Mathematical modeling of buffers used in myocardial preservation

Objective. Buffers added to myocardial preservation solutions are considered to be critical for resisting myocardium pH changes from the accumulation of protons (H⁺). Our hypothesis is that mathematical modeling of three clinically used buffers will define their individual buffering capacities under simulated clinical conditions. Methods. The buffers, tromethamine (THAM), sodium bicarbonate (HCO₃^-), and L-histidine, were compared in terms of their buffering capacity (β) under specific temperatures and concentrations, using a mathematical model. Results. At 37°C, the maximal beta (β_max) occurred at pH 7.75 for THAM, pH 6.10 for HCO₃^-, and pH 5.89 for L-histidine at equimolar concentrations. A decrease in temperature moved β_max to a higher pH value for each buffer. At clinical concentrations, L-histidine provided the greatest buffering capacity followed by HCO₃^- and THAM, respectively. Discussion. This model permitted comparison of the above buffers under simulated clinical conditions. The assumption was that the magnitude of β_max at a given temperature determines which buffer(s) could be most effective for myocardial preservation. Also, the assumption was taken that these buffers are used in a closed system - where there is no continuous blood flow - and that the buffering ability of THAM and L-histidine were not influenced by the accumulation of CO₂ as is HCO₃^-. THAM and L-histidine were more effective at hypothermic temperatures compared with HCO₃^-; however, HCO₃^- provided buffering at normothermic temperatures. Through the theoretical considerations of this study, we propose that combining HCO₃ ^- with THAM or L-histidine could be most efficacious for myocardial preservation during open heart surgery or organ transplantation.