Development of a Rat Model of Hypothermia

Abstract : Hypothermia can significantly impact the outcome of military missions, since it is a seasonal and geographic pervasive physiological phenomenon that reduces not only soldier performance, but may lead to their death. Moreover, military operational stress (MOS) such as exhaustive exercise, caloric restriction and sleep deprivation may enhance soldier vulnerability to hypothermia. Understanding the full influence of MOS on hypothermia morbidity and mortality requires an animal model, since ethical considerations in regards to their health preclude the use of human volunteers. A model of hypothermia was developed that employed rates (male; Sprague-Dawley; 250.5 +/- 7.3 g) immediately exposed to circulating (0.7 +/- 0.3 L/min) cool (10 degrees C) water at a non-full immersion depth of 5 cm. During exposure to cool/wet conditions animals assumed a water avoidance posture of an upright position such that only their hindquarters were exposed to the water. Moderate hypothermia (32 +/- 3 degrees C) was induced within a 2 to 4h timeframe in which animal activity and, core (Tc) and brown adipose tissue (BAT) temperature could be monitored. Thermoregulatory temperature profiles for hypothermia induction and recovery were variable among the animals. Markers of hypothermia induction and recovery were identified as: 1)lowest hypothermia Tc; 2)time to lowest hypothermia Tc; 3)thermoregulatory maintenance time post lowest hypothermia Tc; 4)cooling rate; 5)37 degrees C Tc recovery time from lowest hypothermia Tc; 6)re-warming rate and 7)length of BAT thermogenic response. This rat model features a hypothermia induction vehicle (immediate cool water exposure) and vehicle exposure pattern (lower extremities) that reflects the militarily relevant scenario of sudden soldier exposure to waist-deep cool water. Its military relevance in make this model well suited to characterize the influence of MOS on hypothermia morbidity and mortality.