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Saudi Journal of Kidney Diseases and Transplantation
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EDITORIAL Table of Contents   
Year : 2001  |  Volume : 12  |  Issue : 4  |  Page : 481-486
Adequacy in Peritoneal Dialysis


Renal Unit, Aberdeen Royal Infirmary, United Kingdom

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How to cite this article:
Khan IH. Adequacy in Peritoneal Dialysis. Saudi J Kidney Dis Transpl 2001;12:481-6

How to cite this URL:
Khan IH. Adequacy in Peritoneal Dialysis. Saudi J Kidney Dis Transpl [serial online] 2001 [cited 2019 Jul 16];12:481-6. Available from: http://www.sjkdt.org/text.asp?2001/12/4/481/33540

   Introduction Top


The outcome of patients receiving peritoneal dialysis (PD) has changed considerably since the introduction of this modality for treatment of end-stage renal failure (ESRF). The European Renal Association Registry reported in 1982 that at two years, only about a quarter of the patients commenced on PD survived on this modality of treatment. [1] Patients suffered from high morbidity and rate of technique failure. Over the past two decades, however, with improved technology in the delivery of PD, improved connectology, increased choice of PD solutions and the introduction of prescription dialysis with the option of Automated Home PD (APD), the outcome in PD has improved considerably. Many studies show that survival and morbidity in PD is now comparable to that in hemodialysis (HD). [2],[3] The aim of this review is to discuss methods of assessing adequacy in PD and how this can be improved based on individual patient need using illustrative examples.


   Defining Adequacy in Peritoneal Dialysis Top


Dialysis is the only means by which patients with ESRF can be kept alive until or unless they receive a successful renal transplant. Although it is a life saving treatment, dialysis is not only very expensive but is also an inadequate substitute for normally functioning kidneys. Patients receiving dialysis often require significant dietary and fluid restrictions and are usually on a large number of medications such as phosphate binders, erythropoietin, anti-hypertensive agents, vitamin-D analogues and other drugs. The quality of life in dialysis patients is thus significantly impaired, compared with those with normal kidney function. [4] It is important, therefore, when addressing "adequacy" of dialysis to bear in mind the "inadequacy" of this form of treatment compared with normal renal function. A variety of different criteria have been used to define adequacy. Historically, the Kt/V (a dimensionless derivative) per session in HD and per week in PD has been used to define adequacy in dialysis. The DOQI guidelines which have been published in the United States advise that dialysis should be commenced when the Kt/V of the native kidneys is < 2.0. However, most authorities define adequate dialysis as a Kt/V of 1.0 in HD per session and 1.9/ week in PD. What is remarkable is the fact that a person with a GFR of 120 ml/min with a body surface area of 1.2 sq meter has a weekly Kt/V of approximately 26. In addition to Kt/V, the amount of solute clearance has been used to define adequacy in dialysis. In PD, a weekly creatinine clearance of 60-70 liters is defined as an adequate target (1000 l/wk for a person with normal renal function). The above examples demonstrate the limitations of using Kt/V or solute clearance in isolation to assess adequacy. Nephrologists often define adequacy in terms of these numbers. Whilst such objective measures of quantifying the doses of dialysis are useful, a more pragmatic approach to define adequacy is needed.


   A Pragmatic Definition of Adequacy in Peritoneal Dialysis Top


In clinical practice I would suggest the following, as characteristics of "adequate" PD.

  • Good quality of life - e.g. patient in gainful employment with few restrictions in daily activities.
  • Good nutritional status - e.g. adequate Body Mass Index and anthropometry.
  • Acceptable hemoglobin levels and good left ventricular function.
  • Normotension and absence of fluid overload or dehydration.
  • Satisfactory serum concentration of bicarbonate, phosphate, potassium and calcium.
  • Absence of severe hyperparathyroidism.
  • Weekly target Kt/V and small solute clearance.


The above criteria for adequate dialysis are applicable to both hemo- and peritoneal dialysis patients and if these criteria are met, it is reasonable to expect that patients will enjoy both an acceptable level of quality of life and survival whilst receiving dialysis. The challenge for nephrologists and nurses caring for PD patients is how to achieve these objectives. While the above criteria for adequacy in PD are pragmatic and concentrate on quality of care of the patients, the traditional objective methods of assessing PD adequacy have their place in patient management and help as tools for the nephrologist to tailor PD prescription to the individual patient.


