SI
The stress index (SI, Baevsky's index of regulatory-system tension, "tension index") reflects the degree of stress on the body. It shows how strongly heart-rhythm control is centralised: the more the sympathetic nervous system and central regulation mechanisms dominate over autonomous (vagal) ones, the narrower and taller the distribution of heartbeat intervals becomes, and the higher the SI.
Stress and the heart
Stress is a response of the human body to overstrain, negative emotions, or simply monotonous bustle. During stress the body releases adrenaline and noradrenaline, which mobilise energy and help to find a way out of the situation. Stress in small quantities is needed by everyone — it makes us think and look for solutions. But severe or prolonged stress affects health: immunity decreases, blood pressure and heart rate rise, and the risk of cardiovascular, gastrointestinal and other diseases increases.
At rest the heart rhythm is "soft": the vagus nerve constantly lengthens and shortens the intervals with breathing, so they are spread over a wide range. Under stress the sympathetic system takes over, the heart beats faster and more evenly, and the intervals crowd around one value. Baevsky's stress index turns this change in the shape of the interval histogram into a single number.
How SI is calculated
A sequence of normal heartbeat (NN) intervals — usually a 5-minute recording at rest — is sorted into a histogram with fixed bins (classically 50 ms). Three histogram features are used:
| Component | Definition | Classical interpretation |
|---|---|---|
| Mo, s | Mode — the most frequent NN interval (centre of the tallest bin) | The most probable level of functioning of the cardiovascular system |
| AMo, % | Amplitude of the mode — share of all intervals that fall into the modal bin | Conditional indicator of the activity of the sympathetic link of regulation |
| ΔX, s | Variation range — longest minus shortest NN interval | Degree of variability; reflects mainly parasympathetic (vagal) activity |
Example: Mo = 0.80 s, AMo = 30 %, ΔX = 0.30 s → SI = 30 / (2 × 0.80 × 0.30) = 62.5 conventional units (c.u.). Some programs (for example Kubios HRV) report the square root of this value to make its distribution closer to normal, so numbers from different devices are not always directly comparable.
Reference values
| SI, conventional units | State of regulatory systems |
|---|---|
| < 50 | Low — strong vagal influence (well-trained athletes, sleep) or reduced sympathetic reactivity |
| 50 (80)–150 | Normal at rest — balanced regulation |
| 150–300 | Moderate tension — mild physical or emotional stress raises SI 1.5–2 times |
| 300–600 | Marked tension — values of 400–600 are seen with permanently strained regulatory systems |
| > 600 (up to 1000–1500) | Overstrain — strong stress raises SI 5–10 times; at rest this points to exhaustion of adaptation or disease |
High SI
A high stress index means that heart-rhythm control has shifted towards the central, sympathetic level. Short-term rises are normal during exercise, mental work, pain, emotion or after coffee. A persistently high SI at rest is associated with:
- chronic psychological stress, anxiety, burnout, lack of sleep;
- physical overtraining or insufficient recovery;
- acute illness, fever, dehydration, pain;
- arterial hypertension, heart failure, coronary heart disease;
- diabetes with autonomic neuropathy, thyroid overactivity;
- the use of stimulants (caffeine, nicotine, some medicines).
Long-lasting sympathetic dominance increases heart rate and blood pressure, raises the risk of arrhythmias and reduces immune defence. Very high values at rest (several hundred units and more) suggest that the adaptive reserves are being exhausted.
Low SI
A low SI indicates predominance of autonomous (vagal) regulation. It is typical of rest, sleep and endurance-trained athletes and is usually favourable. Very low values together with slow pulse, weakness or dizziness may reflect excessive vagal tone or reduced sympathetic reactivity; in that case the result should be discussed with a doctor, especially if fainting occurs.
What affects SI and how to lower it
SI depends on posture (it rises on standing), breathing, time of day, recent food, coffee, exercise and talking during the recording. Compare your values recorded in the same position, at the same time of day, after 5 minutes of quiet rest.
- Regular moderate aerobic activity and good recovery between training sessions.
- 7–9 hours of sleep with a regular schedule.
- Slow breathing (about 6 breaths per minute), relaxation, mindfulness.
- Limiting caffeine, alcohol and nicotine.
- Treating the cause: pain, infection, high blood pressure, thyroid disease.
If SI stays above about 300 c.u. at rest for several days, or a high SI is accompanied by chest pain, palpitations, high blood pressure, shortness of breath, severe anxiety or insomnia, consult a doctor.
References
- Baevsky RM, Chernikova AG. Heart rate variability analysis: physiological foundations and main methods. Cardiometry. 2017;(10):66–76.
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
- Tarvainen MP, Niskanen JP, Lipponen JA, Ranta-aho PO, Karjalainen PA. Kubios HRV — heart rate variability analysis software. Comput Methods Programs Biomed. 2014;113(1):210–220. doi:10.1016/j.cmpb.2013.07.024
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258
This article is for general information and does not replace a consultation with a doctor. HRV indices depend on the device, recording length and conditions; interpret them together with a healthcare professional.
Published on 2 June 2019