The model was used to analyze four reported clinical studies [1-4]. The analyses consisted of fitting calculated plasma concentrations to the reported values with the model and the calculation of mass balance as described above and comparing to that reported for dialysate. The serial VCa2+ was estimated as 1/3 of total body water (TBW) considered to average 33 liters. If weight loss was given this was used as total Qf and if not (usually the case) it was assumed to be 2 liters. Discussions of the individual data set analyses follow.
This is the most complete data set available . Nine equally spaced serial values for CpiCa were measured over 4-hour dialyses in 6 patients with three different CdiCa2+ and mean values were reported. Total dialysate was collected during each sequential interval to measure total dialysate removal of Ca. These identical measurements were made using three different levels of CdiCa: 3.50,2.50 and 1.50 mEq/l. Body water and Qfwere not given, so average V = 33 liters and QfT = 2.0 liters were assumed for the calculations. The Ca dialysance (D Ca) was calculated for each sequential intradialytic interval from the reported mean rate of JdCa measured during the interval divided by the Cdi Ca minus mean CpiCa during the interval in accordance with:
recalling that all dialysate Ca is considered ionized and the total Ca concentration in plasma (CpiCaT) expressed in mM is considered numerically equal to CpiCa expressed in mEq/l. Serial values of CpCa were calculated with equation 9 and also with MCa = 0.
The serial measured and model calculated Cpi Ca and CdiCa values are plotted for the Hou data in figure 3. The calculated values agree very closely with values observed with all three dialysate concentrations when MCa is adjusted with equation 9. The powerful effect of M-Ca can also be seen. The values for CpiCa have nearly reached equilibrium with Cdi Ca when MCa = 0 in the calculations. Note that although there is some change in CpCa2+, the plasma concentration is far from equilibrium with CdiCa2+ after 4 h of dialysis with substantial positive and negative Ca concentration gradients. The Ca buffer pool was very effective to minimize change in CpiCa. The properties of this pool will be discussed further below.
Koo et al.: Studies of Ca-Free Dialysate for
Koo et al.  reported CpCaT with CdiCa = 0 values in 7 studies of 6 patients with hypercalcemia and renal failure due to malignancies. They reported CpiCaT values pre-dialysis, at 1 h of dialysis and postdialysis, and Kt/Vu. VCa was estimated as 11 liters and D Ca was estimated to be 50 ml/min in these studies from reported dialyzers and Qb. A representative plot of one of these studies is shown in figure 4. Note that the modeled values correlate closely with measured values and M+Ca is consistently positive showing substantial mobilization of Ca to support the serum Ca with a strongly negative blood to dialysate concentration gradient and minimize the fall in Cpi Ca.
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