The PD modality has an impact on adherence and QOL, which are important considerations in writing a PD prescription. Ultrafiltration, which is important in optimizing volume control and thus patient survival, is dependent on the prescription and peritoneal membrane characteristics. Clearance of middle molecules, while not proved to influence patient survival, should be an important consideration in the prescription.
4.1 The patient's schedule and QOL should be taken into account when prescribing PD.
4.2 To optimize middle-molecule clearance in patients who have minimal RKF, the PD prescription should preferentially include dwells for the majority of the 24-hour day. This is recommended even if small-molecule clearance is above target without the longer dwell.
4.3 As tolerated by the patient, to optimize small-solute clearance and minimize cost, one should first increase instilled volume per exchange before increasing the number of exchanges per day. The exchange volume of the supine exchange(s) should be increased first because this position has the lowest intra-abdominal pressure.
4.4 The patient's record of PD effluent volume should be reviewed monthly, with particular attention to the drain volume from the overnight dwell(s) of CAPD and the daytime dwell(s) of APD.
4.5 A number of techniques can be used to optimize volume and blood pressure control.
- 4.5.1 To achieve the desired volume status, the lowest possible dialysate dextrose concentration should be used.
- 4.5.2 When appropriate, implement dietary sodium and fluid restriction.
- 4.5.3 In patients with RKF, to achieve dry weight, diuretics may be preferred to increasing dialysate dextrose concentration.
- 4.5.4 Drain volume should be optimized during the overnight dwell(s) of CAPD and the daytime dwell(s) of APD to maximize solute clearance and ultrafiltration volume.
- 4.5.5 In patients who are hypertensive or who show evidence of volume overload, ultrafiltration generally should not be negative (ie, no absorption) for any daytime or nighttime exchanges.
As explained in CPGs 2 and 4, the PD prescription requires frequent review to ensure that clearance- and volume status–related guidelines are being implemented. In determining the PD prescription, the required clearances and the effect on volume status are paramount, but other factors that need to be considered are potential effects on middle-molecule clearance and on QOL of patients and their caregivers.
The patient's schedule and QOL should be taken into account when prescribing PD. The PD prescription can be onerous for patients and their caregivers. There is evidence that nonadherence is common and that it is more likely to occur with more demanding prescriptions, such as CAPD with 5 exchanges daily.61 Some patients find larger dwell volumes difficult to tolerate.38 Social factors and “burnout” are recognized as common problems in PD therapy and as causes of technique failure.128 Accordingly, prescriptions should take the personal and social circumstances of patients into account. The implications of additional dwells, increased dwell volumes in CAPD and APD, and longer cycler times in APD should be discussed with patients and/or their caregivers with a view to designing a prescription that can meet both medical and social requirements and maintain reasonable QOL.
To optimize middle-molecule clearance in patients who have minimal RKF, the PD prescription should preferentially include dwells for the majority of the 24-hour day. This is recommended even if small-molecule clearance is above target without the longer dwell. The term “middle molecule” refers to molecules of molecular weight greater than 1,000 kd. There has long been controversy concerning their importance in uremic toxicity in patients with kidney failure generally and in both HD and PD patients. To date, no high-level clinical study has provided conclusive evidence that middle-molecule clearance determines important clinical outcomes in dialysis patients, although there is some weak, but suggestive, evidence for HD patients from the HEMO Study.91 In PD patients, middle-molecule clearance is time dependent and not significantly influenced by dialysate flow rates or dwell volumes.230 Prescriptions, such as standard CAPD or APD with full-duration day dwells, maximize middle-molecule clearance, and this is thought by some to be an advantage of PD over conventional intermittent HD. However, with the increase in popularity of APD in the past decade, there has been widespread use of prescriptions with short dwells or no day dwells at all, particularly in patients with RKF. Such prescriptions may facilitate fluid removal or improve patient QOL in that many APD patients tend to prefer not to carry peritoneal fluid in the daytime. However, there is concern that the dry day may compromise middle-molecule clearance and thus may be harmful to patients.
Such prescriptions often are used in patients with substantial RKF because Kt/Vurea targets can still be achieved easily. In such circumstances, middle-molecule clearance need not be a concern because RKF is a far more substantial contributor to middle-molecule clearance than any PD prescription. If such prescriptions are associated with low Kt/Vurea values, they need to be altered anyway, in accordance with CPG 2.1. The concern about middle-molecule clearance only arises in patients with minimal residual function and a dry day APD prescription that still meets Kt/Vurea targets. This may occur because the patient is small in body size or is a high transporter. In this situation, the disparity between adequate small-solute clearance and low middle-molecule clearance leads to concern. There is no evidence in the PD literature to guide prescriptions in this situation, but in the interests of patient safety, it is recommended that at least low-volume long-duration dwells be prescribed. Given the lack of high-level evidence to support this statement, implementation should be tempered by QOL considerations for the patient and by the risk for mechanical complications, both of which may be affected negatively by long day dwells.
As tolerated by the patient, to optimize small-solute clearance in CAPD and minimize cost, one should first increase instilled volume per exchange before increasing the number of exchanges per day. The exchange volume of the supine exchange(s) should be increased first because this position has the lowest intra-abdominal pressure. In CAPD, the principal methods to increase peritoneal clearance are to either increase dwell volumes, typically from 2 to 2.5 L to 3 L, or increase frequency of exchanges, typically from 4 to 5 daily.212 Both strategies are similarly effective in increasing peritoneal Kt/Vurea, and increased frequency of exchanges may have a greater benefit in enhancing ultrafiltration. However, increasing the dwell volume generally is preferred unless there are mechanical contraindications. This is because adherence to CAPD prescriptions with 5 daily exchanges has been shown to be particularly poor and may be associated with worse QOL. Also, the cost of increasing exchange frequency is greater than the cost of increasing dwell volumes. However, ultimately, patient preferences should be a major determinant of which strategy is followed.
If patients have concerns about tolerating increased dwell volumes in either CAPD or APD, consideration should be given to increasing nighttime dwell volumes initially. The rationale for this strategy is that increases in intraperitoneal pressure (IPP) are less for a given dwell volume in the supine or recumbent position compared with either sitting or standing.
The patient's record of PD effluent volume should be reviewed monthly, with particular attention to the drain volume from the overnight dwell(s) of CAPD and daytime dwell(s) of APD (see CPR 4.2).
A number of techniques can be used to optimize volume and blood pressure control (see CPRs 4.5.1 to 4.5.5).
None of these individual prescription strategies have been shown to produce superior outcomes in RCTs. However, their effects on clearance and fluid removal are well recognized from clinical studies and clinical experience and are not controversial. The relative merits of particular strategies in a given patient need to take into account multiple personal and social factors that will vary among patients. These are not easily studied in clinical trials. In such situations, different strategies may need to be tried in a given patient until an optimal compromise among clearance, ultrafiltration, and QOL requirements is achieved.
With regard to the middle-molecule recommendation, there is too little evidence to offer a firm guideline, but, just as when dealing with small-solute clearances, the best principle is to give the patient the benefit of the doubt and not provide lower clearances than have been shown to be safe by clinical studies. However, given the lack of evidence, weight also should be given to other factors, such as QOL and risk for mechanical complications.
Implementation of these recommendations requires only that patients be carefully evaluated monthly. At the evaluations, ultrafiltration and clearance requirements should be reviewed, with particular attention to how the prescription is affecting QOL and whether the patient is adherent to it. Appropriate changes could then be made.