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    TINN
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    TINN

    TINN: width of the triangular interpolation of the NN interval histogram.

    TINN — the triangular interpolation of the NN-interval histogram — is a geometric heart-rate-variability index. The histogram of all NN intervals is approximated by a triangle, and TINN is the width of the triangle's base in milliseconds. It describes the range over which most heartbeat intervals are distributed and therefore reflects overall variability, in a way that is robust to individual outlying intervals.

    Heart rate variabilityGeometric methodsOverall variability
    DefinitionTINN = M − N
    Unitms
    Best used inlong recordings (≥ 20 min–24 h)
    RelatedHRV triangular index, SDNN

    How TINN is calculated

    A triangle is fitted to the NN-interval histogram: its top is placed at the modal bin, and the points N and M on the time axis are chosen so that the triangle matches the histogram as closely as possible (least-squares fit). The distance between N and M is TINN. Intervals outside the main distribution — artefacts, isolated extrasystoles — hardly influence the fitted triangle, which is the main advantage of geometric methods.

    NN interval histogram approximated by a triangle; the HRV triangular index is the total number of intervals divided by the height of the histogram, and TINN is the width of the triangle base N to M Y = height of histogram (modal bin) TINN = M − N (base of the triangle) NM HRVti = total intervals / Y number of intervals
    Figure 1. The NN-interval histogram approximated by a triangle. TINN is the width of the base (M − N); the HRV triangular index is the total number of intervals divided by the height.
    TINN = M − N (ms), where the triangle q(t) with q(N) = q(M) = 0 minimises ∫ (D(t) − q(t))² dt

    Interpretation

    Narrow base — low TINN

    • Intervals concentrated in a narrow range: rigid rhythm
    • Chronic stress, overtraining, illness, ageing
    • After myocardial infarction, heart failure, diabetic autonomic neuropathy

    Wide base — high TINN

    • Broad distribution of intervals: rich variability, good fitness
    • Large day–night differences in 24-hour recordings
    • Very irregular rhythms (atrial fibrillation) — the histogram is then not a triangle and TINN loses meaning

    There are no universally accepted short-term norms for TINN. It depends strongly on recording length and on the histogram resolution, and is interpreted together with SDNN and the HRV triangular index of the same recording. The Task Force recommends geometric methods mainly for recordings of at least 20 minutes, preferably 24 hours.

    When to see a doctor

    Discuss geometric HRV indices with a cardiologist if they are markedly reduced on Holter monitoring, especially after a heart attack or with heart failure or diabetes.

    References

    1. 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
    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.

    Published on 5 June 2019