RMSSD

To calculate RMSSD, take each pair of neighboring NN intervals NN intervals NN interval An RR interval between two normal heartbeats, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval and find the difference between them. Square each difference, average the squares, then take the square root. The result is in milliseconds.

Because it only compares neighboring beats, RMSSD picks up fast, beat-to-beat changes and ignores slow drifts in heart rate. Those fast changes are driven mainly by the vagus nerve vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve , which is why RMSSD is the main time-domain measure researchers use to estimate vagal influence on the heart. The Task Force standards prefer it to the related pNN50 pNN50 pNN50 The percentage of neighboring NN intervals that differ by more than 50 milliseconds. NN50 is the raw count. Both track fast, vagally driven changes, like RMSSD. More about pNN50 because it has better statistical properties. It is also mathematically identical to SD1 SD1 SD1 and 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. More about SD1 and SD2 from a Poincaré plot 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 .

A single RMSSD value says little on its own. Readings from recordings of different lengths, or taken at different times of day, are not directly comparable, and values differ widely from person to person. Comparing today’s number with your own 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 usually tells you more than comparing it with someone else’s. Some apps show lnRMSSD lnRMSSD lnRMSSD The natural logarithm of RMSSD. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyze. More about lnRMSSD , the natural log of RMSSD, which is easier to track from day to day.

The lesson RMSSD, SDNN and lnRMSSD explained works through an example, and Normal HRV by age covers typical ranges.