HRVti

HRV triangular index: geometric measure of overall variability in 24-hour recordings.

HRVti — the HRV triangular index — is a geometric measure of overall heart-rate variability based on long (typically 24-hour) recordings. It is calculated as the total number of NN intervals divided by the height of the NN-interval histogram — in other words, the integral of the density distribution divided by its maximum. Because it describes the shape of the histogram rather than individual intervals, it is relatively insensitive to occasional artefacts and ectopic beats.

Heart rate variabilityGeometric methods24-hour HolterPrognosis

FormulaN / number in modal bin

Healthy, 24 h (Task Force)37 ± 15

Markedly reduced< 15

Unitdimensionless

How the triangular index is calculated

All NN intervals of the recording are sorted into a histogram with a fixed bin width of 7.8125 ms (1/128 s), the standard sampling resolution of Holter devices. The total number of intervals is then divided by the number of intervals in the tallest bin. A wide, flat histogram — many different interval lengths — gives a high index; a narrow, tall histogram — a monotonous rhythm — gives a low index.

Figure 1. Geometric HRV methods. The NN-interval histogram of a 24-hour recording is approximated by a triangle. The triangular index divides the area by the height; TINN measures the width of the triangle base.

HRVti = total number of NN intervals / number of NN intervals in the modal bin (bin = 1/128 s)

Reference values and interpretation

Short recordings are not suitable: the index needs at least about 20 minutes, preferably 24 hours, to build a stable histogram.
HRV triangular index (24 h)	Interpretation
≈ 37 ± 15	Healthy adults (Task Force reference)
15–20	Moderately reduced variability
< 15	Markedly reduced — associated with high risk of death and arrhythmic events after myocardial infarction

Low values are found after myocardial infarction, in heart failure, diabetic autonomic neuropathy, chronic stress and advanced age.
Because it relies on the overall histogram shape, HRVti is robust to occasional artefacts, but frequent extrasystoles or atrial fibrillation make it meaningless.
HRVti correlates closely with SDNN and total power of the same recording.

When to see a doctor

A markedly reduced triangular index on Holter monitoring — especially after a heart attack or with heart failure — should be discussed with a cardiologist as part of the overall risk assessment.

References

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
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
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

This article is for general information and does not replace a consultation with a doctor.

Published on 11 June 2019