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What is Heart Rate Variability (HRV)?


Glimpse of Facts on HRV

  • Heart Rate Variability (HRV), the most frequently used method for Autonomic Nervous System (ANS) test

  • Our heart pumps 72 cycles per minute on average

  • Within the seemly constant heart rhythm there exists fine, dynamic fluctuations

  • The cyclic fluctuations are caused by ANS modulation of heart rate

  • ANS divides into sympathetic nervous system and parasympathetic nervous system; The former promotes a higher heart rate while the latter a slower rate

  • The heart rate of healthy people is at about 72 beats / min.  Average beat-to-beat interval is 0.833 sec or 833 ms, while its standard deviation is about 40ms. This value is defined as heart rate variability or HRV

  • HRV represents total activity index of ANS; The higher the HRV, the better the ANS control

  • HRV is highly dependent on age. At age of 20, HRV is about 50-60 ms; As people get old, HRV decreases to 20-25 ms.  HRV in female is slightly higher than male, but the difference disappears post-menopause.  See Figure 1 for normal values of HRV as function of sex and age.

  • HRV approaches zero for people (1) with severe diseases and being close to death, due to multiple organ failure (2) wearing a pacemaker, due to a fixed, programmed heart rate, and  (3) with a transplanted 
    heart, due to lack of ANS modulation

Figure 1: HRV-Sex-Age Correlations (Normal Values)


  • The above statistical correlation is established by taking HRV data published in literature for healthy volunteers and conducting regression analysis (30 peer-reviewed journal papers covering world-wide regions and demographics including our own published data; report on file with Taiwan Scientific and  Sun Scientific)

  • 1996 International HRV Standard does provide certain HRV normal ranges.  However, the range is too broad to be useful as a guide for health status (Ref. : Heart rate variability, Standards of measurement, physiological interpretation, and clinical use, Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology, European Heart Journal (1996) 17, 354-381.)

More facts about HRV

A good heart: fast and slow beats alternating!

  • Before 1960’s, it was thought that the heartbeat frequency was constant

  • Lee et al. and other groups discovered heart rate variability

  • Definition of HRV: standard deviation of peak-to-peak intervals

  • The source of HRV comes from ANS modulation on heart rate

  • Sympathetic signals cause heart to beat faster (narrower gaps) while 
    parasympathetic signals opposite (see Figure 2 below)

Reference (1) Hon EH, Lee ST. Electronic evaluations of the fetal heart rate patterns 
preceding fetal death: further observations. Am J Obstet Gynecol. 1965;87:814-826 
(2) Sayers BM. Analysis of heart rate variability. Ergonomics. 1973;16:17-32

Figure 2: Illustration of Heart Rate Variability


HRV - the Golden Standard of Autonomic 
Nervous System Testing

  • HRV has become the golden standard for ANS testing

  • The biggest contributor of such development was the 1996 publication of "Heart 
    Rate Variability - Standards of Measurement, Physiological Interpretation, and 
    Clinical Use" by Task Force of the European Society of Cardiology and  the North 
    American Society of Pacing Electrophysiology in the journal Circulation 
    (Circulation. 1996;93:1043-1065.© 1996 American Heart Association, Inc.)

Power Spectrum Analysis of HRV
- to obtain LF, HF, and LF/HF
1. Obtain peak-to-peak intervals
2. Conduct Fast Fourier Transfer (FFT) analysis
3. Plot power spectrum density distribution


1996 HRV International Task Force Standard

  • HRV:Total ANS activity index (ms); equal to standard deviation of adjacent peak-
    to-peak intervals (also called SDNN)

  • LF(AU):Low frequency (0.04 ~0.15 Hz) (ms2); sympathetic (and some 
    parasympathetic) activity index

  • HF(AU):High frequency (frequency range 0.15~0.4 Hz) (ms2); parasympathetic 
    activity index

  • LF(NU):LF%, [LF/(TP-VLF)]*100; contribution of sympathetic activity

  • HF(NU):HF%, [HF/(TP-VLF)]*100; contribution of parasympathetic activity

  • LF/HF: Ratio of LF to HF; sympathetic-parasympathetic balance index

  • VLF(AU):Very low frequency (0.0001~0.04 Hz) (ms2); physiological meaning not 
    defined by the standard

  • Irregular heartbeats (including cardiac arrhythmias) must be excluded from HRV 

****Beware: Some  HRV analyzers in the market do not conform to the 1996 
international standard

What is Autonomic Nervous System (ANS)?