   Pre-requisites for Peritoneal Dialysis Top


The peritoneal membrane has been defined as the "most durable biological non-sticking surface in existence". [5] The ideal candidate for PD will therefore be a patient with no previous history of abdominal sepsis or operations who has adequate peritoneal surface area and circulation. The key factors for adequate PD are (a) the ability of the peritoneal membrane to transport solutes (diffusion, convection) and (b) the ability of the membrane to transport water (ultrafiltration). The "peritoneal membrane" used for dialysis consists of three layers: the mesothelium, the peritoneal capillaries and the interstitial tissue. About 60% of the peritoneal membrane comprises the visceral peritoneum, 10% the parietal peritoneum and 30%, the mesentry and omenta. The splanchnic blood flow averages 1200 ml/minute of which less than 10% i.e. 100 ml/min is the visceral peritoneal flow. Under basal conditions only 25% of the peritoneal capillaries are perfused but experiments in cats have shown that the instillation of dialysate itself induces an increase in splanchnic blood flow. [6]

Solute transport in PD is mainly a function of diffusion and convection whereas fluid transport across the membrane is determined by hydrostatic and osmotic pressure.

Conventionally glucose has been used as the crystalloid osmotic agent in PD. A 1.36% glucose solution will generate an osmotic gradient of 24 mm Hg whereas a 3.86% solution exerts a net crystalloid osmotic pressure of 105 mm Hg after taking into account the effects of plasma osmolality, capillary hydrostatic and colloid oncotic pressure. The ultrafiltration in PD, however, also depends on surface area of the membrane, the splanchnic blood flow and lymphatic absorption.


   Objective Assessment of PD Adequacy Top


The objective assessment of adequate PD requires two observations, which indicate the individual's ability to transport water and solutes. The first is the measurement of transporter status and the second is solute clearance. Both these observations are of importance to determine whether the patient is receiving adequate dialysis therapy and help in tailoring PD prescription to the individual patient's needs. In addition to these, the residual renal function plays a vital role in determining the success of PD and in particular the dialysis regime which best suits the patient.


   The Importance of Residual Renal Function Top


Successful technique survival in PD is heavily reliant on the preservation of some residual renal function. This has been nicely demonstrated in the CANUSA study in PD. [7] When patients commence PD, over a period of time they lose residual renal function. This is a consequence of a number of factors such as the natural progression of the initial renal disease, exposure to nephrotoxic insults such as aminogly­cosides for peritonitis and periods of hypovolaemia during PD. The CANUSA study was a multicentre prospective cohort study of 680 Canadian and US patients and showed that a decrease of 5 L/wk/1.73m 2 of total creatinine clearance per week was associated with a 5% increase in the relative risk of death. This study showed that while peritoneal dialysis clearance was kept constant, it was the loss of residual renal function over time which led to a total reduction of creatinine clearance over a 24­month period. Considering that 1ml/min of creatinine clearance by the kidneys equates to 10 L/wk, we can appreciate the crucial importance of residual renal function in reducing morbidity and mortality rates in PD. In this study, a weekly creatinine clearance of 60 litres and a Kt/V urea of 2.1 per week were associated with a 78% two­year probability of survival in PD.

The following example illustrates the importance of residual renal function in achieving these targets.

Example: A 70 Kg man on 4 two-litre exchanges has a drain volume of 10.5 L and dialysate/plasma (D/P) urea 0.95. His peritoneal Kt/V is calculated as follows:

Calculation of Kt/V in PD

Kt = Drain volume X D/P urea, volume of distribution (V) = total body water (TBW)

Kt = 10.5 X 0.95 = 9.98

Kt/V = 9.98 / 42 = 0.24 (V~42 litres)

Kt/Vp (weekly) = 1.68 (PD Kt/V)

Kruf (residual GFR) = 2.0 ml/min (renal GFR)

Kt/V r (weekly) = {(2/1000)X60X24 X7}/42

= 0.48 (Renal Kt/V)

Kt/V total = 2.16

This example illustrates the importance of residual renal function. If this patient had no residual renal function, his total Kt/V would only have been 1.68 (desirable 2.1), but due to the presence of residual renal function his total Kt/V per week was 2.16, thus meeting the desirable target. In patients with reduced residual renal function the use of increased volume of dialysate from 2 to 2.5 or even 3 litres in large patients might achieve the target Kt/V.


