Variable heat shock response model for medical laser procedures.

According to the standard Arrhenius relation, tissue damage is linearly dependent on the duration of exposure to elevated temperatures and exponentially dependent on the temperature itself. However, recently published measurements of damage threshold temperatures at extremely short exposure times (commonly present during laser treatments) exhibit a shift to temperatures that are higher than what would normally be expected from a single-process Arrhenius model. A novel variable heat shock (VHS) response model was developed that takes into account the observed deviation from the single-process Arrhenius relation, by assuming that the cell viability can be described as the combined effect of two biochemical processes that dominate cell survival characteristics at very short and very long exposure times. The potential implications of the VHS model are explored theoretically through an example of non-ablative laser resurfacing. The VHS model shows that under the appropriate conditions, very high temperature heat shocks can be generated within the superficial epithelium tissue layer without causing irreversible tissue damage. A mechanism of action for tissue regeneration by means of non-ablative resurfacing with the Er:YAG laser is proposed, which involves indirect triggering of tissue regeneration through intense heat shock to the epithelia, in addition to the tissue regeneration mechanism by means of direct thermal injury to deeper lying connective tissues.