Regulation of cardiac rhythm in hibernating mammals

The dramatic fall in heart rate exhibited by mammals entering hibernation b egins before there is any noticeable fall in body temperature. The initial, progressive decrease in heart rate is the result of a cyclic parasympathet ic activation that induces skipped beats and regular asystoles as well as s lows the even heart beat. As body temperature subsequently falls, the paras ympathetic influence is progressively withdrawn and periods of parasympathe tic and sympathetic dominance alternate and give rise to regular periods of arrhythmia (tachycardia followed by bradycardia), and occasional long asys toles or periods of highly irregular cardiac activity. Superimposed on this is a vagally-mediated, respiratory sinus arrhythmia that is accentuated in species that breathe episodically. These events give way to a uniform hear t rate in deep hibernation at low temperatures where both parasympathetic a nd sympathetic tone appear absent. The complete absence of tone is not a fu nction of reduced temperature but is reflective of the state of deep, stead y state hibernation. The elevation in heart rate that accompanies the onset of arousal is the result of dramatic increases in sympathetic activation t hat precede any increases in body temperature. As body temperature then ris es, sympathetic influence is slowly withdrawn. Arrhythmias are also common during natural arousals or shifts from lower to warmer hibernation temperat ures as periods of parasympathetic and sympathetic dominance again alternat e en route to re-establishing a steady state in euthermia. The mechanism be hind, and the biological significance of, cardiac changes mediated through orchestrated arrhythmias remain unknown. (C) 1999 Elsevier Science Inc. All rights reserved.