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

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 (ΔPTH) and CMB.

We measured ΔPTH and CMB (calcium flux from the dialysate to the patient) in 10 stable patients on haemodiafiltration. All patients underwent two treatments differing in DHCO₃ (26 versus 32 mmol/L). The dialysate calcium concentration was 1.25 mmol/L for all treatments.

We found significant difference in ΔPTH, which decreased with 26_DHCO₃ and slightly increased with 32_DHCO₃ (-110.5 versus +19.7 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).

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.