D/Pcreatinine ratio at 4 h
Transporter type
D/Pcreatinine 0.81–1.03
High
D/Pcreatinine 0.65–0.81
High average
D/Pcreatinine 0.50–0.65
Low average
D/Pcreatinine 0.34–0.5
Low
Higher transporters achieve the most rapid and complete equilibrium for solutes creatinine and urea because they have a relatively large effective peritoneal surface area or low membrane resistance. However high transporters rapidly lose their osmotic gradient for ultrafiltration because the dialysate glucose diffuses into the blood through the highly permeable membrane. They also have higher dialysate protein losses and so tend to have lower serum albumin levels.
Conversely, despite the slow movement of solutes in low transporters and need for longer dwell times, lack of ultrafiltration is rarely an issue due to longer maintenance of the osmotic gradient. High average and low average transporters have intermediate values for these ratios and for ultrafiltration and protein losses (Fig. 2.1). In practice, high transporters do best on PD regimens that involve frequent short duration dwells, so that ultrafiltration is optimized while the low transporters do best on regimens based on long high volume dwell times so that diffusion is maximized.
Other testing modalities include the Fast PET, an abbreviated test which only samples dwell and plasma creatinine and glucose at the 5 h mark and the Modified PET, which evaluates ultrafiltration failure using an osmotic challenge by utilizing a more concentrated dextrose solution (4.25% 2 L dextrose). Ultrafiltration less than 400 mL during a Modified PET is diagnostic for peritoneal membrane failure [4, 23, 24].
Adequacy of Dialysis
In the broadest sense, adequacy is the overall assessment of the efficacy of dialysis and is an expression of the overall gestalt of a patient’s well-being [25, 26]. Ultimately the goal of dialysis is to maintain a patient’s subjective quality of life. In practice this is difficult to quantify.
While being secondary goals, outcomes such as solute clearance, acid base maintenance, maintenance of electrolyte equilibrium, and mineral bone disease prevention, are objective and easier to quantify. Urea and its clearance became the most commonly used markers for adequacy after 1981, when a study performed by the National Cooperative Dialysis Study noted that significantly better outcomes were noted in patients with lower BUN levels for patients undergoing hemodialysis [27].
In the clinical setting, urea clearance is normalized to body water levels (Kt/Vurea) and is measured at 1 month after starting PD concurrently with the PET then subsequently every 4 months thereafter [23, 24]. Urea clearance is comprised of clearance from peritoneal dialysis itself and clearance from any residual kidney function and have to be individually calculated, and then added together.
The components of Kt/Vurea are as follows:
K – This is the daily clearance of urea, and can be obtained by the D/Purea ratio multiplied by the total volume of dialysate (dwell plus ultrafiltrate) for the PD portion. For assessing urea clearance of the kidneys, D (dialysate) is substituted by U (urine). Correspondingly this is multiplied by the volume of urine produced rather than dialysate volume.
t – This is time, in days. Standard measurements of Kt/Vurea is expressed as a weekly value, so this number is typically 7.
V – This is the volume of distribution of urea, and can be obtained by the Watson formula which is available online. It is roughly 60% of a patient’s ideal body weight.
Survival on PD has documented associations with maintaining higher Kt/V levels. Initial studies recommended maintaining weekly Kt/V levels of greater than 2.0 though this has since relaxed. The most recent guidelines from KDOQI committee recommends maintaining the total Kt/Vurea (peritoneal and urinary clearance) above 1.7 [25].
Peritoneal Dialysis Modalities
Peritoneal dialysis is divided into two main modalities: continuous ambulatory peritoneal dialysis (CAPD) and automated peritoneal dialysis (APD). The choice of peritoneal dialysis to use depends on numerous factors including transport phenotype; however, by and large patient and caregiver preference is the main determinant. There is no difference in survival outcomes between CAPD and APD.
Continuous Ambulatory Peritoneal Dialysis (CAPD)
Developed and refined by Drs. Popovich, Moncrief and Nolph in the 1970s, CAPD was the dominant modality worldwide for a number of years though automated peritoneal dialysis (APD) has since caught up [1, 15, 28]. CAPD requires a continued dwell throughout the day, with exchanges between used dialysate and fresh solutions performed four times per day. Fluid volume per exchange is 2 L. Residual kidney function, patient size, and transport characteristics of the patient will influence subsequent fluid volumes, exchange frequency as well as solution types though these changes are typically made after formal PET and adequacy measurements are performed a month after PD has started.
CAPD offers the advantage of portability, and longer dwell times to facilitate better solute clearance and ultrafiltration. In patients whose residual kidney function is terminal or are low transporters with poor ultrafiltration, CAPD may be the only feasible peritoneal dialysis option. The main disadvantages is the inconvenience of performing exchanges throughout the day, as well as higher rates of peritonitis due to persistent fluid in the peritoneal cavity.
Automatic Peritoneal Dialysis (APD)
The use of cyclers became more mainstream in the early 1980s. APD is now the most common form of PD in the United States. The cycler attempts to compress a patient’s dialysis during their least active part of the day – during sleep. This is an especially attractive option in patients who have active schedules, or who require help from persons whose time is limited.
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