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Part 4: Technical Advances in PD |
Division of Nephrology, Department of Medicine and Therapeutics, Prince of Wales Hospital, Chinese University of Hong Kong, Hong Kong SAR, PR China
Correspondence: P.K.T. Li, Department of Medicine and Therapeutics, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong SAR, PR China. philipli{at}cuhk.edu.hk
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ABSTRACT |
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This article examines the roles of continuous ambulatory peritoneal
dialysis (CAPD) versus automated peritoneal dialysis (APD) as first-line renal
replacement therapy. To date, no high-quality large-scale randomized
controlled studies have compared CAPD with APD as first-line therapy. However,
a discussion on this issue is important so that nephrologists can decide and
patients can have a choice of modality on which to start dialysis, especially
in the context of health care economics. We review the literature and present
Hong Kong as the model of a "CAPD first" policy, an appealing,
cost-effective approach for any country.
An ideal renal replacement therapy should provide optimal survival,
lowest possible risk for comorbidity, highest level of quality of life, and
equally important, acceptable cost to society. When we consider this subject
in the context that all patients should be started on one first-line modality,
the data suggest that a "CAPD first" policy has all these
advantages, with APD probably having the edge only with regard to patient
preference. The present review highlights preservation of residual renal
function, removal and balancing of sodium, incidence of peritonitis,
peritoneal membrane transport status, patient rehabilitation, and financial
issues in demonstrating that a "CAPD first" policy is the model
that should be adopted.
KEY WORDS: Continuous ambulatory peritoneal dialysis; automated peritoneal dialysis; first-line therapy; cost; residual renal function.
Technologic advances occur rapidly in the field of peritoneal dialysis (PD), to the extent that medical evidence sometimes lags behind (1). One of the prime examples is the lack of data comparing continuous ambulatory PD (CAPD) with automated PD (APD). Although performance evaluation has become a central issue in medicine, PD clinicians and patients alike are choosing between these two modalities of renal replacement therapy based on less-than-robust medical evidence. At times, the choice between these two options for PD treatment is driven largely by budgetary considerations and funding or reimbursement policies. In particular, the question of whether APD should uniformly be considered the first-line dialysis therapy remains unsettled. In the discussion that follows, "APD" refers to the use of a cycler for any of a variety of regimens, including nightly intermittent PD (NIPD), continuous cycling PD (CCPD), and tidal PD.
The uncertainty lies in whether APD and CAPD exhibit intrinsic difference that might suggest the superiority of one form of dialysis over the other. Unfortunately, available knowledge on this subject is too limited to allow this fundamental question to be answered with certainty. And so the question becomes which option to use first, rather than which one is best. When no hard clinical data are available to prove or refute the superiority of one treatment option over another, discussion should be based on a decision tree that balances the pros, the cons, and the cost considerations. The discussion that follows reviews the available information about APD and CAPD to help health care professionals and patients optimize prescription of a PD modality.
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DISCUSSION |
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TOPABSTRACTDISCUSSIONCONCLUSIONSREFERENCES |
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Although the deleterious effects with APD are not consistently observed (8,9), there are biologically plausible reasons to suggest a link between APD and a faster rate of decline of residual renal function. One proposed explanation is the less stable fluid and osmotic load, together with intermittent nature of APD. Because the osmotic load varies markedly with each nightly APD session, hemodynamic status may be adversely affected. Adverse hemodynamic effects with episodes of dehydration have been postulated during APD, analogous to the association between intradialytic hypotensive episodes and the quicker loss of residual renal function in hemodialysis patients (5).
Selby et al. (10) recently reported on the hemodynamic effects of APD and documented in detail the physiologic changes in 8 stable CAPD patients who started on APD. A consistent reduction in stroke volume and cardiac output was found. Also, blood pressure fell during two of the three drain-and-fill periods when dialysate was drained from the peritoneal cavity; blood pressure rose again on instillation of dialysis fluid (10). Furthermore, a progressive rise in total peripheral resistance was noted, possibly attributable to a cooling effect as each drain-and-fill cycle further cooled the mesenteric vessels, leading to vasoconstriction. All of these hemodynamic changes have the potential to affect renal perfusion—and thus the rate of decline of residual renal function.
Residual renal function is now recognized to be crucial in maintaining dialysis adequacy; it confers a survival benefit in PD populations (11). For this reason, CAPD may be a better option than is APD as first-line renal replacement therapy, because incident patients frequently have residual renal function when they are started on maintenance dialysis.
SODIUM REMOVAL AND BALANCE
Maintenance of sodium and water balance is a crucial element in the
management of patients on PD, especially anuric patients. Failure to achieve
adequate daily ultrafiltration causes devastating consequence for dialysis
patients and has been identified as an independent predictor of mortality in
more than one prospective cohort study
(12,13).
For example, the European APD Outcome Study (EAPOS) showed that failure to
achieve daily ultrafiltration of more than 750 mL was associated with
increased mortality in anuric patients on APD
(13).
During PD, sodium balance correlates strongly with net fluid removal (14). Moreover, it is a function of diffusive transport, convection across the peritoneal membrane, and fluid absorption. Sodium sieving during convective transport lowers the sodium concentration in the dialysate, thereby increasing the sodium diffusion gradient and rendering ultrafiltration a less effective form of convective solute transport. The short-dwell schedule in nightly APD results in significant sodium sieving, and hence, less efficient sodium removal. In addition, recumbent posture during APD may increase peritoneal solute transport (15), which may be associated with a reduced capacity for ultrafiltration. Furthermore, the mechanics of the recumbent position may hinder proper drainage of dialysate in many APD patients, and many PD cyclers are programmed to limit drainage time as much as possible, which further increases the likelihood of incomplete emptying at the end of every exchange. As a matter of fact, at least two previous studies have demonstrated significantly lower sodium removal in APD than in CAPD: 87 mmol versus 195 mmol daily (14) and 91 mmol versus 210 mmol daily respectively (16). However, it should be noted that no definitive effect of mode of PD or sodium removal rates on clinical or cardiovascular outcomes has been observed (7).
