HRV triangular index

The HRV triangular index is the total number of NN intervals NN intervals NN interval An RR interval between two normal heartbeats, in milliseconds, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval divided by the number in the fullest bin of their histogram. It has no unit.

Why this measure exists

The geometric measures came out of the 24-hour Holter era, when Marek Malik and colleagues wanted numbers that would survive a day’s worth of noisy beats. Sort 100,000 intervals into bins and draw the histogram: a variable heart makes a wide, low hill, a steady one a tall, narrow spike. The 1996 Task Force, which Malik co-chaired, defines the index as the integral of the histogram (the number of intervals) divided by its height, and notes that the geometric measures are relatively insensitive to the quality of the interval series: a few bad beats add a few far-off bins without touching the peak. To make results comparable across programs it fixes the bin width at 1/128 s, 7.8125 ms, the resolution of the ECG ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG equipment of the time.

The formula

$$\mathrm{HTI} = \frac{N}{\max_k\, n_k}$$

Sort the $N$ intervals into bins of 7.8125 ms and count the intervals $n_k$ in each; divide $N$ by the largest count. A recording whose intervals all fall in one bin scores 1; the more bins the intervals spread across, the higher the index.

Worked example

Sort the beats into bins

Five minutes of the teaching recording

Intervals in each bin 500 to 1300 ms

500 700 900 1100 1300

Intervals

341

In the fullest bin

39

Triangular index

8.7

TINN

125 ms

SDNN

28.7 ms

Rested: 341 intervals sorted into 17 bins of 7.8 ms. The fullest bin holds 39, so the triangular index is 341 ÷ 39 = 8.7. The triangle that fits the histogram best runs from 816 to 941 ms, so TINN is 125 ms. Both numbers describe the shape of this histogram, not the order of the beats.

Rested Stressed Drifting Ectopic beats

Bin width 7.8 ms (1/128 s, the Task Force bin)

The controls work when JavaScript is on.

A bar chart of RR intervals from 500 to 1300 ms sorted into bins of 7.8 ms. The tallest bar is highlighted and a triangle is drawn over the bars from its left foot to its apex to its right foot. Readouts give the number of intervals, the count in the fullest bin, the HRV triangular index, TINN with its two feet, and SDNN, followed by a sentence explaining the numbers. A slider sets the bin width and four buttons choose a rested, stressed, drifting or ectopic recording.

Five minutes of the teaching recording give 341 intervals, 39 of them in the fullest bin, so the index is 341 ÷ 39 = 8.7. Widen the bins to 50 ms and the fullest bin holds far more, so the index drops to 1.9: the number only means something at the agreed bin width. Switch to the ectopic preset: three premature beats take the index from 8.7 to 9.0 while SDNN SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN jumps from 28.7 to 56.4 ms.

What moves it

Overall variability of every speed, like SDNN, and above all the recording length: the Task Force intends the index for 20 minutes at the least and ideally 24 hours, and its table of approximate normal values gives 37 ± 15 for 24-hour recordings of healthy adults aged 40 to 69. Shaffer and Ginsberg list 5 minutes as the conventional short epoch, and 120 seconds as the proposed ultra-short minimum.

When it misleads

In short recordings the histogram is jagged and the fullest bin is a matter of luck, so the index is noisy. The bin width must be 7.8125 ms or the number is not comparable; some programs round to 8 ms, some let you change it. Like SDNN it grows with recording length. It cannot tell a breathing swing from a slow drift, because the histogram forgets the order of the beats.

Where you will see it

The Task Force’s name is HRV triangular index. Kubios HRV prints it as “HRV triangular index”, unitless, next to TINN TINN TINN The width, in milliseconds, of the base of the triangle that best fits the histogram of NN intervals. A geometric measure of overall variability from 24-hour Holter analysis. More about TINN , with the 1/128 s bin recommended; Holter reports list it among the 24-hour measures. Wearables do not show it. The Baevsky stress index Baevsky stress index Baevsky stress index A number built from the histogram of heartbeat intervals, with no unit. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index is built from the same histogram with 50 ms bins.