ABSTRACT
Heart rate variability (HRV) is an expression of the immense complex interplay of various biological systems and subsystems. Heart rate (HR) is strongly modulated by the combined effects of the sympathetic and parasympathetic nervous systems that are affecting heartbeat generation in the sinoatrial node. In healthy subjects, the sinoatrial node located at the posterior wall of the right atrium initiates each beat of the heart. Due to the unstable membrane potential of the myocytes located in this region, action potentials are generated periodically at a fairly constant frequency. The measurement of HRV from the electrocardiogram (ECG) or pulse curve is a bedside or an ambulant, noninvasive, low-cost, and simple to perform method, requiring standard medical equipment and dedicated software. HRV analyses in the time domain evaluate variability by determining the variations of normal-to-normal RR intervals over a period of time.