   Peritoneal Membrane Transport Characteristics Top


The Peritoneal Equilibriation Test (PET) provides useful information regarding peritoneal transport characteristics for small molecules. This test is useful to enable nephrologists to prescribe the most appropriate PD regime. The PET test has been described elsewhere [8] and requires a 4­hour dwell with a 2.27% glucose solution. It allows patients' transport characteristics to be divided into Low, Low Average, High Average and High, depending on the amount of glucose absorbed during the dwell and the creatinine removed at the same time. High transporters tend to absorb glucose rapidly and low transporters have the opposite characteristic. A high D/D0 (dialysate to plasma) concentration of glucose indicates low transport status as does a low D/P (dialysate to drain) concentration of creatinine. Patients with a high transport status tend to suffer from problems of fluid retention and may benefit from rapid exchanges or the use of colloidal osmotic agents such as extraneal. It is desirable, if such patients have reasonable residual renal function which allows better fluid and blood pressure control. Low transporters tend to have poor dialysate clearances and may suffer from inadequate dialysis. In such patients, colloid osmotic agents may achieve convectional solute removal but such patients if they have little or no residual renal function, do best on HD. High or low average transporters may benefit from conventional CAPD using four exchanges.

The information used from the PET test can be used for prescribing PD according to the following algorithm.

  • Baseline PET test: Type of PD
  • High transporter: Nocturnal or daytime automated PD; consider colloidal solutions
  • High Average: Nocturnal automated PD with day-time exchange or CAPD
  • Low Average: High volume CAPD or CCPD
  • Low: High-dose CCPD only if residual renal function present, or HD



   Solute Clearance in PD Top


Solute removal in PD can be measured, either by Kt/V (urea) or total creatinine clearance. The NKF-DOQI guidelines [9] recommend a weekly Kt/V of 2.0 or greater and a weekly creatinine clearance of 60/L/1.73m 2 . There is now considerable evidence that achieving these targets is associated with decreased morbidity and mortality in patients on PD. [7],[10],[11] At present, it is not clear which solute clearance should be measured to assess adequacy in PD. It is recommended that both Kt/V urea and creatinine clearance should be used. If these targets are not achieved, then depending on the PET test, the algorithm described above could be used to increase clearance of solutes. An increase in solute volume, use of colloid solution to increase convectional clearance and the use of high volume automated PD with daytime exchanges are all methods which can be applied to achieve these targets. There is, however, one caveat. Patients who repeatedly suffer from peritonitis and those who are on PD for a considerable time may develop an increase in the transporter status. While solute clearance might appear good, such patients usually develop problems with loss of ultra­filtration, hypertension and the consequences of fluid overload. [12]


   Importance of Nutrition Top


Patients receiving PD are generally malnourished and particular attention needs to be given to adequate nutritional intake. Patients who are uremic generally have a poor appetite. They tend to have easy satiety due to fluid in the peritoneal cavity, they are usually on dietary and fluid restrictions and have altered smell and taste sensation. A study recently showed that hypoalbuminemia, which is a marker of malnutrition was significantly associated with mortality in 666 patients on CAPD.[13] It is recommended that PD patients should have a dietary intake of 30 kcal/kg/day and the total daily protein intake should be at least 1.2 g/kg.


   Recommendations to Ensure Adequate Peritoneal Dialysis Top


The following suggestions may guide the nephrologist to ensure adequacy in patients receiving PD.

  • Proper selection of patients. Anephric, large patients or those with a history of abdominal scarring and previous operations tend to have poor adequacy in PD. Patient with good residual renal function are easy to treat with PD.
  • Perform a baseline PET test 3 months after commencing PD to establish patient transporter status. This will enable the nephrologist to tailor prescription accordingly.
  • The PET should be repeated at least yearly or following unexplained loss of ultra­filtration or repeated episodes of peritonitis.
  • Solute clearance using both weekly creatinine clearance and weekly Kt/V urea should be performed 6-monthly. The residual renal function should always be included in aiming for a total weekly creatinine clearance of at least 60 litres and a Kt/V of 2.1.
  • Regular nutritional assessments using anthropometry and subjective global assessments and, where appropriate, dietary supplementation should be carried out in all PD patients.
  • Continuing patient education regarding compliance with exchanges, diet and drugs is an essential component of delivering adequate PD.
  • Fluid and blood pressure control, treatment of hyperlipidemia and general measures such as the importance of avoiding smoking and regular exercise should not be ignored. One should bear in mind that cardiovascular disease is the most common cause of morbidity and mortality in PD.



