Ca Mass Balance over the Complete Dialysis Cycle

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Ca mass balance (accumulation in the body) will be determined by the net intake of Ca minus the removal of Ca as schematically depicted in figure 1. The volume of distribution for ionized Ca (VCa2+) is defined as being anatomically equal to the extracellular fluid volume (VECW). The ultrafiltrate during dialysis is considered uniformly removed from VCa, a well-mixed pool of ionized, diffusible Ca 2+. There is virtually no quantitative understanding of the magnitudes of intake, removal and accumulation with current dialysis therapy. We hope that we can learn to predict and control mass balance and the risk of Ca accumulation in the vascular system through use of kinetic modeling of Ca in dialysis therapy.

The Hou data reviewed here reported only total plasma Ca as mM (see Appendix) which is commonly the case with clinical data since ion-selective Ca electrodes are still not widely used. Consequently the interrelationships between the various units of the Ca concentration must be discussed. The total plasma Ca (C CaT) is the sum of approximately equal moieties CCaB and CCa2+ expressed as mass units, mM or mg/dl. Thus CCaT reported in mM can be taken as equal to the ionized, diffusible fraction, C Ca2+ mEq/l. This is very useful since C CaT expressed as m M provides a numerical value for C Ca2+ mEq/l, assuming the protein-binding ratio remains constant which has been assumed in the analyses developed below, and provides more realistic estimates of mass balance than calculations based on measurements of ionized Ca by ISE as will also be shown below.

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