Calcium mass transfer in CAPD: the role of convective transport

One hundred and fifty calcium (Ca) balance studies were performed in 50 patients treated with CAPD using dialysate with a 1.75 mmol/l (7 mg/dl) Ca content, in order to calculate the peritoneal balance of Ca by measuring the Ca in all the effluent for a 24-h period, and looking at the influence of serum ionized Ca and the ultrafiltration rate in the calcium balance. Of the 150 balance studies, 77 were made using four exchanges of dialysate per day and 73 using three exchanges per day. The serum ionized Ca was 1.17 +/- 0.09 mmol/l, the ultrafiltration 844 +/- 723 ml/day and the peritoneal Ca transfer 39 +/- 46 m/day. The net Ca abortion with four exchanges was less than that with three exchanges per day. There was a strong negative correlation between the peritoneal Ca absorption and the ultrafiltration (r = -0.7, P < 0.00001) and with the ionized Ca (r = -0.49, P < 0.0001). Thirty-three peritoneal balance studies showed a negative Ca balance and in all 33 cases ultrafiltration was greater than 350 ml/day. We conclude that the peritoneal balance of Ca depends not only on the serum ionized Ca, but also on ultrafiltration. The lesser Ca gain observed with four dialysis exchanges per day is due to greater ultrafiltration rates present in this setting.