Dialysate Calcium Loss Using a Calcium Free Dialysate

Abstract

Using a dialysate that contained no calcium, we studied the amount of calcium lost and the clearances of urea, creatinine, phosphate and calcium in fifteen different dialyzers at blood flow/min (Q_B) of 200 and 300. Five of these dialyzers, covering the functional range of the fifteen, were also tested at Q_B of 100, 150 and 400. The collection period was divided into the first twenty minutes and the remaining 220 minutes. Clearance was calculated using the initial twenty minutes and for all dialyzers the creatinine, phosphate and calcium clearances were linearly related to the measured urea clearnce, r = 0.946, 0.810 and 0.808 respectively. Calcium recovered in the dialysate varied, depending on the dialyzer and Q_B, however the amount found plateaued at Q_B 200 and did not increase further, unlike creatinine, phosphate and urea. Total dialysate calcium loss for a four-hour dialysis averaged 49.9 mmol (range 42.2 -63) or almost 2g of elemental calcium for Q_B of 200 ml/min and 48.9 mmol (range 31.1 -65.5) or 1.95 g of elemental calcium for a Q_B of 300 ml/min. Decreasing the calcium infusion replacement rate did not decrease the calcium dialysate loss in equal amounts as the blood to dialysate gradient falls leading to a smaller than expected dialysate loss. Measured urea clearance in these fifteen dialyzers was approximately 20% less than specified by the manufacturer and almost 30% less for creatinine and phosphate. This difference is probably due to in vivo measurements with blood whereas most of the specifications are obtained using saline in vitro. The data given provide a basis for calcium replacement when using a calcium free dialysis and a variety of dialyzers and blood flow rates.