total annual
exposure to glucose was calculated from the dialysis regime reported every 6
mo. The product of the volume and the glucose concentration for each exchange
was calculated. For example, for an individual who was using 4 x 2 L
exchanges (2 x 1.36%, 1 x 2.27%, and 1 x 3.86%), there would
be 54.4 + 45.4 + 77.2 = 176.8 g of glucose per day. This is equivalent to
64,532 g of glucose per year. The second method was a simple tally of the
number of hypertonic (3.86%) exchanges used per day.
Dialysis Dose (Kt/V)
The dialysis dose was assessed by calculating the weekly
Kt/Vurea from the 24-h urinary and dialysate clearance, by direct
measurement of urea in urine and each dialysate exchange. The volume of
distribution for urea was calculated as 58% of the body weight. Results are
expressed as the total weekly Kt/Vurea (peritoneal component) or
for the RRF alone.
Peritonitis
Peritonitis was defined as a dialysate white cell count >100 cells/
µ1, and all episodes were treated empirically with intraperitoneal
vancomycin and gentamicin until sensitivities were known. Dialysate white cell
count was documented on presentation, day 3, and day 12. Definitions of
recurrence, same organism within 30 d, and clusters of episodes within 3 mo
were used as described previously
(4).
Analytical Methods
Plasma and dialysate concentrations of urea, creatinine, and glucose were
determined on an automated discrete random access analyzer (DAX 72, Bayer
Instruments, Basingstoke, UK). Urine and dialysate total protein estimations
were made using the biuret method. Plasma albumin levels were measured using
the Bromocresol green method.
Statistical Analyses
Comparison of demographic data between multiple groups was made using ANOVA
(parametric) and Kruskal-Wallis (nonparametric variables, e.g.,
comorbid score). Comparison between groups 1 and 2 of solute transport,
glucose exposure, and peritoneal Kt/V was made using an unpaired t
test, and for RRF and use of hypertonic exchanges using the Mann-Whitney test.
Longitudinal changes in solute transport were tested for using a paired
t test.
| Results |
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Of the 25 patients who were on PD for 5 yr at censor, 22 had been on treatment without significant interruption, i.e., period on hemodialysis or with functioning transplant. Of these, 13 had solute transport kinetics that were within 0.1 of that at the start of treatment (group 1) and 9 had a sustained increase of >0.1, equivalent to >15% above baseline (group 2). The demographic details of these two groups at the start of treatment, which did not differ significantly, are shown in Table 1.
The longitudinal changes in solute transport are shown in Figure 2A. It can be seen that the mean solute transport in group 1 is stable throughout the 5-yr period, being 0.67 (±0.1) at the start and 0.67 (±0.08) at 5 yr. In group 2, for the first 2 to 3 yr of treatment the mean solute transport remains relatively low, followed by an increase from 0.56 (±0.08) at the start to 0.78 (±0.09). As would be anticipated from the definition of this group, the increase was statistically significant (P < 0.01). There were no significant differences in plasma albumin or dialysis-delivered Kt/V between groups throughout the study.
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The details of peritonitis, including intensity of inflammation, causative organism, and clinical outcome, are summarized in Table 2. It can be seen that the overall severity of the peritonitis record was similar in both groups. If anything, the overall picture is slightly worse for the stable group 1 patients, with a higher peritonitis rate and a tendency to be associated with recognized pathogens.
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One of the most important differences between these two patient groups was their RRF, in terms of both urea clearance (Figure 3A) and urine volumes (Figure 3B). Patients in group 2 had less RRF on commencing treatment, and they lost what was remaining at an earlier stage of treatment. Patients who were on this dialysis program before the censor in January 1998 did not have their PD dose increased to compensate for losses in residual renal clearances. It is apparent, however, that they did have their dialysis regimes altered in terms of glucose tonicity to manipulate achieved peritoneal ultrafiltration. The summary of peritoneal clearances (Kt/V), urine volumes, and achieved peritoneal ultrafiltration in Figure 3 indicate that the loss in urine volume was compensated for by an increase in ultrafiltration. It is of interest that both groups had remarkably similar total fluid losses, which were sustained throughout the 5-yr treatment period.
