Ionized calcium plays a pivotal role in controlling blood pressure during haemodialysis.

The influence of dialysate bicarbonate concentration on ionized plasma calcium as well as on haemodynamics have been studied prospectively. In 3 consecutive weeks, 7 chronic patients were dialysed with a dialysate containing low bicarbonate (30 mEq/l), medium bicarbonate (35 mEq/l) and high bicarbonate (39.5 mEq/l), respectively. Notwithstanding the same considerable influx of calcium during high, medium and low bicarbonate dialysis (Bc-D), a significant increase in ionized calcium from 1.09 to 1.41 mmol/l was observed during low Bc-D only. During medium and high Bc-D no significant increase in ionized calcium occurred. In multivariate analysis the only significant predictive variable for the changes in ionized calcium was the change in total CO2 (r = 0.70; p less than 0.001). During low Bc-D the mean arterial pressure remained almost the same, while during medium and high Bc-D a significant decrease of mean arterial pressure was observed. Multivariate analysis revealed that only the change in ionized calcium had a significant predictive effect on the changes of blood pressure (r = 0.69; p less than 0.001). In conclusion, during dialysis an increasing ionized plasma calcium level was counteracting the negative effect of fluid removal on blood pressure. The results of this study underline the pivotal role of ionized plasma calcium for the regulation of blood pressure.