Despite the foregoing caveats, the benefit of sodium removal should be an important consideration in the choice of CAPD as a first-line renal replacement therapy.
PERITONITIS INCIDENCE
Treatment-related infections remain a serious complication in PD patients
(17). In some previous small
studies, the choice of APD appeared to confer an advantage in the form of a
reduced incidence of peritonitis. However, a recent retrospective study based
on data from the United States Renal Data System and involving 11,975 patients
nationwide (18) called into
question previous suggestions that APD be used in preference to CAPD because
of a lower risk of peritonitis. In contrast to previous investigations, this
broad-based study observed a statistically significantly lower risk of first
peritonitis (6%) after 9 months, with a longer time to first peritonitis (17.1
months vs 16.1 months respectively), in CAPD patients than APD patients. The
results favoring CAPD during the observation period (1994–1997) remained
even after adjustment for a higher entry-period peritonitis rate
(18).
That important finding also accords with more recent reports (19,20) of favorable peritonitis outcome in CAPD patients as compared with outcome in APD patients. The reasons for these disparate findings remain unclear, but may pertain to improvements in the CAPD technique. Our group recently demonstrated peritonitis rates of 1 episode in 37–45 patient–months with the use of double-bag systems in CAPD—a very respectable and acceptable figure (21).
On the whole, the literature now provides clear evidence against an association between APD (as compared with CAPD) and lower risk of peritonitis. Based on these observational data, and in the absence of solid evidence based on prospective randomized studies, the indications for APD—as in the European best practice guidelines (22)—do not include desire for a reduction in the peritonitis rate.
PERITONEAL MEMBRANE TRANSPORT STATUS
Apart from a need to avoid high intraperitoneal pressure, high transport
status of the peritoneal membrane is currently the strongest medical
indication for APD (22). High
transport implies either a structural or a functional alteration of the
peritoneum—namely, larger effective peritoneal surface area or higher
intrinsic membrane permeability—leading to rapid equilibration of small
solutes including creatinine and urea. Similarly, because of rapid absorption
of glucose from the dialysate, high transporters are prone to lose the osmotic
gradient required for sustained ultrafiltration. As a consequence, and
subsequent to a decrease in ultrafiltration capacity, high transporters
benefit from the short dwells of APD.
However, a note of caution is necessary, because APD is often inferior to CAPD in patients with slow peritoneal transport, especially in terms of creatinine clearance and clearance of larger solutes (the "middle molecules")—particularly when APD is used in patients without residual renal function. Evidently, broad use of APD as first-line treatment regardless of peritoneal transport status might not be the best approach.
Automated PD is perhaps more justified in high transporters in terms of clinical outcome. The recently published Caring for Australians with Renal Impairment (CARI) guidelines specifically state that patients with low peritoneal membrane transport are less well suited to APD, particularly in the setting of poor residual renal function (23).
PATIENT REHABILITATION
A number of unproven psychosocial benefits of APD (largely in the form of
NIPD) over CAPD have been suggested
(24,25),
including fewer connections, more frequent use of reduced fill volumes, and
patient independence from dialysis during the daytime. In one report that
explicitly assessed health-related quality of life in APD patients as compared
with CAPD patients, APD treatment appeared to be associated with better mental
health (less anxiety and depression) after adjustment for case-mix variables
(25). Taken at face value,
such findings seem to suggest certain advantages of APD, but in observational
studies, the influence of therapy selection cannot be excluded. For instance,
the apparent differences could have been related to the fact that APD patients
were treated at only 3 of the participating centers, and CAPD patients were
selected from 13 other centers where APD was less available
(25). In another series, APD
patients scored worse for physical function, despite better mental function,
than the CAPD patients did
(26). Sleep problems are also
more marked in APD patients
(24). Furthermore, the
potential benefits of APD might be negated by the necessity for many patients
to perform at least 1 daytime exchange to meet small-solute clearance
targets.
From the perspective of patient rehabilitation, CAPD remains an attractive treatment. A previous survey by our group noted that 89% of prevalent CAPD patients were able to continue normal activities without special care, and 72% reported participation in social activities most of the time (27).
An association of APD with improved quality of life remains controversial. As summarized in the CARI guidelines (23), data from controlled studies to suggest that APD offers any advantages or disadvantages over CAPD with respect to quality of life (other than additional social time) are lacking.
FINANCIAL ISSUES
Although prescription of APD has been increasingly adopted by PD units in
North America (28), no
reliable comparative expenditure studies on the cost-effectiveness of APD
versus CAPD are available. Clinical experience suggests that the choice of PD
modality seems to be influenced by financial considerations or non-medical
factors
(29,30).
Some studies suggest that APD requires an increase of 60% in PD fluid volume.
Most costing studies report that APD is 8%–36% more expensive than CAPD
(23,24,31)
and probably even more costly where dialysis fluid is more expensive per unit
volume.
Apart from the additional costs for the machine, greater volumes of dialysate are employed, special tubing and connection sets are used, and nursing training for APD takes longer than training for CAPD does (32). Nevertheless, firm conclusions cannot be drawn with respect to decision-making, because no data are available on the cost-effectiveness ratio, including cost per life-year saved and quality-adjusted life-years saved. Also, although APD patients reported significantly more time for work, family, and social activities (24), uncertainty about the incremental cost-effectiveness ratio—that is, the increase in the financial cost that accompanies this benefit—remains.
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CONCLUSIONS |
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ACKNOWLEDGMENTS |
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
