Determining cold strain in cold air: a comparison of two methods of partitional calorimetry to calculate heat storage and debt in cold air with mild hypothermia

We compared two methods of partitional calorimetry to calculate heat storage and heat debt during cold air (0°C) exposure causing mild core cooling. Twelve participants performed a 5 min baseline in thermoneutral conditions (∼22.0°C, ∼50% relative humidity) followed by cold air exposure (∼0°C) until rectal temperature was reduced by ∆-0.5°C. Partitional calorimetry was used to calculate avenues of heat exchange (radiative, convective, and evaporative), heat storage, and heat debt continuously throughout cold exposure. We compared deriving these variables using prediction equations based on environmental and participant characteristics (PCAL_{Equation Method}) versus using measurement tools such as humidity sensors and heat flux discs (PCAL_{Heat Flux Method}). There were significant differences between methods (all p ≤ 0.001) for determining heat exchange, heat storage, and heat debt. At ∆-0.5°C, PCAL_{Heat Flux Method} had greater levels of radiative and convective heat exchange (PCAL_{Heat Flux Method}: -143.0 ± 16.8 W∙m² vs PCAL_{Equation Method}: -123.0 ± 12.9 W∙m², p ≤ 0.001), evaporative heat exchange (PCAL_{Heat Flux Method}: -9.0 ± 1.7 W∙m² vs PCAL_{Equation Method}: -4.1 ± 0.0 W∙m², p ≤ 0.001), heat storage (PCAL_{Heat Flux Method}: -15.0 ± 31.0 W∙m² vs PCAL_{Equation Method}: +6.0 ± 25.9 W∙m², p = 0.020), and heat debt (PCAL_{Heat Flux Method}: -692.0 ± 315.0 kJ vs PCAL_{Equation Method}: -422.0 ± 136.0 kJ, p ≤ 0.001). Overall, this study found the largest discrepancies between the two methods were when the environmental conditions and skin temperature were in high flux, as well as when core temperature was reduced by ∆-0.5°C. The use of PCAL_{Heat Flux Method} may be more advantageous to use in the cold to provide a higher resolution measurement of cold strain.