Evaluation of Thermal Efficacy of Hypothermic Tissue Preservation Methods

Abstract

The Two-Layer Method (TLM) of hypothermic preservation applied to cadaveric pancreata has been reported to permit improved islet cell yield and functional quality over simple immersion preservation in University of Wisconsin (UW) solution or perfluorocarbon (PFC) fluid alone. The mechanistic basis for improved islet cell outcomes with TLM preservation is argued by proponents of the TLM to be due to improved oxygen diffusion from PFC to tissue. However, no rigorous evaluation of this or alternative hypotheses has been reported. Because UW solution and PFC have significantly different thermal properties, the current study postulates that these characteristics could more likely than oxygen diffusion contribute to the salutary outcomes reported for the TLM. In the current study, an isotropic block of agarose gelatin is used as a tissue surrogate. The similarity in thermal behavior of agarose gelatin and pancreas tissue is validated. Thermal measurements are made in the tissue surrogate, when cooled by immersion in UW solution, PFC, and by TLM. Numerical calculations are performed to solve natural convection for all three methods. A multiphase model is used to solve for heat transfer between the two different fluid phases in the TLM. Results are expressed in terms of tissue cooling rates and surface heat transfer. At 30 min immersion, the instantaneous difference in temperature within the tissue surrogate immersed in UW solution and PFC is 8°C, while the same between UW solution and TLM is 4°C. The temporal heat transfer rate in the tissue surrogate immersed in UW solution is 1.8 times and 1.4 times higher than that achieved in PFC alone and TLM, respectively, at 15 min of preservation. The spatial and time averaged heat transfer coefficient in UW solution is 2 and 3 times higher than that of TLM and PFC alone, respectively, during the first 60 min of tissue immersion. The variation in heat transfer coefficients could thus contribute significantly to the outcomes achieved by tissue preservation using these three methods.