Time-domain methods
These are based on the beat-to-beat or NN intervals, which are analysed to give variables such as:
SDNN, the standard deviation of NN intervals. Often calculated over a 24-hour period. SDANN, the standard deviation of the average NN intervals calculated over short periods, usually 5 minutes. SDNN is therefore a measure of changes in heart rate due to cycles longer than 5 minutes. SDNN reflects all the cyclic components responsible for variability in the period of recording, therefore it represents total variability.
RMSSD ("root mean square of successive differences"), the square root of the mean of the squares of the successive differences between adjacent NNs.
SDSD ("standard deviation of successive differences"), the standard deviation of the successive differences between adjacent NNs.
NN50, the number of pairs of successive NNs that differ by more than 50 ms.
pNN50, the proportion of NN50 divided by total number of NNs.
NN20, the number of pairs of successive NNs that differ by more than 20 ms.[25]
pNN20, the proportion of NN20 divided by total number of NNs.
EBC ("estimated breath cycle"), the range (max-min) within a moving window of a given time duration within the study period. The windows can move in a self-overlapping way or be strictly distinct (sequential) windows. EBC is often provided in data acquisition scenarios where HRV feedback in real time is a primary goal. EBC derived from PPG over 10-second and 16-second sequential and overlapping windows has been shown to correlate highly with SDNN.[26]