The EKG of black bears (Ursus americanus) in hypothermia compared with that of humans

Abstract

Hearts of hibernators can resist lethal arrhythmias seen in hypothermic non‐hibernators. We present data on the effects of hypothermia on hearts of large hibernators, black bears. Hypothermia is used in cardiac surgery and survivors of cardiac arrest, but can also be fatal, leading to arrhythmias such as ventricular fibrillation and asystole. We compare electrophysiologic changes the heart undergoes in hypothermia between black bears and humans. Specifically, we compare heart rate and QT interval (QTi) with core body temperature. The last part of the QTi is the time during which the heart electrically repolarizes and mechanically relaxes. During this time the heart is vulnerable to fibrillation. Animal data was obtained from radio capsules implanted in 3 black bears that were made progressively hypothermic. Human data came from published literature. Black bears and humans show a direct relationship between heart rate and body temperature. Both bears and humans developed J (Osborne) waves, the amplitude of which correlated with the degree of hypothermia. As heart rate and body temperature decrease, QTi increases. However, 2 of the bears demonstrated a shortening of the QTi as their core temperature reached 21.8°C. After this point, the QTi began to prolong. This perturbation was not seen in human subjects, and was an unexpected finding. Does the hibernating heart attempt to resist hypothermia? Supported by ONR, NSF.