Glossary
SD1 and SD2
Also called SD1, SD2, SD1/SD2
Two measures of the spread of dots in a Poincaré plot. SD1 is the width and tracks short-term variability (it equals RMSSD); SD2 is the length and tracks overall variability.
A Poincaré plot Poincaré plot A scatter plot of each heartbeat interval against the one before it. The shape of the cloud of dots shows short-term and overall variability at a glance. More about Poincaré plot puts each heartbeat interval against the one before it, so the dots form a cloud along the diagonal. SD1 and SD2 describe the size of that cloud, both in milliseconds.
- SD1 is the spread of the dots across the diagonal, the width of the cloud. It measures short-term, beat-to-beat variability.
- SD2 is the spread along the diagonal, the length of the cloud. It measures short- and long-term variability together.
- SD1/SD2 is the ratio of the two, sometimes used as a measure of how unpredictable the rhythm is.
SD1 and RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD are the same measurement written two ways. A 2017 paper by Ciccone and colleagues showed this both on real data and with the math. Many studies still report both as if they were separate findings, so when you see both, count them as one result. SD2 is a separate measure that correlates with LF power LF power The share of heart rate variability in slow rhythms of 0.04 to 0.15 Hz, about one cycle every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power .
So if a tool shows SD1, you can read it as you would RMSSD. The lesson RMSSD, SDNN and lnRMSSD explained covers the measures these relate to.