DIFFUSIVE MASS-TRANSPORT COEFFICIENTS ARE NOT CONSTANT DURING A SINGLE EXCHANGE IN CONTINUOUS AMBULATORY PERITONEAL-DIALYSIS

Mass transport coefficients usually are assumed to be constant during single 6 hr exchanges of dialysis fluid in continuous ambulatory perit oneal dialysis (CAPD). To check this assumption, the authors estimated diffusive mass transport coefficients, K-BD, for five low molecular w eight solutes in 34 dwell studies with glucose 3.86% (20 studies), glu cose 2.27% (nine studies), and glucose 1.36% (nine studies) dialysis f luids for time periods 3-30, 30-60, 60-90, 90-120, 120-180, 180-240, a nd 240-360 min. Dialysate volume and the rate of fluid reabsorption we re measured using radiolabeled serum albumin (RISA) as a marker. Conve ctive transport was described using a sieving coefficient of 0.55 for all solutes. The K-BD values were constant for sodium, but higher at t he beginning (3-30 min) than at the end (180-360 min) of the exchanges by an average of approximately 50% for urea, creatinine, and glucose, and by approximately 120% for potassium with all three dialysis fluid s. This initial increment did not depend upon the concentration of glu cose in dialysis fluid, except for urea. The steady state value of K-B D was reached at 120 min for all solutes. The time patterns of K-BD va lues for urea, creatinine, glucose, and potassium were well described by an exponential decay function, with the decay constant approximatel y 0.02 min(-1). The patterns were similar for electrically neutral sol utes, but different for electrolytes. The initial increments in K-BD v alues mean that clearances during short dwell time (30-60 min) may be higher by 5-15% than clearances calculated from the steady state K-BD values.