Glimpse of Facts on ANS

  • The autonomic nervous system (ANS ) is a part of the peripheral nervous system that acts as a physiological feedback control system

  • ANS functions below the level of consciousness.  Its actions are mostly involuntary and automatic.

  • ANS controls 90% of body and mental activities, including heart rate, respiration rate, digestion, and sexual arousal (see Table 1 below)

  • ANS is divided into two subsystems: the sympathetic nervous system (SNS) and 
    parasympathetic nervous system (PNS).  Both are opposing in general, but sometimes complimenting and cooperating to meet physiological needs (see Figure 1)

  • ANS responds to “fight or flight” and acts as an accelerator, while PNS responds to “rest and digest” and acts as a brake

  • The primary function of the immune system is to keep virus, bacteria, and foreign matters from invading the body;  In contrast, ANS’s main functions is to help body cope with internal and external changes

  • Some people manage stress better than others.  An efficient ANS system might be the 

  • Many complex symptoms  of unknown origins are due to ANS imbalance or dysautonomia

Figure 1: Autonomic Nervous System


Table 1: Main Functions of Autonomic Nervous System


Understanding of ANS 

Autonomic Nervous System (see Figure 2 below)

  • The autonomic nervous system conveys sensory impulses from the blood vessels, the heart and all of the organs in 
    the chest, abdomen and pelvis through nerves to other parts of the brain (mainly the medulla, pons and hypothalamus).

  • These impulses often do not reach our consciousness, but elicit largely automatic or reflex responses through the efferent autonomic nerves

  • Thereby eliciting appropriate reactions of the heart, the vascular system, and all the organs of the body to variations in environmental temperature, posture, food intake, stressful experiences and other changes to which all 
    individuals are exposed.

  • The above information is cited from Wilson-Pauwels ; Stewart•Akesson: Autonomic Nerves (1997)

Figure 2: The Feedback Loop of Autonomic
    Nervous System


Homeostasis, ANS, and HRV


  • When resting, our body maintains a constant and stable condition called 
    “Homeostasis”; Body temperature, blood pressure, cardiac output, respiration, 
    and endocrine secretions are controlled by negative feedback mechanisms

  • ANS is the primary system responsible for these feedback mechanisms


  • At resting and under homeostasis, ANS is at balance with LF/HF close to 1.0

  • When the body conducts a specific activity, ANS provides timely and proper 
    support (Sympathetic nerve activated during exercise; Parasympathetic nerve 
    activated during recovery)

  • When the body is off balance or unhealthy, ANS becomes over-active or under-
    active in certain branches (Depression causes parasympathetic withdrawal; 
    Lack of sleep decrease overall ANS function)


  • HRV parameters reflect the present activities exerted by ANS

  • Deviation of any HRV parameter from normal or baseline value is an indication of  abnormal ANS activity

  • Homeostatic patient parameters (such as BP, heart rate, body temperature) 
    may still be normal under the above conditions

  • HRV parameters are early indicators of body stress and disease

ANS Imbalance or Dysautonomia

Conventional definition of ANS imbalance /dysautonomia

  • fast heart rate, shortness of breath, dizzy/lightheaded, weak, fatigued/exhausted 
    ,  memory loss, chest discomfort/pain, anxiety, palpitations,  nausea, vomiting, 
    bloating, abdominal pain, diarrhea, and constipation

Definition of ANS imbalance / dysautonomia by HRV

  • Any severe deviation of HRV parameters from normal values can be judged as 
    ANS imbalance or dysautonomia

  • Normal values can be obtained from the HRV-Age-Sex correlation (see Figure 3)

Cause of ANS imbalance / dysfunction

  • ANS exhibits unusual or extraordinary activities in order to support, supplement, 
    or compensate other organ systems in unhealthy states - Most frequently 
    encountered (about 80% of dysautonomia cases in our clinical observation)

  • ANS itself has incomplete, imbalanced, deteriorated, or damaged functions 
    (insufficient or  excessive secretion of neurotransmitters, defected receptors, 
    neuropathy, spinal lateral bending) – Less frequent (about 20% among 
    dysautonomia cases)

  • Genetic factors play an important role

Figure 3: HRV-Sex-Age Correlations (Normal Values)



  • The above statistical correlation is established by taking HRV data  published in literature for healthy volunteers and conducting regression analysis (30 peer-reviewed journal papers covering world-wide regions and demographics including our own published data; report on file with Sun 

  • 1996 International HRV Standard does provide certain HRV normal ranges.  
    However, the range is too broad to be useful as a guide for health status

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