[Kinetic of calcium, phosphate, magnesium and PTH variations during hemodiafiltration]

Hemodiafiltration (HDF) is a technique resulting from coupling of diffusive and convective transport and thereby increase the elimination of small and middle molecules. However, may induce a convective loss from others substances such as calcium and magnesium. The aim of this study was to evaluate the effects of Ultrafiltration on the kinetics of calcium, phosphate, magnesium and parathyroid hormone. A total of thirteen patients (7 males and 6 females) on hemodialysis, were studied. Each patient was randomly dialyzed with the same dialysate calcium concentration and three different ultrafiltration rate. Schedule A: High flux hemodialysis, schedule B: HDF with 10% of weight body and schedule C: HDF with 20% of weight body. The others parameters were kept identical. Total Ultrafiltration was 2,6+/-0,9 L (9,78+/-3,78 ml/min) in A, 9,3+/-1,7 L (34,54+/-6,22 ml/min) in B and 16,3+/-3,3 L (60,94+/-12,63 ml/min) in C. Replacement fluid during dialysis was 6,85+/-1,42 and 13,65+/-2,9 L. in C and C respectively. Postdialysis total,ionized calcium and magnesium were significantly lower in schedules B and C versus A. PTH levels did not differ significantly. However, PTH changes during dialysis was -36.6+/-38.6%, 6.3+/-69.8% and 32.2+/-63.2% in A, B and C, respectively (p<0.05 A vs. C). A significant inverse correlation was found between total Ultrafiltration and postdialysis levels of total calcium (r:-0.56, p<0.001), ionized calcium (r:-0.65, p<0.001) and magnesium (r:-0.47, p<0.01). No differences were observed in pre and postdialysis phosphate levels, neither mass transfer and clearance of phosphate. We concluded that high ultrafiltration flow rates and substitution fluid without divalent cations induces a negative calcium and magnesium balance. These changes may stimulate PTH secretion during HDF. This technique did not resulted in a higher clearance or phosphate removal.