AR index

AR, autonomic rate index 1/(Mo x dX): meaning, calculation and interpretation.

AR — the autonomic rate index (vegetative rhythm index, VRI) — is one of R. M. Baevsky's heart-rate-variability indices. It combines the most frequent heartbeat interval (Mo) with the total spread of intervals (ΔX) and reflects the autonomic balance: the higher AR, the more the rhythm is driven by the sympathetic system; the lower AR, the stronger the parasympathetic (vagal) influence.

Heart rate variabilityBaevsky indicesAutonomic balance

FormulaAR = 1 / (Mo · ΔX)

Units⁻² (conventional units)

Typical value at rest≈ 6–9

Grows withfaster, more rigid rhythm

What AR shows

AR uses two features of the NN-interval histogram. The mode (Mo) is the most frequent interval and reflects the usual heart rate. The variation range (ΔX) — the difference between the longest and shortest interval — reflects how much the vagus nerve modulates the rhythm, mainly with breathing.

When the sympathetic system dominates, the heart beats faster (Mo shortens) and more evenly (ΔX narrows): both factors make the product Mo·ΔX smaller, so AR increases. When vagal influence prevails, the heart beats slower and with larger beat-to-beat changes, and AR falls.

Figure 1. AR is the reciprocal of the product of the mode (Mo) and the variation range (ΔX, red) of the NN-interval histogram.

AR = 1 / (Mo (s) × ΔX (s))

Example: Mo = 0.80 s, ΔX = 0.21 s → AR = 1 / (0.80 × 0.21) ≈ 6.0. Published guide values for healthy adults at rest vary between about 3 and 10, mostly because programs use different units and recording lengths. Compare your values with the reference range of the device you use.

Interpretation

Low AR

Predominance of parasympathetic regulation
Rest, sleep, good aerobic fitness
Very low values with slow pulse and dizziness may mean excessive vagal tone

High AR

Shift of autonomic balance towards the sympathetic system
Physical or emotional stress, fatigue, pain, fever, dehydration
Persistently high values at rest: chronic stress, overtraining, hypertension, heart failure, autonomic neuropathy

Because AR depends strongly on heart rate, it should be interpreted together with heart rate itself and with variability measures that are less rate-dependent (RMSSD, HF power) as well as with the stress index.

What affects AR

Heart rate: any acceleration (exercise, standing, caffeine) raises AR.
Breathing: slow deep breathing widens ΔX and lowers AR.
Ectopic beats and artefacts artificially widen ΔX and must be removed.
Age, medicines (beta-blockers lower AR, stimulants raise it), recording length.

When to see a doctor

Consult a doctor if AR stays high at rest together with fast pulse, high blood pressure or palpitations, or if a very low AR is accompanied by a slow pulse, weakness, dizziness or fainting.

References

Baevsky RM, Chernikova AG. Heart rate variability analysis: physiological foundations and main methods. Cardiometry. 2017;(10):66–76.
Baevsky RM, Ivanov GG, Chireykin LV, et al. Analysis of heart rate variability using different electrocardiographic systems (methodological recommendations). Vestnik Aritmologii. 2001;(24):65–86.
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
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. Reference values of Baevsky indices are approximate and depend on the device, histogram settings and recording conditions.

Published on 5 June 2019

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