> ## Documentation Index
> Fetch the complete documentation index at: https://docs.medera.info/llms.txt
> Use this file to discover all available pages before exploring further.

# HRV

> SDNN, RMSSD, LF/HF, SD1, SD2

HRV metrics are computed by `_compute_hrv_metrics(peaks, fps)` from rPPG inter-beat intervals.

| Metric           | Field             | Description                                     |
| :--------------- | :---------------- | :---------------------------------------------- |
| Time-domain      | `hrv_sdnn`        | Standard deviation of NN intervals (ms)         |
| Time-domain      | `hrv_rmssd`       | Root mean square of successive differences (ms) |
| Frequency-domain | `hrv_lf_hf_ratio` | Low-frequency / high-frequency ratio            |
| Nonlinear        | `hrv_sd1`         | Poincaré plot short-term variability (ms)       |
| Nonlinear        | `hrv_sd2`         | Poincaré plot long-term variability (ms)        |

Minimum duration: 60 s (`min_hrv_duration`). Lower SDNN and elevated LF/HF correlate with sympathetic dominance.