   Conclusion Top


There have been great improvements in the technique of PD over the past two decades. The future holds even more promise. Some of the newer developments on the horizon are increasing use of APD as 'third modality', increasing use of non-glucose osmotic agents and ability to provide sustained ultrafil­tration, the introduction of non-lactate buffers in PD solution and continued improvements in connectology.

Whilst PD is a life saving modality of renal replacement, it is unphysiological. Patients on PD are exposed to high glucose concen­trations, low pH with lactate and are at constant risk of infection and malnutrition. The management of these patients is a major commitment on the part of the nephrologists, nurses and paramedical staff involved in their care. One should remember that the most "adequate" type of renal replacement therapy are two functioning native kidneys. It is therefore impossible to overdialyse with any other modality. It is also important not to just look at "numbers" when assessing adequacy but to examine and see the patient regularly.

 
   References Top

1.Wing AJ, Broyer M, Brunner FP, et al. Combined report on regular dialysis and transplantation in Europe, XIII, 1982. Proc Eur Dial Transplant Assoc 1983;20:2-75.  Back to cited text no. 1  [PUBMED]  
2.Nolph KD. Why are reported relative mortality risks for CAPD and HD so variable? (inadequacies of the Cox propor­tional hazards model). Perit Dial Int 1996; 16(1):15-8.  Back to cited text no. 2    
3.Davies SJ, Phillips L, Griffiths AM, Russell LH, Naish PF, Russell GI. What really happens to people on long-term peritoneal dialysis? Kidney Int 1998;54(6): 2207-17.  Back to cited text no. 3    
4.Khan IH, Garratt AM, Kumar A, et al. Patients' perception of health on renal replacement therapy: evaluation using a new instrument. Nephrol Dial Transplant 1995;10:684-9.  Back to cited text no. 4  [PUBMED]  [FULLTEXT]
5.Gokal R, Khanna R, Krediet R, Nolph K. In: Textbook of Peritoneal Dialysis. 2nd ed 2000. Kluwer Academic Publishers Dord­recht, Netherlands.  Back to cited text no. 5    
6.Granger DN, Ulrich M, Perry MA, Kvietys PR. Peritoneal dialysis solutions and feline splanchnic blood flow. Clin Exp Pharmacol Physiol 1984;11:473-81.  Back to cited text no. 6  [PUBMED]  
7.Churchill DN. Implications of the Canada­USA (CANUSA) study of the adequacy of dialysis on peritoneal dialysis schedule. Nephrol Dial Transplant 1998;13(Suppl 6):158-63.  Back to cited text no. 7  [PUBMED]  [FULLTEXT]
8.Twardoski ZJ, Nolph KD, Khanna R, et al. Peritoneal equilibriation test. Perit Dial Bulletin 1987;7:138-47.  Back to cited text no. 8    
9.NKF-DOQI clinical practice guidelines for peritoneal dialysis adequacy. Am J Kidney Dis 1997;30(Suppl 2):S67-136.  Back to cited text no. 9    
10.Lameire NH, Vanholder R, Veyt D, Lambert M, Ringoir S. A longitudinal, five year survey of urea kinetic parameters in CAPD patients. Kidney Int 1992;42:426-32.  Back to cited text no. 10    
11.Adequacy of dialysis and nutrition in continuous peritoneal dialysis: association with clinical outcomes. Canada-USA (CANUSA) Peritoneal Dialysis Study Group. J Am Soc Nephrol 1996;7:198-207.  Back to cited text no. 11  [PUBMED]  
12.Davies SJ, Phillips L, Griffiths AM, Russell LH, Naish PF, Russell GI. Impact of peritoneal membrane function on long-term clinical outcome in peritoneal dialysis patients. Perit Dial Int 1999;19(Suppl 2):S91-4.  Back to cited text no. 12  [PUBMED]  [FULLTEXT]
13.Abdo F, Clemente L, Davy J, Grant J, Ladouceur D, Morton AR. Nutritional status and efficiency of dialysis in CAPD and CCPD patients. Adv Perit Dial 1993;9:76-9.  Back to cited text no. 13  [PUBMED]  

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Correspondence Address:
Izhar H Khan
Renal Unit - Ward 23/24 Aberdeen Royal Infirmary Foresterhill, Aberdeen, Scotland AB29 2ZD
United Kingdom
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    Introduction
    Defining Adequac...
    A Pragmatic Defi...
    Pre-requisites f...
    Objective Assess...
    The Importance o...
    Peritoneal Membr...
    Solute Clearance...
    Importance of Nu...
    Recommendations ...
    Conclusion
    References
 

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