Experimental Hyperthermia during Cardiac Arrest and CPR Is Associated with Severe Spontaneous Hypothermia in Mice

Background: Since genetically engineered mice are becoming more and more available, these animals become of high interest to study physiologic and pathophysiologic pathways of brain ischemia. The aim of this study was to examine body temperature (Tb), physical activity variation and neurohistopathology in mice exposed to normothermic and hyperthermic cardiac arrest and cardiopulmonary resuscitation (CA/CPR). Methods: Male C57Bl/6 mice weighing 22 - 27 g were implanted intraperitoneally with a radio telemeter and subjected to 10 min cardiac arrest followed by cardiopulmonary resuscitation. Normothermia (37.5°C) or hyperthermia (39.0°C) was induced by controlling pericranial temperature during the arrest period. Results: Hyperthermia during the arrest resulted in a Tb decrease during early recovery to a nadir of 28°C ± 0.8°C (mean ± SE) and partially recovered to 34.4°C ± 1°C 36 hrs after CA/CPR. With normothermia during the arrest, Tb depression was less pronounced (nadir of 32.3°C ± 0.3°C) and recovered to physiologic levels within 24 hrs. Coupling of physical activity and body temperature was absent in all animals after CA/CPR. Neuronal injury in the caudoputamen was greater in the hyperthermia group. Conclusions: This study demonstrates that CA/CPR eliminates normal connectivity between body temperature and physical activity and induces long-lasting hypothermia, the depth of which is related to severity of brain injury. Long term temperature monitoring is required in survival murine experiments, if body temperature is a study variable.