Phosphate kinetics during high-flux hemodialysis.

Phosphate clearance by polysulfone (PS) and cuprophane (CU) membranes and the relationship of peridialytic changes in serum phosphate with those of serum-ionized calcium, parathyroid hormone (PTH), and insulin were studied in six stable patients undergoing chronic hemodialysis (HD). Dietary phosphate intake was 25.7 mmol/day, and total dose of elemental calcium was 3.2 g/day. Patients were dialyzed for 2 to 4 h, once with each membrane. Serum phosphate levels fell precipitously during the first hour of HD with both dialyzers, from 1.42 and 1.49 mM to nadirs of 0.53 and 0.69 mM for PS and CU, respectively. Phosphate levels began to increase either late in HD or at the end of HD and, by 4 h post-HD, did not differ from predialysis values. Total mass transfer of phosphate was greater during the first hour (3.4 and 3.2 mmol for PS and CU, respectively) than during the remainder of HD (2.4 and 2.6 mmol/h). There were no significant differences in intradialytic serum phosphate changes, postdialytic phosphate rebound, or total phosphate removal between the two dialyzers. Ionized calcium increased by 0.11 mM, and PTH was suppressed to 40 to 50% of baseline values during dialysis with either membrane. Although phosphate removal continued for the duration of dialysis, serum phosphate did not continue to decrease, either reaching an apparent steady state or beginning to rebound, even during dialysis. This suggests active phosphate mobilization from a pool other than the extracellular fluid and demonstrates the inadequacy of a one-compartment model to explain these data. Further, these data do not support the regulation of intradialytic phosphate mobilization by serum PTH or insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Initial clinical experience with a new model of mass transfer for peritoneal dialysis.

A clinical pilot study compared predictions of a new model of peritoneal dialysis mass transfer to measured weekly KT/V urea (KTu/V) and weekly creatinine clearance (Ccr) in liters per 1.73 m2 in 50 patients from five centers (40 CAPD, 10 CCPD). The Robertson et al. model is unique in that it does not require a 24-hour collection of dialysate. Instead, model predictions are based on the results of a standard 4-hour peritoneal equilibration test (PET) and appropriate demographic data. Analysis revealed 12 collection errors, 8 affecting the PET and 4 affecting 24-hour dialysate volume. PET drainage volume was low in six cases, excessive in two; 24-hour volume was incomplete in three, excessive in one. Similar errors were not found in the remaining 38 patients. In the 38 patients with correctly performed PET and dialysate collections, agreement between predicted and measured values was excellent.

Vancomycin elimination during high-flux hemodialysis: kinetic model and comparison of four membranes.

Vancomycin clearance was measured in five patients during dialysis with cuprophane (CU), polysulfone (PS), cellulose triacetate (CT), and polyacrylonitrile (PAN) dialyzers. Vancomycin was significantly cleared during routine high-flux (HF) hemodialysis (HD) with the latter three membranes, but not by CU. Postdialytic rebound of serum vancomycin concentrations was noted following HF dialysis, necessitating use of a two-compartment pharmacokinetic model. Measurement of serum vancomycin concentration immediately postdialysis significantly overestimates intradialytic removal, possibly resulting in inappropriate dose adjustment. Vancomycin infusion during HF HD results in significant drug removal during its administration to the patient, complicating the calculation of an adequate dose.