[A mathematical calculation model of replacement fluid and citric acid infusion in continuous veno-venous...

To design a mathematical calculation model for better understanding and grasping the logical problem of replacement fluid and citric acid anticoagulant infusion in continuous veno-venous hemofiltration (CVVH).

(1) Parameter definition: A, B, and T were respectively called the main part of pre-replacement fluid, 5% sodium bicarbonate solution, and 4% sodium citrate infused before filter. And a and b were respectively called the main part of post-replacement fluid, and 5% sodium bicarbonate solution infused after filter. (2) Logic conversion: The liquid in back terminal (Z) was artificially divided into two parts. One (X) was the original residual plasma after filtration. The second (Y) was the part excluding the plasma, including the left part of pre-replacement fluid with sodium citrate, and the post-replacement fluid. (3) The mathematical formulas of liquid volume and electrolyte concentration at X, Y and Z in unit time were listed according to the principle of CVVH and the screening coefficient of filter for different substances. (4) The calculation formulas were entered into Excel form, and a mathematical calculation model was made, and a simulation calculation with examples was carried out.

An Excel model was established by inserting the calculation formulas of volume, electrolyte, and total calcium at X, Y and Z. And it was found that the concentration of Na⁺, K⁺, Cl^-, HCO₃^- at Y point remained unchanged only when A, B and (or) a, b was kept in same side and proportion even with the change of blood flow and other parameters without sodium citrate as anticoagulant. Once any of the parameters (such as blood flow, replacement fluid volume, etc.) were adjusted in other infusion methods (such as different ratios, different directions of the same year, etc.), the calculation results at Y would vary, and the electrolyte concentration at Z would change accordingly. A change of dilution model or parameter would result in the change of the electrolyte concentration at Y and Z with sodium citrate as anticoagulant. The concentration of total calcium scarcely changed no matter in what model and parameters.

All kinds of infusion ways could be included in the Excel model. The infusion results of all kinds of infusion matching could be intuitively evaluated. It is helpful for the medical staff to make a logical analysis and risk prediction in CVVH.