HRV Measurement: SDNN and RMSSD


Someone last week mentioned about measurement methods of HRV, so I wanted to investigate a bit more, and find the wearables that measure in each method.

Time domain measures

As HRV is a measure of variability, there are numerous ways to sample a user’s heartbeats to produce meaningful HRV values. The most common and well-tested methods are Root Mean Square of Successive Differences (RMSSD), Standard Deviation of the N-N intervals (SDNN), Number of pairs of intervals that differ by more than 50ms (NN50), and proportion of NN50 over all N-N intervals expressed as a percentage (pNN50)

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