FCS index

FCS, mode (Mo): the most probable level of functioning of the cardiovascular system.

FCS — the most probable level of functioning of the cardiovascular system — is the mode (Mo) of the heartbeat-interval histogram in R. M. Baevsky's variational pulsometry: the duration of the heartbeat interval that occurs most often during the recording. It describes the level at which the sinus node — the heart's natural pacemaker — most probably works, and corresponds to the "usual" heart rate of the moment.

Heart rate variabilityBaevsky indicesHeart rateMo

DefinitionMo — most frequent NN interval

Unitseconds

Approximate norm at rest≈ 0.76–0.84 s

Heart rate equivalent60 / Mo ≈ 71–79 bpm

What FCS shows

Unlike the mean interval, which can be shifted by a few very long or very short beats, the mode shows the value around which the rhythm is concentrated. A short Mo means that the heart most probably works at a high rate — under sympathetic influence or increased demand. A long Mo means a slow, economical rhythm — typical of rest, sleep and vagal predominance.

Figure 1. FCS is the position of the tallest histogram bar on the time axis (Mo, red): the heartbeat interval that occurs most often.

FCS = Mo — the centre of the most populated 50-ms bin of the NN-interval histogram (s); heart rate ≈ 60 / Mo

Reference values and interpretation

Approximate values for 5-minute recordings at rest in adults; children have shorter Mo.
FCS (Mo), s	Heart rate, bpm	Interpretation
< 0.60	> 100	Short — fast rhythm: exercise, stress, fever, anaemia, dehydration, hyperthyroidism, heart failure; at rest — tachycardia that needs evaluation
0.60–0.76	79–100	Shortened — sympathetic predominance or increased demand
0.76–0.84	71–79	Typical at rest — balanced regulation (mean in healthy adults ≈ 0.80 ± 0.04 s)
0.84–1.20	50–71	Long — vagal predominance, good fitness, rest, sleep
> 1.20	< 50	Very long — bradycardia: normal in trained athletes; with dizziness or fainting — needs evaluation

FCS is one of the three building blocks of Baevsky's indices: together with AMo and ΔX it enters the stress index (SI), the autonomic rate index (AR) and the index of adequacy of regulatory processes (ARP).

What affects FCS

Body position (Mo shortens on standing), physical and emotional load.
Breathing, time of day, temperature, recent food, caffeine, alcohol, nicotine.
Fitness level: trained people have a longer Mo at rest.
Medicines: beta-blockers and some antiarrhythmics lengthen Mo; stimulants and thyroid hormones shorten it.
Arrhythmias (atrial fibrillation, frequent extrasystoles) make Mo unreliable.

When to see a doctor

Consult a doctor if FCS at rest is repeatedly below 0.60 s (pulse above 100) or above 1.20 s (pulse below 50) with symptoms such as palpitations, breathlessness, dizziness, weakness or fainting. Seek urgent care for fainting or chest pain.

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

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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