Intradialytic alkalinization is a neglected factor affecting calcium mass balance and parathyroid hormone level during haemodiafiltration

Abstract

The diffusion gradient between ionized calcium (iCa) in the inlet dialysate and blood is considered to be the main driving force of calcium mass balance (CMB). The intradialytic change of parathyroid hormone (PTH) level corresponds to the change in plasma iCa. In contrast to the widely discussed calcium concentration of dialysis solution, the dialysate pH and bicarbonate concentration (DHCO3), important factors affecting the level of iCa, have not been studied with respect to the intradialytic change of plasma PTH level (DPTH) and CMB. Methods. We measured DPTH and CMB (calciumflux from the dialysate to the patient) in 10 stable patients on haemodiafiltration. All patients underwent two treatments differing in DHCO3 (26 versus 32mmol/L). The dialysate calcium concentration was 1.25mmol/L for all treatments. Results. We found significant difference in DPTH, which decreased with 26-DHCO3 and slightly increased with 32-DHCO3 (110.5 versusp19.7 pg/mL, P<0.01). CMB was negative for both DHCO3, but with higher DHCO3 there was a trend to minor intradialytic loss of calcium (108 versus 309 mg). Conclusions. DHCO3 increase at first glance leads to contrasting phenomena: the intradialytic rise of PTH and calcium gain. Both processes are caused by a pH-dependent decrease of plasma iCa, resulting in parathyroid stimulation and intradialytic increase of iCa diffusion gradient.We found no significant correlation between CMB and intradialytic change of plasma total Ca. With respect to plasma PTH level and CMB, the bicarbonate concentration should always be taken into account when selecting the optimal dialysis solution.