SDSD

SDSD is the standard deviation of the successive differences between 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 , in milliseconds.

Why this measure exists

The 1996 Task Force lists SDSD next to RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD as a second way of summarising the differences between neighbouring beats. RMSSD squares the differences around zero; SDSD squares them around their own mean. In a resting recording the differences average almost exactly zero, because every speed-up is matched by a slow-down, so the two numbers nearly coincide. SDSD matters mostly because it links the time domain to the Poincaré plot Poincaré plot Poincaré plot A scatter plot of each heartbeat interval against the one after it, in milliseconds on both axes. The shape of the cloud of dots shows short-term and overall variability at a glance. More about Poincaré plot : Brennan and colleagues showed in 2001 that $\mathrm{SD1}^2 = \tfrac{1}{2}\,\mathrm{SDSD}^2$.

The formula

$$\mathrm{SDSD} = \sqrt{\frac{1}{N-2}\sum_{i=1}^{N-1}\left(\Delta_i-\Delta_{\mathrm{mean}}\right)^2},\qquad \Delta_i = \mathrm{RR}_{i+1}-\mathrm{RR}_i$$

Take the successive differences $\Delta_i$, subtract their mean $\Delta_{\mathrm{mean}}$ from each, square, add up, divide by one less than the number of differences, and take the square root. $\mathrm{RR}_i$ is the $i$-th normal-to-normal interval and $N$ the number of intervals, so there are $N-1$ differences.

Worked example

SDSD 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

SDSD of these 10 intervals is 27.5 ms.

Step 5 of 5

Back Start at step 1

1. Step 1 of 5 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 5 Subtract each interval from the next

   Beat 2 − beat 1	−54 ms
   Beat 3 − beat 2	−14 ms
   Beat 4 − beat 3	+11 ms
   Beat 5 − beat 4	+25 ms
   Beat 6 − beat 5	+29 ms
   Beat 7 − beat 6	+3 ms
   Beat 8 − beat 7	−14 ms
   Beat 9 − beat 8	−38 ms
   Beat 10 − beat 9	−13 ms

   Plus means the heart slowed down for a beat; minus means it sped up.

3. Step 3 of 5 Find the mean difference

   −65 ÷ 9	−7.22 ms

   Over a long resting recording the differences average almost zero, which is why SDSD and RMSSD nearly agree.

4. Step 4 of 5 Subtract the mean difference and square

   Difference 1	2188.16
   Difference 2	45.94
   Difference 3	332.05
   Difference 4	1038.27
   Difference 5	1312.05
   Difference 6	104.49
   Difference 7	45.94
   Difference 8	947.27
   Difference 9	33.38
   Sum	6047.56

5. Step 5 of 5 Divide by one less than the number of differences, then take the square root

   6047.56 ÷ 8	755.94
   √755.94	27.5 ms

   SDSD 27.5 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 SDSD as numbered steps: start with the intervals; subtract each interval from the next; find the mean difference; subtract the mean difference and square; divide by one less than the number of differences, then take the square root. The final step reads SDSD 27.5 ms. Back and Next buttons move between the steps.

The ten beats of the teaching recording give an SDSD of 27.5 ms against an RMSSD of 26.9 ms. The gap between them comes from the small non-zero mean of the nine differences; over a longer recording it shrinks to nothing.

What moves it

Whatever moves RMSSD: breathing depth and rate, vagal activity vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , heart rate, posture, sleep, alcohol and illness.

When it misleads

Treat SDSD and RMSSD as one measure: a paper that reports both as separate findings has counted the same thing twice. In a recording where heart rate trends steadily in one direction the differences no longer average zero, and the two drift apart slightly. 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 inflate it exactly as they inflate RMSSD.

Where you will see it

The Task Force’s name is SDSD. Kubios’s documentation uses it to define SD1 SD1 SD1 and SD2 Two measures of the spread of dots in a Poincaré plot, in milliseconds. SD1 is the width and tracks short-term variability, like RMSSD; SD2 is the length and tracks overall variability. More about SD1 and SD2 , and some older papers report it instead of RMSSD. SD1 on a Poincaré plot is SDSD divided by the square root of two.