The physiological strain index applied to heat-stressed rats

A physiological strain index (PSI) based on heart rate (HR) and rectal temperature (T(re)) was recently suggested to evaluate exercise-heat stress in humans. The purpose of this study was to adjust PSI for rats and to evaluate this index at different levels of heat acclimation and training. The corrections of HR and T(re) to modify the index for rats are as follows: PSI = 5 (T(ret) - T(re0)) · (41.5 - T(re0))^-1 + 5 (HR(t) - HR₀) · (550 - HR₀)^-1, where HR(t) and T(ret) are simultaneous measurements taken at any time during the exposure and HR(0) and T(re0) are the initial measurements. The adjusted PSI was applied to five groups (n = 11-14 per group) of acclimated rats (control and 2, 5, 10, and 30 days) exposed for 70 min to a hot climate [40°C, 20% relative humidity (RH)]. A separate database representing two groups of acclimated or trained rats was also used and involved 20 min of low-intensity exercise (O₂ consumption ~50 ml · min^{- 1} · kg^-1) at three different climates: normothermic (24°C, 40% RH), hot- wet (35°C, 70% RH), and hot-dry (40°C, 20% RH). In normothermia, rats also performed moderate exercise (O₂ consumption ~60 ml · min^-1 · kg^-1). The adjusted PSI differentiated among acclimation levels and significantly discriminated among all exposures during low-intensity exercise (P < 0.05). Furthermore, this index was able to assess the individual roles played by heat acclimation and exercise training.