Low cardiac output as physiological phenomenon in hibernating, free-ranging Scandinavian brown bears (Ursus arctos) - an observational study

Abstract

Background: Despite 5-7 months of physical inactivity during hibernation, brown bears (Ursus arctos) are able to cope with physiological conditions that would be detrimental to humans. During hibernation, the tissue metabolic demands fall to 25% of the active state. Our objective was to assess cardiac function associated with metabolic depression in the hibernating vs. active states in free-ranging Scandinavian brown bears.

Methods: We performed echocardiography on seven free-ranging brown bears in Dalarna, Sweden, anesthetized with medetomidine-zolazepam-tiletamine-ketamine during winter hibernation in February 2013 and with medetomidine-zolazepam-tiletamine during active state in June 2013. We measured cardiac output noninvasively using estimates of hemodynamics obtained by pulsed wave Doppler echocardiography and 2D imaging. Comparisons were made using paired T-tests.

Results: During hibernation, all hemodynamic indices were significantly decreased (hibernating vs. active state): mean heart rate was 26.0 (standard deviation (SD): 5.6) beats per min vs. 75.0 (SD: 17.1) per min (P=0.002), mean stroke volume 32.3 (SD: 5.2) ml vs. 47.1 (SD: 7.9) ml (P=0.008), mean cardiac output 0.86 (SD: 0.31) l/min vs. 3.54 (SD: 1.04) l/min (P=0.003), and mean cardiac index 0.63 (SD: 0.21) l/min/kg vs. 2.45 (SD: 0.52) l/min/ m2 (P<0.001). Spontaneous echo contrast was present in all cardiac chambers in all seven bears during hibernation, despite the absence of atrial arrhythmias and valvular disease.

Conclusion: Free-ranging brown bears demonstrate hemodynamics comparable to humans during active state, whereas during hibernation, we documented extremely low-flow hemodynamics. Understanding these physiological changes in bears may help to gain insight into the mechanisms of cardiogenic shock and heart failure in humans.

Figure 1

Differences in heart rate, stroke volume, cardiac output, and cardiac index between the hibernating and active states of free-ranging brown bears. First row shows repeated measurements from each bear, second row shows mean values and standard deviations. Levels of significance: *P < 0.05, **P < 0.01 and ***P < 0.001. During hibernation, all assessed hemodynamic parameters were significantly decreased as an adaptation to low energy demands. During hibernation, the cardiac index, which is cardiac output related to body size, had a level that would imply very severe cardiogenic shock in humans and, thus, would be largely incompatible with life.

Figure 2

Comparison of 2D echocardiographic images from the hibernating and active states of free-ranging brown bears. An arrow indicates the presence of spontaneous echo contrast as echo-dense shadows during hibernation. Also see Additional files 1 and 2.