TINN

TINN is the baseline baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline width, in milliseconds, of the triangle that fits the NN interval NN interval 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 histogram best.

Why this measure exists

The HRV triangular index HRV triangular index HRV triangular index The number of NN intervals in a recording divided by the height of the tallest bar of their histogram. A unitless geometric measure of overall variability that a few bad beats barely move. More about HRV triangular index reads the histogram’s height. TINN reads its width, in a way meant to ignore the ragged tails that artifacts artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact and ectopic beats ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat leave at the edges. The 1996 Task Force defines it as the baseline width of the minimum-square-difference triangular interpolation of the highest peak of the histogram: a triangle is stood on the tallest bar and its two feet are slid along the axis until the triangle matches the bars as closely as possible. Whatever lies outside the triangle, such as a few far-off bins from bad beats, barely moves the feet. Both geometric measures were designed for 24-hour recordings, where a day’s worth of intervals makes a smooth hill.

The formula

$$\mathrm{TINN} = N_t - M_t,\qquad (M_t, N_t) = \arg\min_{M,N}\sum_k \left(n_k - T_{M,N}(k)\right)^2$$

Build the histogram with counts $n_k$ in bins of 7.8125 ms. Stand a triangle $T$ with its apex on the fullest bin, its left foot at $M$ and its right foot at $N$ on the interval axis, and choose the feet that make the squared differences between the bars and the triangle as small as possible, counting bins outside the triangle as error too. TINN is the distance between the two feet.

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 a triangle whose feet sit at 816 and 941 ms, so TINN is 125 ms. Switch to the drifting preset: the slide in heart rate smears the intervals across more bins, the hill widens, and TINN grows to 172 ms. Switch to the ectopic preset and the three stray bins hardly move the feet.

What moves it

Overall variability of every speed and, above all, recording length: a day’s slow changes make a much wider hill than 5 minutes do. The Task Force intends it for 20 minutes to 24 hours; Shaffer and Ginsberg list 90 seconds as a proposed ultra-short minimum, with the warning that two artifacts in a 5-minute segment can distort it.

When it misleads

In short recordings the histogram is jagged and the fit is unstable, so TINN jumps between bin widths. Programs place the feet on different grids and give different answers for the same beats. It depends on the 7.8125 ms bin like the triangular index does. And it says nothing about speed: the histogram has forgotten the order of the beats.

Where you will see it

The Task Force’s name is TINN. Kubios HRV prints it in ms next to the HRV triangular index; Holter reports list both among the 24-hour measures. Wearables do not show it.