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 peritoneal dialysis (CAPD). To check this assumption, the authors estimated diffusive mass transport coefficients, KBD, for five low molecular weight solutes in 34 dwell studies with glucose 3.86% (20 studies), glucose 2.27% (nine studies), and glucose 1.36% (nine studies) dialysis fluids for time periods 3-30, 30-60, 60-90, 90-120, 120-180, 180-240, and 240-360 min. Dialysate volume and the rate of fluid reabsorption were measured using radiolabeled serum albumin (RISA) as a marker. Convective transport was described using a sieving coefficient of 0.55 for all solutes. The KBD values were constant for sodium, but higher at the 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 fluids. This initial increment did not depend upon the concentration of glucose in dialysis fluid, except for urea. The steady state value of KBD was reached at 120 min for all solutes. The time patterns of KBD values for urea, creatinine, glucose, and potassium were well described by an exponential decay function, with the decay constant approximately 0.02 min-1. The patterns were similar for electrically neutral solutes, but different for electrolytes. The initial increments in KBD values mean that clearances during short dwell time (30-60 min) may be higher by 5-15% than clearances calculated from the steady state KBD values.