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    SD2
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    SD2

    SD2: Poincare plot length, long-term overall heart rate variability.

    SD2 is the standard deviation of points along the line of identity on the Poincaré plot (scatterogram) of heartbeat intervals. It describes the length of the point cloud and measures longer-term variability of the heart rhythm — slow changes of heart rate over many beats — which reflects the combined influence of the sympathetic and parasympathetic systems, the baroreflex and other slow regulatory mechanisms.

    Heart rate variabilityPoincaré plotOverall variabilityNonlinear analysis
    FormulaSD2 = √(2·SDNN² − SD1²)
    Unitms
    Example (5 min)SDNN 50, SD1 30 → SD2 ≈ 64 ms
    Reflectssympathetic + vagal, slow regulation

    What SD2 shows

    When the heart rate slowly rises and falls over tens of seconds — with changes in blood pressure, posture, attention or temperature — successive intervals remain similar to each other but the whole sequence drifts. On the Poincaré plot such drifts move the points along the diagonal, stretching the cloud into a "comet". SD2 measures this stretching. It is closely related to SDNN: while SD1 captures fast changes only, SD2 contains both slow and fast components.

    Poincaré plot of RR n against RR n plus 1 with a fitted ellipse; SD1 is the width across the line of identity and SD2 the length along it; a large ellipse in a healthy person and a small round cloud under stress SD1SD2 RRₙ, msRRₙ₊₁, ms line of identity Rest, healthy: large ellipse Stress / disease: small, shifted to short RR
    Figure 1. Poincaré plot. SD2 (green) is the length of the point cloud along the line of identity; SD1 (red) is its width across the line.
    SD2 = √(2 · SDNN² − ½ · SDSD²) = √(2 · SDNN² − SD1²)

    Example: SDNN = 50 ms and SD1 = 30 ms give SD2 = √(2 × 2500 − 900) ≈ 64 ms. Like SDNN, SD2 depends on recording length: 24-hour values are much larger than 5-minute values and cannot be compared directly. There are no separate international norms for SD2; it is interpreted together with SDNN and SD1.

    Interpretation

    Low SD2 — short cloud

    • Reduced overall variability and adaptability of heart-rate regulation
    • Chronic stress, overtraining, illness, ageing, sedentary lifestyle
    • Persistently low values: heart failure, after myocardial infarction, diabetes with autonomic neuropathy — associated with higher cardiovascular risk

    High SD2 — long cloud

    • Good overall variability: healthy, physically active people
    • Changes of activity or posture during a long recording
    • Falsely high values: artefacts, pauses, arrhythmias

    The SD1/SD2 ratio is sometimes used as an index of the balance between fast (vagal) and slow regulation. A very small ratio means that slow fluctuations dominate; a ratio approaching 1 means the cloud is round, which can occur with irregular rhythms.

    When to see a doctor

    Discuss the results with a doctor if SD2 and SDNN remain low for your age over a long period, or if the scatterogram has an unusual shape (several clusters, a fan or scattered points), which may indicate arrhythmia.

    References

    1. Brennan M, Palaniswami M, Kamen P. Do existing measures of Poincaré plot geometry reflect nonlinear features of heart rate variability? IEEE Trans Biomed Eng. 2001;48(11):1342–1347. doi:10.1109/10.959330
    2. 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
    3. 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 24 June 2019