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These differences in achieved peritoneal ultrafiltration were associated with an increased use of hypertonic (3.86% glucose) exchanges in group 2. The use of hypertonic exchanges is summarized in Figure 2, along with the overall resulting glucose exposure to the peritoneum. Seventy percent of the stable group 1 patients used no hypertonic glucose solutions during the first 4 yr of treatment. The greater use of hypertonic glucose and thus peritoneal glucose exposure in group 2 patients preceded the rise in solute transport and continued to increase as solute transport increased.
| Discussion |
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Whereas the data for this study were collected prospectively, the categorization of patients into those with stable as opposed to increasing solute transport had to be made in retrospect. The definition of stable solute transport, a D/Pcreat remaining within 0.1 (approximately 15%) of that at the start of treatment, was chosen to account for the reproducibility of the test (4.8%) (15) and the observation that changes within this range are rarely of clinical significance. Modest fluctuations in solute transport did occur in stable patients during the 5-yr period of treatment, but none had a significant trend with time. The observation that patients who experienced changes in solute transport had an increase rather than a decrease with time reflects the general trend of the whole patient cohort and the experience of other authors (3).
Peritonitis was reported previously to alter membrane function (4,18). It was important, therefore, to examine closely the peritonitis record in these two patient groups. Changes in membrane function related to peritonitis are associated with recurrent clusters of infection and usually occur during the first 2 yr of treatment (4). These patients had, by definition, avoided excessive peritonitis, which remains the principal cause for treatment dropout. It seems unlikely, therefore, that peritoneal infection is the explanation for the difference in these patient groups.
The principal difference between these two patient groups was their RRF. It is unlikely that renal function has a direct effect on solute transport. There is no relationship between peritoneal transport status and RRF at the beginning of PD treatment, and their influence on patient and technique survival is independent (1,19,20). If anything, there is a selection advantage of low solute transport on long-term survival on PD, which is most apparent in those who lose RRF early. Indeed, this is probably the explanation of the observation in this analysis that patients who had increasing transport (group 2) had persistently lower solute transport during the first 2 to 3 yr of treatment.
The factors that determine solute transport are complex. The distributed model, which describes the diffusive transport across the peritoneal membrane for molecules the size of creatinine, defines three major factors (21,22). These include the area of the peritoneum in contact with dialysate, the diffusive mass transport through the capillary wall, and the diffusive mass transport through the interstitium. It is not known which of these factors is responsible for the changes seen in solute transport with time on dialysis. It is tempting to speculate that the diabetiform changes, particularly new vessel formation that has been described, cause an increase in solute transport by augmenting mass transport through the capillary wall. For a molecule such as creatinine, which passes relatively freely through intercellular pores, the capillary surface area will influence solute transport (23). If solute transport increases disproportionately to the peritoneal ultrafiltration coefficient, then ultrafiltration failure will ensue. The case, therefore, that the long-term use of glucose as the primary component to dialysis fluid is detrimental to the peritoneal membrane is growing. The in vitro toxicity of glucose to cellular components (24), the demonstration of AGE deposition in the peritoneal membrane consequent on their enhanced formation from glucose degradation products (7,9), and the findings of advanced vascular changes typical of diabetes in peritoneal biopsy samples are concerning (13). To these must now be added functional changes in the peritoneal membrane, both in the present study and from the observation that glucose-free dialysis results in a modest reduction in solute transport (10).
It should be remembered, however, that even low-strength glucose solutions (1.36%) are far from being physiologic. Another way of expressing the data presented in this study is that it was not possible to demonstrate functional changes in the peritoneal membrane over 5 yr of exposure, provided hypertonic exchanges (3.86%) were avoided. This is encouraging for the continued use of glucose as the predominant low molecular weight osmotic agent in dialysis fluid. It also suggests that it is the extreme hypertonicity of 3.86% solutions that is the problem, and this may be true of other osmotic agents. This will need to be kept in mind during the development and testing of alternatives to hypertonic glucose solutions.
In summary, in a selected group of patients, we demonstrated that the early use of hypertonic glucose exchanges is associated with subsequent increases in solute transport. The possibility that hypertonic glucose, perhaps through its effects on the peritoneal capillary vessels, can alter membrane function is supported. This reinforces the need for the development of alternative hypertonic dialysis solutions. Equally, the use of 1.36% glucose solutions over 5 yr was associated with stable membrane function.
| Acknowledgments |
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| References |
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