In the 18th century, Albrecht von Haller1– made the initial observation that the beat of a healthy heart is not absolutely regular. Heart rate and rhythm are governed by the intrinsic automaticity of the sinoatrial node and the modulating influence of the autonomic nervous system. Vagal tone dominates under resting conditions,2– and rhythmic variations in heart rate are largely dependent on vagal modulation.3– The vagal and sympathetic nervous system constantly interact. The stimulation of the vagal afferent fibers leads to the reflex excitation of vagal efferent activity and the inhibition of sympathetic efferent activity.4– The opposite reflex events are mediated by the stimulation of sympathetic afferent activity.5– Central oscillators (ie, vasomotor and respiratory centers) and peripheral oscillators (ie, oscillation in arterial pressure and respiratory movements) can further modulate the efferent sympathetic and vagal activities that are directed to the sinus node.6 These oscillators generate rhythmic fluctuations in efferent neural discharge that are manifested as short-term and long-term oscillation in beat-to-beat intervals and periodic heart rates.6Heart rate variability (HRV) is a conventionally accepted term that is used to characterize these heart rate fluctuations. The analysis of HRV permits inferences to be made about the state and function of the central oscillators, autonomic efferent activity, humoral factors, and the sinus node.