An Overview of HRV Norms and Metrics

Healthy biological systems exhibit complex patterns of variability that can be described by mathematical chaos. Heart rate variability (HRV) consists of changes in the time intervals between consecutive heartbeats called interbeat intervals (IBIs). A healthy heart is not a metronome. The oscillations of a healthy heart are complex and constantly changing, which allow the cardiovascular system to rapidly adjust to sudden physical and psycho-logical  challenges  to  homeostasis.  This  article  briefly  reviews  current  perspectives  on  the mechanisms that generate 24 h, short-term (~5 min), and ultra-short-term (<5 min) HRV,  the  importance  of  HRV,  and  its  implications  for  health  and  performance.  The  authors provide an overview of widely-used HRV time-domain, frequency-domain, and non-linear metrics. Time-domain indices quantify the amount of HRV observed during monitoring  periods  that  may  range  from  ~2  min  to  24  h.  Frequency-domain  values  calculate  the  absolute  or  relative  amount  of  signal  energy  within  component  bands.  Non-linear measurements quantify the unpredictability and complexity of a series of IBIs. The  authors  survey  published  normative  values  for  clinical,  healthy,  and  optimal  per-formance populations. They stress the importance of measurement context, including recording period length, subject age, and sex, on baseline HRV values. They caution that 24 h, short-term, and ultra-short-term normative values are not interchangeable. They encourage  professionals  to  supplement  published  norms  with  findings  from  their  own  specialized  populations.  Finally,  the  authors  provide  an  overview  of  HRV  assessment  strategies for clinical and optimal performance interventions.

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