Heart rate variability (HRV) is a measure of the continuous interplay between sympathetic and parasympathetic influences on heart rate that yields information about autonomic flexibility and thereby represents the capacity for regulated emotional responding.
The sympathetic (SNS) and parasympathetic (PNS) branches of the autonomic nervous system (ANS) antagonistically influence the lengths of time between consecutive heartbeats. Faster heart rates, which can be due to increased SNS and/or lower PNS activity, correspond to a shorter interbeat interval while slower heart rates have a longer interbeat interval, which can be attributed to increased PNS and/or decreased SNS activity.
The frequency-based HRV analyses are based on the fact that the variations in heart rate produced by SNS and PNS activity occur at different speeds, or frequencies. SNS is slow acting and mediated by norepinephrine while PNS influence is fast acting and mediated by acetylcholine.