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    SER index
    Home > Parameters

    SER index

    SER, coefficient of variation CV = SDNN/mean NN x 100%: relative heart rate variability.

    SER — the normalized index of summary effect of regulation — is the coefficient of variation (CV) of heartbeat intervals used in R. M. Baevsky's approach to heart-rate-variability analysis. It expresses the overall variability of normal heartbeat intervals (SDNN) as a percentage of their mean duration, so that variability can be compared between people and situations with different heart rates.

    Heart rate variabilityBaevsky indicesCoefficient of variationTime domain
    FormulaSER = SDNN / RRNN × 100 %
    Unit%
    Example45 ms / 850 ms = 5.3 %
    Estimated short-term range≈ 3.5–10 %

    What SER shows

    SDNN — the standard deviation of normal-to-normal (NN) intervals — describes how much the heart rhythm varies in absolute terms (milliseconds). But the same absolute variation means different things at different heart rates: 45 ms of variation at a slow pulse of 60 beats per minute is relatively smaller than 45 ms at a fast pulse of 100 beats per minute. SER removes this dependence by dividing SDNN by the mean NN interval (RRNN). It therefore characterises the relative variability of the rhythm — the total, combined effect of all regulatory influences on the heart, normalised to the average length of the cardiac cycle.

    How SER is calculated

    SER = CV = SDNN / RRNN × 100 %
    SDNN = √[ Σ (NNᵢ − NN̄)² / (N − 1) ] · RRNN = Σ NNᵢ / N · HR = 60000 / RRNN
    SymbolMeaningUnit
    SER (CV)Normalized index of summary effect of regulation%
    SDNNStandard deviation of NN intervalsms
    RRNNMean duration of NN intervalsms
    NNumber of NN intervals in the recording—
     SDNN and RRNN must be calculated from the same set of artefact-free NN intervals.

    Example: SDNN = 45 ms and RRNN = 850 ms (heart rate ≈ 71 beats per minute) give SER = 45 / 850 × 100 ≈ 5.3 %.

    Two people with the same SDNN of 45 milliseconds but different mean RR intervals have different SER values: 4.5 percent at 1000 milliseconds and 7.5 percent at 600 milliseconds Person A: pulse 60 bpmPerson B: pulse 100 bpm RRNN = 1000 msRRNN = 600 ms SDNN = 45 msSDNN = 45 ms SER = 4.5 %SER = 7.5 % Same absolute variability (SDNN), different relative variability (SER)
    Figure 1. SER normalises SDNN to the mean heartbeat interval. With identical SDNN, a person with a faster heart rate (shorter RRNN) has a higher SER.

    Reference values

    SER depends on the recording length in the same way as SDNN, so 5-minute and 24-hour values cannot be compared directly. For short-term (5-minute) recordings at rest, published values for SDNN in healthy adults (about 32–93 ms) together with a typical mean interval of about 0.9 s correspond to an SER of roughly 3.5–10 %. This range is an estimate derived from SDNN norms, not an independently validated diagnostic threshold; use the reference range shown by your measuring device.

    How to interpret SER

    Lower SER

    • Less relative variability of NN intervals
    • Often seen with stress, fatigue, illness, ageing and low physical fitness — together with low SDNN and RMSSD
    • A low value alone does not prove sympathetic overactivity or heart disease

    Higher SER

    • More relative variability of NN intervals
    • Often seen at rest, during slow breathing and in well-trained people
    • Falsely high with extrasystoles, atrial fibrillation or artefacts; a high value alone does not prove healthy regulation
    Important

    SER is a mathematical descriptor of relative variability. It is not a direct measurement of sympathetic or parasympathetic activity and has no generally accepted cut-off that defines disease. It is most useful for following changes in the same person under the same recording conditions, together with SDNN, RMSSD, the stress index and spectral indices.

    What affects SER

    • Recording length, posture, breathing pattern, talking or moving during the recording.
    • Time of day, recent meals, caffeine, alcohol, nicotine and exercise.
    • Age, fitness, medication (for example beta-blockers) and acute illness.
    • Ectopic beats and artefacts, which must be removed before calculation.
    When to see a doctor

    Discuss your results with a doctor if SER and other HRV indices stay low over a long period, or if changes are accompanied by palpitations, an irregular pulse, chest pain, breathlessness, dizziness or fainting.

    References

    1. Baevsky RM, Chernikova AG. Heart rate variability analysis: physiological foundations and main methods. Cardiometry. 2017;(10):66–76.
    2. 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
    3. Nunan D, Sandercock GRH, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol. 2010;33(11):1407–1417. doi:10.1111/j.1540-8159.2010.02841.x
    4. 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 values depend on the device, recording length and conditions.

    Published on 5 June 2019