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    SD1
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    SD1

    SD1: Poincare plot width, short-term vagal variability (RMSSD/sqrt2).

    SD1 is the standard deviation of points perpendicular to the line of identity on the Poincaré plot (scatterogram) of heartbeat intervals. It describes the width of the point cloud and measures short-term, beat-to-beat variability, which is controlled mainly by the vagus nerve. Mathematically SD1 is equivalent to RMSSD divided by √2.

    Heart rate variabilityPoincaré plotVagal toneNonlinear analysis
    FormulaSD1 = RMSSD / √2
    Unitms
    Estimated 5-min range≈ 13–53 ms
    Reflectsparasympathetic activity

    What SD1 shows

    On a Poincaré plot every heartbeat interval RRn is plotted against the next interval RRn+1. If two successive intervals are equal, the point lies on the diagonal (line of identity). The farther a point lies from the diagonal, the larger the difference between neighbouring beats. SD1 summarises how far the points spread across the diagonal — that is, how large the quick, beat-to-beat changes of the rhythm are.

    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. SD1 (red) is the width of the point cloud perpendicular to the line of identity; SD2 (green) is its length along the line.
    SD1 = √(½ · Var(RRₙ₊₁ − RRₙ)) = SDSD / √2 ≈ RMSSD / √2

    Because SD1 is a scaled version of RMSSD, both indices carry the same physiological information. In short-term recordings, healthy adults have RMSSD of about 19–75 ms, which corresponds to SD1 of roughly 13–53 ms. This estimate is derived from RMSSD norms; use the reference values of the device that measured your data.

    Interpretation

    Low SD1 — narrow cloud

    • Weak vagal modulation, sympathetic predominance
    • Stress, fatigue, lack of sleep, acute illness, high heart rate
    • Ageing and low physical fitness
    • Persistently low values: heart failure, after myocardial infarction, diabetic autonomic neuropathy — associated with higher cardiovascular risk

    High SD1 — wide cloud

    • Strong vagal tone: rest, sleep, good aerobic fitness
    • Slow breathing during the recording
    • Falsely high values: extrasystoles and atrial fibrillation scatter points far from the diagonal — check the ECG

    The ratio SD1/SD2 describes the shape of the cloud: a low ratio (long, narrow "comet") is typical of healthy regulation with dominant slow fluctuations; a ratio close to 1 (round cloud) can indicate random-like variability, for example in arrhythmias.

    When to see a doctor

    Consult a doctor if SD1 stays low for your age over a long period, or if the Poincaré plot shows scattered points or several clusters, which may reflect extrasystoles or atrial fibrillation.

    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. 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
    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