Comparing hypothermia in the human and the black bear (Ursus americanus)

Abstract

Hearts of small hibernators can resist lethal arrhythmias seen in hypothermic non-hibernators. While studies have confirmed this in small mammals, there are no data on large hibernators in hypothermia. We present data on the effects of hypothermia on hearts of large hibernators, black bears. We compare the electrophysiologic changes the heart undergoes in hypothermia between black bears and humans while also considering data obtained from hibernating bears. Specifically, we compare heart rate (HR) and QT interval with core body temperature. The last part of the QT is the time during which the heart electrically repolarises and mechanically relaxes. During this time the heart is vulnerable to fibrillation. Animal hypothermia data were obtained using human EKG equipment on three black bears that were made progressively hypothermic. Human data came from the published literature. Data from hibernating bears were obtained via implanted radio capsules. Black bears and humans show a direct relationship between heart rate and body temperature. Hypothermic bears and humans all developed J (Osborn) waves, the amplitude of which correlated with the degree of hypothermia. Hibernating bears did not develop J waves. As heart rate and body temperature decrease, QT increases. This held for hibernating bears. However, two of the hypothermic bears demonstrated a temporary shortening of the QT as their core temperature reached 21.8°C. After this point, the QT began to prolong. This perturbation was not seen in human subjects, and was an unexpected finding. This may represent the hibernating heart's attempt to decrease the risk of fibrillation in hypothermia.

Keywords:

• hypothermia
• QT interval
• cardiac electrophysiology
• hibernation

Acknowledgements

We thank Kevin C. Campbell, M. Bridget Zimmerman, Richard C. Simmonds, M.J. Krantz and C.M. Lowery, and the Arctic Aeromedical Laboratory.