Table 1: Descriptions of HRV measures and their tendency under stress [12].

Domain Measure Description Features trend under stress
Time-domain AVNN Average of NN intervals (ms)
- SDNN Standard Deviation of all NN intervals (ms) ↓↑
- RMSSD Root mean square of differences of successive NN intervals (ms)
- pNN20 NN variations above 20 ms (%)
- pNN50 NN variations above 50 ms (%)
Frequency-domain LF/HF Describes the ratio of Low Frequency and High Frequency power bands.

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