Cardiorespiratory Variability: Fractals, White Noise, Nonlinear Oscillators, and Linear Modeling. What’s to Be Learned

Abstract

Over the last 20 years, the study of heart rate (HR) variability has progressed from a phenomenological description of the spontaneous oscillations of HR to an understanding that relatively complex physiological control systems underlie the observed beat-to-beat fluctuations. Many of the current concepts have evolved from the use of either linear analysis techniques or linear systems modeling to evaluate classical physiological control systems such as respiratory sinus arrhythmia or the arterial baroreflex. Despite these efforts, linear techniques have only been successful at explaining a portion of the total variance in HR and arterial blood pressure (ABP). The remaining variance must be accounted for in one of two ways: 1) more complex linear models, or 2) nonlinear system elements which may contribute both locally and globally to the process of physiologic equilibrium known as homeostasis. This paper attempts to briefly examine both possibilities.