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. The transient, but profound, intradialytic hypophosphatemia may serve as a stimulus for as-yet-uncharacterized homeostatic mechanisms.