Mathematical modeling of buffers used in myocardial preservation

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.

The buffers, tromethamine (THAM), sodium bicarbonate (HCO3-), and L-histidine, were compared in terms of their buffering capacity (beta) under specific temperatures and concentrations, using a mathematical model.

At 37 degrees C, the maximal beta (betamax) 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 betamax to a higher pH value for each buffer. At clinical concentrations, L-histidine provided the greatest buffering capacity followed by HCO3- and THAM, respectively.

This model permitted comparison of the above buffers under simulated clinical conditions. The assumption was that the magnitude of betamax 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.