AB index

AB, autonomic balance index (AMo/dX): balance of sympathetic and parasympathetic regulation.

AB — the autonomic balance index (index of vegetative balance, IVB) — is one of R. M. Baevsky's heart-rate-variability indices. It is the ratio between the activity of the sympathetic link of regulation (AMo) and the activity of the parasympathetic link (ΔX), and shows which branch of the autonomic nervous system prevails in heart-rhythm control.

Heart rate variabilityBaevsky indicesSympathetic / parasympathetic

FormulaAB = AMo / ΔX

Unit%/s (conventional units)

Typical value at rest43 % / 0.21 s ≈ 200

Grows withsympathetic dominance

What AB shows

In Baevsky's model of heart-rhythm regulation the amplitude of the mode (AMo) reflects sympathetic, "stabilising" influences: the more of the heartbeats fall into one narrow interval range, the stronger the sympathetic drive. The variation range (ΔX) reflects parasympathetic influences: the vagus nerve constantly lengthens and shortens the intervals, so a wide range means strong vagal modulation.

AB divides one by the other. A high AB means that sympathetic influences prevail over parasympathetic ones; a low AB means the opposite. It is a simple, time-domain counterpart of the spectral LF/HF ratio, and both should be interpreted with the same caution.

Figure 1. AB is the ratio of the height of the modal bin (AMo, red) to the width of the histogram (ΔX).

AB = AMo (%) / ΔX (s)

Example: AMo = 43 %, ΔX = 0.21 s → AB ≈ 205. With AMo = 60 % and ΔX = 0.10 s (marked stress), AB = 600; with AMo = 25 % and ΔX = 0.40 s (deep rest, trained person), AB ≈ 63. Absolute values depend on bin width and units, so use the reference range of the device that measured them.

Interpretation

AB	Autonomic balance	Typical situations
Low	Parasympathetic predominance	Rest, sleep, endurance training; excessive vagal tone (slow pulse, low blood pressure, fainting tendency)
Within range	Balanced regulation	Healthy person at rest
High	Sympathetic predominance	Stress, anxiety, pain, fever, lack of sleep, overtraining, hypertension, heart failure

A single value of AB is less informative than its trend in the same person and its agreement with other indices — the stress index, AR, ARP, RMSSD and spectral measures. AB cannot be calculated meaningfully in atrial fibrillation or with frequent extrasystoles.

What affects AB

Posture: standing up raises AB within a minute.
Breathing: slow breathing widens ΔX and lowers AB.
Emotions, mental work, caffeine, nicotine, alcohol, recent exercise and meals.
Age: ΔX narrows with age, so AB tends to rise.
Medicines: beta-blockers lower AB; sympathomimetics and anticholinergic drugs raise it.

When to see a doctor

If AB remains high at rest over several days together with poor sleep, palpitations, high blood pressure or exhaustion — or if it is very low together with a slow pulse, dizziness or fainting — discuss the results with a doctor.

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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