Prediction of Tissue Perfusion and Heat Generation Rates by an Improved Heat Clearance Technique

Abstract

Thermodilution (heat clearance) was introduced 50 years ago for prediction of blood flow rate in tissue. The method lost favour because of the size of the temperature probes, technical requirements for generation of a tissue temperature perturbation, and lack of appreciation of interplay of conduction, heat storage, and convective transport. Recent progress has eliminated several of these objections. Tissue temperature shifts may be reliably sensed to ± 0.05°C with commercially available probes 0.8 mm in diameter. A heat balance in tissue has been developed, called the bio-heat transfer equation, which allows explicit account of all heat transport mechanisms.1 The biological medium is assumed to have homogeneous and isotropic thermal properties. Tissue blood flow is non-directional at the capillary level, that is, the capillaries are assumed to be randomly oriented with respect to their arteriolar and venular connections. Convective heat exchange occurs only in this capillary system; arteriovenous anastomoses and other arteriovenous heat exchange mechanisms play no role. Tissue blood flow is assumed so low that tissue and end-capillary temperatures are in equilibrium. Tissue blood flow may not vary in the thermal influence region of the probe. Local heat production is ignored, but can be introduced with little increase in mathematical complexity. Under these conditions the one dimensional bio-heat transfer equation in tissue is Open image in new window