Autonomic influences on Heart rate variability Self-organized criticality: Insights of Zipf's law

Cardiovascular self-organized criticality has recently been demonstrated. A better characterization of this cardiovascular dynamics is needed. We studied a model of autonomic nervous system changes according to body position and exercise training to better characterize heart rate variability self-organized criticality. We counted and classified bradycardia episodes according to their length in number of beats. We checked whether these episodes are distributed according to Zipf's law, a feature of self-organized criticality. Bradycardia episodes are distributed according to Zipf's law with no influence of autonomic changes. Zipf's law is broken for delays shorter than four beat intervals. Zipf's law could also be broken by training in some subjects. Zipf's law confirms the self-organized nature of heart rate variability. However, Zipf's law could be broken, the significance of which remains unclear.