Figure 416

Creatinine and urea clearances rates. These rates are estimated by dividing the amount of solute removed per unit of time by the plasma solute concentration. Alternatively, clearance also can be estimated by multiplying the solute equilibration rate per unit of time by the volume of dialysate into which equilibration occurred over the same unit of time. By convention, the creatinine clearance rate is normalized to body surface area.

The urea clearance is normalized to total body water (volume of urea distribution in the body) and is expressed as Kt/V. The Kt/Vvalue is a number without a unit ([mL/min X min]/ mL). During intermittent dialysis, with a dialysate flow rate of 30 mL/min, the typical urea clearance is about 18 to 20 mL/min [18]. Increasing the dialysate flow rates to 3.5 to 12 L/h by rapid exchanges increases the urea clearance rate to a maximum of 30 to 40 mL/min. Beyond this maximum rate, the clearance rate begins to decrease owing to the loss of membrane-fluid contact time with infusion and drainage; inadequate mixing may also occur [19-22]. Clearance could be enhanced by increasing the membrane-solution contact [23]. Continuous dialysate flow techniques using either two catheters or double-lumen catheters also have enhanced the urea clearance rate to a maximum of 40 mL/min. At these high flow rates, poor mixing, channeling, abdominal pain, and poor drainage limit successful application. Maintaining a fluid reservoir in the peritoneal cavity (called tidal peritoneal dialysis) and then replacing only a fraction of the intraperitoneal volume rapidly, increases clearance rates by about 30% compared with the standard technique using the same doses owing to maintaining fluid-membrane contact at higher dialysis-solution flow rates [24-29]. During continuous ambulatory peritoneal dialysis (CAPD) in adults, the optimum volume that ensures complete membrane-solution contact is about 2 L [30,32]. Successful use of 2.5-and 3.0-L volumes has been reported in adult patients undergoing CAPD; however, hernial complications are increased [32,33].

Mass-transfer area coefficient

The diffusive mass transfer is estimated by A

where M = diffusive mass transfer:

A = effective membrane surface area; I = coefficient of proportionality; R = sum of all resistances; Cp = solute concentration in the potential capillary blood; and Cd = solute concentration in the dialysate

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