A physiological strain index (PSI), based on rectal temperature (Tre) and heart rate (HR), capable of indicating heat strain online and analyzing existing databases, has been developed. The index rates the physiological strain on a universal scale of 0–10. It was assumed that the maximal Tre and HR rise during exposure to exercise heat stress from normothermia to hyperthermia was 3°C (36.5–39.5°C) and 120 beats/min (60–180 beats/min), respectively. Tre and HR were assigned the same weight functions as follows: PSI = 5(Tre t − Tre0) ⋅ (39.5 − Tre0)−1+ 5(HR t − HR0) ⋅ (180 − HR0)−1, where Tre t and HR t are simultaneous measurements taken at any time during the exposure and Tre0 and HR0 are the initial measurements. PSI was applied to data obtained from 100 men performing exercise in the heat (40°C, 40% relative humidity; 1.34 m/s at a 2% grade) for 120 min. A separate database representing seven men wearing protective clothing and exercising in hot-dry and hot-wet environmental conditions was applied to test the validity of the present index. PSI differentiated significantly ( P < 0.05) between the two climates. This index has the potential to be widely accepted and to serve universally after extending its validity to women and other age groups.

A multidimensional, finite difference numerical scheme for the freezing process of biological tissues during cryosurgery is presented, which is a modification of an earlier numerical solution for inanimate materials. The tissues are treated as nonideal materials, freezing over a temperature range and possessing temperature-dependent thermophysical properties, blood perfusion, and metabolic heat generation. The numerical scheme is based on the application of an effective specific heat, substituting the intrinsic property, to include the latent heat effect within the phase transition temperature range. Results of the numerical solution were verified against an existing exact solution of a one-dimensional inverse Stefan problem in Cartesian coordinates. Results were further validated against experimental data available from the literature. The utility of the numerical solution for the design and application of cryodevices is demonstrated by parametric studies of the freezing processes around spherical and cylindrical cryoprobes. The parameters studied are the cryoprobe cooling power and the dimensions of the frozen region. Results are calculated for typical thermophysical properties of soft biological tissues, for angioma and for water.

The wind-chill index (WCI), developed in Antarctica in the 1940s and recently updated by the weather services in the USA and Canada, expresses the enhancement of heat loss in cold climates from exposed body parts, e.g., face, due to wind. The index provides a simple and practical means for assessing the thermal effects of wind on humans outdoors. It is also used for indicating weather conditions that may pose adverse risks of freezing at subfreezing environmental temperatures. Values of the WCI depend on a number of parameters, i.e, temperatures, physical properties of the air, wind speed, etc., and on insolation and evaporation. This paper focuses on the effects of various empirical correlations used in the literature for calculating the convective heat transfer coefficients between humans and their environment. Insolation and evaporation are not included in the presentation. Large differences in calculated values among these correlations are demonstrated and quantified. Steady-state wind-chill-equivalent temperatures (WCETs) are estimated by a simple, one-dimensional heat-conducting hollow-cylindrical model using these empirical correlations. Partial comparison of these values with the published "new" WCETs is presented. The variability of the estimated WCETs, due to different correlations employed to calculate them, is clearly demonstrated. The results of this study clearly suggest the need for establishing a "gold standard" for estimating convective heat exchange between exposed body elements and the cold and windy environment. This should be done prior to the introduction and adoption of further modifications to WCETs and indices. Correlations to estimate the convective heat transfer coefficients between exposed body parts of humans in windy and cold environments influence the WCETs and need to be standardized.

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A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.