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Good understanding of the factors that determine peritoneal mass transfer and the practical maneuvers to enhance clearance is the formula to reach the therapeutic goals. As RRF is lost, it is necessary to adjust the dialysis dose to maintain adequacy. The maneuvers to reach the adequacy goals are:
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Increase dialysate flow rate (DFR). This is one of the most effective means of increasing solute removal. This can be achieved by either increasing the number and frequency 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 45 to 50 ml/min clearance plateaus or diminishes when Intermittent Techniques are used 1-3. The most likely explanation for this phenomenon is that during frequent fast exchanges the dialysis solution spends a significant amount of time (non-dialytic time) in transit in and out of the peritoneal cavity than in contact with the membrane.
Ratio of transit time to total dialytic time
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Increase the exchange volume. An increase in Vip significantly increases the effective peritoneal surface area and the mass transfer area coefficient (MTAC)4, 5.
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Supine position. Both position and Vip affect MTAC. The improvements in mass transfer observed by assuming the supine position is also probably related to an increase in effective peritoneal transfer area6.
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Optimize dwell time. Attention must also be given to optimal timing of the exchanges. First, we must emphasize the importance of continuous therapy throughout the day and night for most patients with the exception of those with very high solute transport. This is particularly important in order to maintain a high clearance of larger solutes, the so-called 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 blunting of the osmotic gradient. Dwell times in excess of 6 hours require glucose concentrations in the 2.5 to 4.25% range in order to maintain negative UF and result in higher absorption of glucose. Whenever possible, patients on APD should incorporate an additional manual or automated exchange in the afternoon or evening in order to optimize both clearances and UF.
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Optimize catheter function. Adequate catheter flows cannot be over emphasized since they are intimately related to DFR.
Specific recommendations according to transport characteristics:
Low transport states
In general low transporters require long dwell, high volume exchanges to achieve clearance targets. Ideal prescriptions might include:
CAPD - with 4 x 2.5-3 liter exchanges daily
CAPD - with 5 x 2.5-3 liter exchanges daily, with the fifth delivered by a night exchange device
APD - with a small number of large volume cycles at night and one or more day dwells [e.g., 3 x 3 liters over 9 hours plus 2 x 2.5 liter day dwells done using the cycler pause option].
High transport states
High transporters do best with more frequent short cycles that achieve clearance targets without compromising UF. Ideal prescriptions might include:
APD - with 4-7 x 2.5-3 liter cycles over 9 hours plus or minus a short [3-4 hr] day dwell done using the cycler pause option. |