In order to reach peritoneal dialysis therapeutic goals, a good understanding of the factors that determine peritoneal mass transfer and enhance clearance is required. As residual renal function or RRF is lost, it is necessary to adjust the dialysis dose to maintain adequacy, defined by the NFK/KDOQI guidelines as a minimal weekly total Kt/Vurea ≥ 1.71). In patients who fall short of this goal, prescription parameters can be adjusted by the physician accordingly. These adjustments may include:
1. Increasing dialysate flow rate (DFR)
The term dialysate flow rate refers to the total volume of dialysate exchanged over time. Increasing DFR is one of the most effective means of increasing solute removal. This can be achieved by either increasing the number of exchanges or by increasing the intraperitoneal volume (Vip). It is known that there are limitations to the use of high DFR in terms of clearance. Several studies have shown that when the DFR is greater than 2.7 L to 3.0 L/hr, clearance plateaus or diminishes when intermittent techniques are used (2–4). The most likely explanation for this phenomenon is that during frequent fast exchanges, the dialysis solution spends significantly more time (non-dialytic time) in transit, i.e., in and out of the peritoneal cavity, rather than in contact with the membrane. Keshaviah and colleagues reported that the optimal intraperitoneal volume is approximately 1500 mL/m2 of body surface area (BSA) in sitting adults(5)Durand et al. further suggest that the optimal prescribed volume depends on the maximum tolerated volume and the intraperitoneal pressure (IPP, not exceeding 18 cmH2O)It is therefore important that the Vip prescribed is individualized for each patient, taking into consideration the patient’s BSA, tolerance of fill volumes (measurement of IPP), expected ultrafiltration (each 500 mL of dialysate increases IPP by 1 cmH20 and increases fluid absorption by 35 mL/hr), and drain profile.
2. Increasing the exchange volume
An increase in Vip significantly increases the effective peritoneal surface area and the mass transfer area coefficient (MTAC)(5,7)An increase in volume also means that a higher amount of solute can be cleared until equilibrium is reached.
3. Performing PD in the supine position
Both position and Vip affect MTAC. The improvements in mass transfer observed by assuming the supine position provides an increase in effective peritoneal transfer area(8) and could result in greater clearance during that time. Dialyzing in supine position may also allow an increased fill volume while staying within the limits of IPP
4. Optimizing dwell time
Attention must also be given to optimal timing of the exchanges. Continuous therapy throughout the day and night is needed by most patients, with the exception of those with very high solute transport(9) This is particularly important in order to maintain a high clearance of larger solutes such as the middle molecules. Larger solutes are more dependent on time and peritoneal surface area than dialysate flow rate (DFR). It is important to avoid very long dwell times, since UF diminishes due to glucose absorption and attenuation of the osmotic gradient. Dwell times in excess of 6 hours require higher glucose concentrations such as 2.5 or 4.25%(10) or polyglucose solutions in order to prevent negative UF|(11). If needed, patients on automated PD (
Optimizing dwell time for different solutes might also be accomplished by using methods developed by Fischbach and colleaguesThey hypothesized that the sequential use of shorter dwells with smaller intra-peritoneal volumes (IPV) and longer dwells with larger IPVs are superior to uniform cycles. They suggested that short-dwell exchanges with small volumes could lead to greater UF capacity, and long-dwell exchanges with large volumes would favor “saturation” of the dialysate with creatinine and phosphate. Thus, sequential use of both could provide more effective clearances and UF at lower glucose absorption i.e., at lower metabolic costs. Such a regimen would be especially valid in average to high (fast) transporters.
5. Optimizing catheter function
Adequate catheter flows cannot be over emphasized since they are intimately related to DFR(14). It is therefore important to monitor the patient for factors that may impede flow e.g., obstructions or kinks in the tubing.
Specific recommendations according to transport characteristics:
Low transport states
In general, low transporters require long dwells and high volume exchanges to achieve clearance targets. Adequate prescriptions might include manual peritoneal dialysis, better known as continuous ambulatory PD or CAPD, which is performed 24 hours a day, 7 days a week. There are several manual exchanges per day, where each exchange is typically 4 to 6 hours in duration and uses 2 to 3 L of dialysate. In total, patients may use between 8 and 12 L over a 24-hour period(15) With CAPD, most of the exchanges are performed during the day. Thus, a daily regimen might consist of three 5-hour day dwells with one longer exchange for approximately 9 hours at night. During the short daytime exchanges, PD solutions containing glucose are usually used. For the long nighttime exchange a higher glucose concentration or a polyglucose PD solution may be needed to maintain an effective ultrafiltration gradient. APD, with a small number of large volume cycles at night and possibly one or more day dwells, could also be performed in these patients (15)
High transport states
Good candidates for automated PD or APD include those patients withinsufficient clearance on CAPD such as high or fast transporters Short dwells can enhance fluid removal. In APD, the cycler performs the exchanges. Typical regimens include continuous cycling PD or CCPD, nighttime intermittent PD or NIPD, PD plus and Tidal PD. Each of these modalities comprise multiple exchanges during the night. In CCPD, there is a long daytime dwell. In NIPD, the abdomen is dry during the daytime. In PD plus, there is an additional morning or evening dwell.
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