THE RECENT USE OF HEART RATE VARIABILITY FOR RESEARCH

Heart rate variability (HRV) is a non-invasive method that analysis cardiac autonomic regulation 1,2 .The use of this method was previously validated in physiological experiments 3,4 ,which performed pharmacological stimulation of the sympathetic and parasympathetic nervous systems and made relationship with the traditional time and frequency domain analysis of HRV, confirming the use of this method to evaluate cardiac autonomic regulation.Moreover, HRV has also been used to predict mortality after cardiacevents 5 and it has been considered as a marker of stress 6 .An easy and economic access to HRV analysis through heart rate monitors, which are well recognized in the literature 7,8 helped researches to use this method to evaluate cardiac autonomic regulation.In the last 15 years this method has received great attention.After a careful search on Pubmed/Medline, the term "heart rate variability" was present in 1,013 manuscripts in 2014, in 2013 this number was 972 and in 2012 it was 729 9 .We realize that each year the number of groups that use HRV in their studies has considerably increased.In this context, I raise points that are worth to mention.Firstly, advantages includeadditional information for cardiac autonomic responses in pathological situations [10][11][12] , during physiological stress 13,14 , association with social function 15 for instance.It is currently clear that autonomic nervous system is related to several conditions.Nevertheless, before starting to study HRV analysis,relevant details of this method should be discussed in order to avoid misunderstanding of results.For example, HRV consider the interval between consecutive heart beats 2 , reflecting parasympathetic and sympathetic cardiac autonomic modulation, it does not directly measures sympathetic or parasympathetic activity.There are different methods that quantify it, i.e. plasma levels