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Thermal damage in heated bovine myocardial tissue is assessed from measured changes in total reflection and transmission of light. Mathematical expressions, based on random walk analysis of light propagation within tissue slabs, are used to relate the diffuse reflection and transmittance to the absorption coefficient, [[micron].sub.a], and effective scattering coefficient, [[micron][prime].sub.s] for samples of myocardial tissue which were subjected to rapid step changes in temperature. Time-dependent changes in [[micron][prime].sub.s] indicate two processes, one with a fast and temperature-dependent rate the other with a slow and apparently temperature-independent rate. For final temperatures above 56.8 [degrees] C and for the first 500 s after the temperature change, the optical parameters are well fit by exponential forms that exhibit temperature-dependent time constants as predicted by Arrhenius reaction rate theory of thermal damage. The scattering changes are associated with an apparent activation energy, [Delta]E, of 162 kJ/mole and a frequency constant, A, of 3x [10.sup.23] [s.sup.-1]. This method provides a means for estimating optical coefficients which are needed to assess laser tissue dosimetry.

Electrical conductivities in the bidomain model of cardiac tissue are expressed as functions of four parameters. These expressions allow simulations to be performed using nominal, equal, and reciprocal anisotropy without introducing undesired effects, such as length constant variations. Relative values of the bidomain conductivities are estimated to be: [[Sigma].sub.iL] = 1, [[Sigma].sub.iT] = 0.1, [[Sigma].sub.eL] = 1, and [[Sigma].sub.eT] = 0.4. Index Terms - Anisotropy, bidomain, cardiac tissue, conductivity.

The analyses made by A.T. Winfree regarding critical cardiac curvatures failed to take into account the influence of propagation fronts caused by boundary effects on the isotropic properties of cardiac tissue. The author did not recognize that the value for critical curvatures obtained by rescaling to anisotropy is not the same as that for critical curvatures that are theoretically present on either axis in the heart. Winfree's conclusions can have detrimental consequences on the design of small pacing electrodes for the cardiovascular system.