[Non-pharmacological calcium metabolism control in patients undergoing hemodialysis]

Calcium overload has been claimed to be involved in the increased vascular morbidity and mortality of dialysis patients. Conversely, calcium depletion can worsen secondary hyperparathyroidism and reduce bone mass. When residual renal function is null or negligible, the calcium balance is mainly determined by the calcium absorbed from the intestine, and that gained or lost from or into the dialysate. This article aims to review the assessment and mechanisms of calcium flux during hemodialysis (HD) and hemodiafiltration (HDF). Calcium mass transfer in HD is mainly dependent on the ionized calcium concentration gradient between dialysate and blood, ultrafiltration volume (Qf) and treatment time. Calcium flux in HDF is also affected by the infusion mode. In post-dilution HDF, the calcium balance is comparable to that in HD for a given concentration gradient between dialysate and blood. Conversely, in pre-dilution HDF the dialysate calcium concentration should be increased by about 0.25 mmol/L to maintain comparable balances. For a given dialysate total calcium concentration, the ionized fraction changes according to the pH and the bicarbonate concentration of the solution. Thus, the dialysate ionized calcium is higher (94% of total calcium) in acetate-free biofiltration (AFB, a HDF modality where the dialysate does not contain bicarbonate or acetate) than in standard HDF (dialysate bicarbonate and acetate concentration of 31 mmol/L and 5 mmol/L, respectively), where the dialysate ionized calcium fraction is about 84% of the total calcium. As a consequence, calcium mass transfer is more positive in AFB compared to post-dilution HDF at similar dialysate total calcium concentrations. Clinical studies and kinetic models have shown that the calcium mass transfer during HD is neutral when the ionized calcium gradient and Qf are zero. This suggests that patients dialysed against a dialysate total calcium concentration of 1.25 mmol/L (corresponding to ionized calcium of 1.25 x 0.84 = 1.05 mmol/L) should have a serum ionized calcium concentration of 1.05 mmol/L and no weight loss to achieve a neutral calcium flux during HD treatment. In patients with serum ionized calcium in the normal range and an average Qf of 2-3 L, calcium losses of about 5-12 mmol (200-480 mg) have been documented during a single HD treatment with a dialysate calcium concentration of 1.25 mmol/L. Such losses might counterbalace the calcium gained by intestinal absorption (about 4-5 mmol/day or 160-200 mg/day in patients with an oral daily calcium intake of 1-1.5 g and normal vitamin D status) and ensure an overall approximately neutral calcium balance. The development and validation of kinetic models able to prescribe the dialysate calcium concentration necessary to achieve a neutral calcium balance in relation to dialysis treatment time, Qf, serum ionized calcium concentration, oral calcium intake, and vitamin D therapy is desirable.