The heart as a chronotaxic system — Why its rate variability is both complex and simple: Theory and analysis methods

The heart rate variability (HRV) is analysed under the framework of a new class of self-sustained oscillators known as chronotaxic systems. These oscillators have time-varying frequencies that can be perturbed by noise but remain stable, resulting in dynamics that can appear both complex and simple. Methods developed for the inverse approach to these systems are applied to an HRV signal recorded under quasi-periodically forced respiration. The analysis suggests that the respiratory-related oscillation in the signal has the properties of a chronotaxic oscillator and an addition to an existing HRV model is proposed.