CALCIUM BALANCE AND SERUM IONIZED CALCIUM FLUCTUATIONS IN ONLINE HEMODIAFILTRATION IN RELATION TO ULTRAFILTRATION RATE AND DIALYSATE CALCIUM-CONCENTRATION

The use of high ultrafiltration rates in haemodiafiltration (HDF) has been suggested for improving the clearance of small and large molecules. This strategy has become economically applicable with the development of safe techniques for on-line production of sterile infusate from dialysate, which enables us to infuse large substitution fluid volumes without further increasing the cost of the sessions. The effect of increasing the ultrafiltration rate in HDF on electrolyte balance has not yet been evaluated. The aim of this study was to evaluate the effects of variations of the ultrafiltration rate on calcium kinetics in HDF using three different dialysate calcium concentrations. Since the increase in ultrafiltration rate augments the convective calcium loss, variations of intrasession calcium balance could result from modifications of the ultrafiltration rate. In the present study we found no significant variations in calcium balance and serum ionized calcium (iCa) levels during on-line HDF treatment when increasing the mean ultrafiltration rate from 60 to 100 ml/min in the presence of an adequate and corresponding increase in the infusion rate (from 2.5 to 5 l/h). During the balance studies, pretreatment serum iCa was on the average 1.32 mmol/l and weight loss 3.2 kg. Mean calcium loss during treatment was 2.8 and 3.3 mmol at infusion rates of 2.5 and 5 l/h with 1.63 mmol/l of calcium in both the dialysate and infusate; calcium loss rose to 5.9 and 11.2 mmol at infusion rates of 2.5 l/h and to 5.7 and 14.2 mmol at infusion rates of 5 l/h when the dialysate and infusate calcium was reduced respectively to 1.5 and 1.25 mmol/l. Serum iCa significantly increased at the end of the session with the higher dialysate concentration, while it decreased to 1.28 mmol/l and 1.20 mmol/l with the lower two concentrations. Linear regression analysis showed no variation in serum iCa during treatment when the iCa concentration in the dialysate was equal to pretreatment serum iCa. A neutral calcium balance would be expected using 1.75 mmol/l of calcium in the dialysate. In conclusion, increasing the ultrafiltration rate from 60 to 100 ml/min did not significantly affect calcium kinetics in on-line HDF. The main factors affecting calcium mass transfer and serum iCa fluctuation during treatment were the dialysate calcium concentration and the iCa gradient between dialysate and pretreatment serum levels.