The Autonomic Nervous System (ANS) consists of two opposite pathways (sympathetic and parasympathetic) that have action both on the atrial and ventricular myocardium, as well as on atrioventricular and sinus nodes, thus exerting influence on the variation of heart rate (HR). The increase in HR is a consequence of the greater action of the sympathetic pathway and the lower parasympathetic activity (vagal inhibition), while its reduction basically depends on predominance of vagal activity. The influence of ANS on the heart is dependent on information that departs, among others, from baroceptors, chemoreceptors, changes in the respiratory system, vasomotor system, renin-angiotensin-aldosterone system and thermoregulatory system.

The discovery of the relationship between ANS and mortality from cardiovascular diseases has led to several studies, which have proven the existence of reduced parasympathetic and increased sympathetic activity in several pathologies of the cardiovascular system. These findings made it necessary to develop quantitative markers of cardiac autonomic activity, and heart rate variability (HRV) is the most promising autonomic marker . Historically, their clinical interest arose in 1965, when Hon and Lee demonstrated a well-defined clinical application of HRV in the area of fetal distress monitoring. In 1977, Wolf et al., showed an association between low HRV and higher risk of mortality after acute myocardial infarction and Kleiger et al., in 1987, confirmed that HRV was a potent and independent predictor of mortality after acute myocardial infarction. In 1996, in an age-stratified cross-sectional study, Barbosa et al., reported >18 % per decade reduction in HRV in healthy adults and >25 % per decade reduction in subjects two years post-myocardial infarction with persevered left ventricular systolic function. Additionally, the authors observed that aging related HRV decrease was mainly related to reduction in spontaneous vagal activity. In 2002, Nunes studied HRV variability in preschool healthy children, defining normality ranges for HRV variables in this population.

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