SDANN

SDANN is the standard deviation of the average 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 in each 5-minute segment of a long recording, in milliseconds. It is meant for 24-hour recordings.

Why this measure exists

A 24-hour 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 mixes two different things: the fast swings inside each few minutes and the slow drift between morning, afternoon, evening and sleep. The 1996 Task Force split them. Average every 5-minute piece first and the fast swings vanish; the standard deviation of those 288 averages, SDANN, is left with only the slow changes. Its partner, the SDNN index SDNN index SDNN index The mean of the SDNN of each 5-minute segment of a 24-hour recording, in milliseconds. It keeps the variability inside each segment and drops the slow drift between them. More about SDNN index , keeps the opposite half. Shaffer and Ginsberg note that SDANN is highly correlated with ULF power ULF power ULF power The variance of the heartbeat intervals below 0.003 Hz, in ms²: rhythms slower than one cycle every 5 minutes, including the day-night cycle. It can only be measured from a 24-hour recording. More about ULF power , the slowest band of the spectrum, and that it is not a substitute for SDNN.

The formula

$$\mathrm{SDANN} = \sqrt{\frac{1}{K-1}\sum_{k=1}^{K}\left(\mathrm{RR}_{\mathrm{mean},k}-\mathrm{RR}_{\mathrm{mean}}\right)^2}$$

Cut the recording into $K$ pieces of 5 minutes, take the mean interval $\mathrm{RR}_{\mathrm{mean},k}$ of each piece, and take the standard deviation of those $K$ means around their grand mean $\mathrm{RR}_{\mathrm{mean}}$. A full day gives $K = 288$.

Worked example

SDANN step by step

Ten beats, every number shown

The ten intervals, in ms (300 to 2000)

Beat 1 Beat 2 Beat 3 Beat 4 Beat 5 Beat 6 Beat 7 Beat 8 Beat 9 Beat 10

SDANN of these 10 intervals is 9.3 ms.

Step 4 of 4

Back Start at step 1

1. Step 1 of 4 Start with the intervals

   Beat 1	880 ms
   Beat 2	826 ms
   Beat 3	812 ms
   Beat 4	823 ms
   Beat 5	848 ms
   Beat 6	877 ms
   Beat 7	880 ms
   Beat 8	866 ms
   Beat 9	828 ms
   Beat 10	815 ms

   10 intervals between 11 beats, in milliseconds.

2. Step 2 of 4 Cut the recording into pieces of 2.5 seconds

   Piece 1	880, 826, 812 ms
   Piece 2	823, 848, 877 ms
   Piece 3	880, 866, 828 ms

   A day is cut into 5-minute pieces (288 of them). Ten beats are cut into 2.5-second pieces here to show the same steps; the last, incomplete piece is dropped.

3. Step 3 of 4 Take the mean interval of each piece

   Piece 1	839.3 ms
   Piece 2	849.3 ms
   Piece 3	858.0 ms

4. Step 4 of 4 Take the standard deviation of those means

   Mean of the means	848.9 ms
   (piece 1 − 848.9)²	91.31
   (piece 2 − 848.9)²	0.20
   (piece 3 − 848.9)²	83.01
   Sum ÷ 2	87.26
   √87.26	9.3 ms

   Only the slow change from piece to piece survives; everything that happens inside a piece is averaged away.

   SDANN 9.3 ms

With JavaScript on, the steps show one at a time and the beats can be changed.

A row of ten editable heartbeat intervals in milliseconds, then the calculation of SDANN as numbered steps: start with the intervals; cut the recording into pieces of 2.5 seconds; take the mean interval of each piece; take the standard deviation of those means. The final step reads SDANN 9.3 ms. Back and Next buttons move between the steps.

Ten beats cannot be cut into 5-minute pieces, so the example cuts them into 2.5-second pieces of three beats and runs the same steps: three piece means, and their standard deviation, 9.3 ms. On a real day the pieces are 5 minutes long and the number is larger, because a day holds a lot of slow change.

What moves it

The shape of your day: the drop in heart rate during sleep against its rise with activity, meals, caffeine and stress. The Task Force’s table of approximate normal values gives 127 ± 35 ms for 24-hour recordings, from 274 healthy adults aged 40 to 69. A day spent in bed with a flat heart rate gives a small SDANN even in a healthy heart.

When it misleads

It says nothing about beat-to-beat variability: a heart with very little variation from beat to beat can still have a normal SDANN. It needs the whole day; from a 5-minute recording there is nothing to compute. And because it depends on what you did that day, two days from the same person can differ a lot.

Where you will see it

The Task Force’s name is SDANN. Kubios HRV reports it for recordings long enough to hold several 5-minute segments; Holter reports after a heart attack list it beside SDNN. Wearables do not show it.