Many people would presume the first graph belonged to a patient with severe heart problems, but they would be incorrect. In fact, the first one is that of a healthy person at sea level, the second of a healthy person at high altitude, the third of a person with obstructive sleep apnia and the fourth of a person with ventricular fibrillation. . The key observation here is that the variability of time intervals between heartbeats decreases as the subject's cardiac and respiratory functions become more unstable. Variability is not the same as randomness, however, and is actually the product of an underlying complex mathematical structure. We can describe it as "deterministic chaos", where the initial conditions of the sympathetic and parasympathetic nervous systems (influenced by thermo-regulation, hormones, sleep-wake cycles, meals, stress, etc.)  give rise to beat time intervals that are locally unpredictable, but exhibit a globally identifiable pattern. It is the intricate "music" that results from a complex orchestra of underlying instruments and notes, rather than the "noise" which would result from random tones. .
The time series of a healthy human heartbeat is a fractal structure, since it is an irregular pattern with fractional dimensions that exhibits "self-similarity" at different scales of resolution. We observe fractal structures in many different natural systems, whether it be a continental coast line, branching tree, human circulatory system or even impulses generated from biological processes. The fractal structures that arise from processes of the heart dynamically interact with all other rhythms of the body and help maintain a stable lifeform. Although these rhythms each exhibit a deterministic variability of their own, they also synchronize with each other so that different parts of the body can work together while staying within a bounded range of operation. An unhealthy human heart, on the other hand, is characterized by a collapse of inter-communication with other signals and a return to a regular, intrinsic rhythm that has lost its emergent order. .
|Fractal Blood Vessels|