Issues in modeling thermal alterations in tissues

Thermal injury in living tissues is commonly modeled as a rate process in which cell death is interpreted to occur as a function of a single kinetic process. Experimental data indicate that multiple rate processes govern the manifestation of injury and that these processes may act over a broad spectrum of time domains. Injury is typically computed as a dimensionless function (omega) of the temperature time history via an Arrhenius relationship to which numerical values are assigned based on defined threshold levels of damage. However, important issues central to calculation and interpretation of the omega function remain to be defined. These issues include the following: how is temperature identified in time and space within a tissue exposed to thermal stress; what is the biophysical and physiological meaning of a quantitative value for omega; how can omega be quantified in an experimental system; how should omega be scaled between graded levels of injury; and what are the differences in injury kinetics between unit volume- and unit surface area-governed processes of energy deposition into tissue to cause thermal stress? This paper addresses these issues with the goal of defining a more rigorous and comprehensive standard for modeling thermal injury in tissues.