Over the course of the past two decades, numerous studies (on both animals and human beings) have shown a significant relationship between the autonomic nervous system (ANS) and cardiovascular mortality, particularly in patients with myocardial infarction and congestive heart failure. Perturbations of the ANS and its imbalance consisting of either increased sympathetic or reduced vagal activity may result in ventricular tachyarrhythmias and sudden cardiac death, which is nowadays one of the leading causes of cardiovascular mortality.
There have emerged numerous methods available for assessing the status of the ANS, which include cardiovascular reflex tests, and biochemical and scintigraphic tests. In recent years, noninvasive techniques based on the electrocardiogram (ECG) have been used as markers of autonomic modulation of the heart. These include heart rate variability (HRV), baroreflex sensitivity (BRS), QT interval, and heart rate turbulence (HRT), a new method based on fluctuations of sinus rhythm cycle length after a single premature ventricular contraction. Among these techniques, analysis of HRV has emerged as a simple, noninvasive method to evaluate the sympatho-vagal balance at the sinoatrial level.
Heart rate variability is the physiological phenomenon of the variation in the time interval between consecutive heartbeats in milliseconds. A normal, healthy heart
does not beat evenly like a metronome, but instead, when looking at the milliseconds between heartbeats, there is constant variation. Heart rate variability is a noninvasive electrocardiographic marker reflecting the activity of the sympathetic and vagal components of the ANS on the sinus node of the heart. It expresses the total amount of variations of both instantaneous HR and R-R intervals (intervals between QRS complexes of normal sinus depolarisations). HRV, in other words, measures the heart’s contractile variation from beat to beat by evaluating the consistency between R waves (R-R intervals), as seen on an electrocardiogram (ECG) reading and gives information about the tonic baseline of autonomic nervous system function.
The following selected articles describe HRV as it pertains to bioregulation and the autonomic nervous system.