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The rhythm of the heart is influenced by both the sympathetic and parasympathetic branches of the autonomic nervous system. There are also some feedback mechanisms modulating the heart rate that try to maintain cardiovascular homeostasis by responding to the perturbations sensed by baroreceptors and chemoreceptors. Another major influence of the heart rate is the respiratory sinus arrhythmia: the heartbeat synchronization with the respiratory rhythm. All these mechanisms are responsible for continuous variations in the heart rate of a healthy individual, even at rest. These variations are referred to as heart rate variability (HRV). Subtle characteristics of these small variations conceal information about all the mechanisms underlying heart rate control, and hence about the health status of the individual.

Since the 1960s, researchers have developed a wide range of algorithms to extract the information hidden in …