Table 1 Time and frequency-domain metrics derived from HRV

AV_NN

[58, 70, 76, 78]

Average value of NN intervals within a specific time window.

\({AV}_{NN}=\frac{1}{N}\sum_{i=1}^N{t_{NN}}_i\)

SD_NN

[20, 60, 62, 73, 75, 77, 78, 81, 86]

Standard deviation of NN intervals within a specific time window.

\({SD}_{NN}=\sqrt{\ \frac{1}{N}\sum_{i=1}^N{\left({t_{NN}}_i-{AV}_{NN}\right)}^2}\)

RMS_SD

[60, 62, 73, 79, 86]

Root mean square of successive differences between successive NN intervals.

\({RMS}_{SD}=\sqrt{\ \frac{1}{N}\sum_{i=1}^N{{t_{NN}}_i}^2}\)

C_HRV

[20]

Coefficient of heart rate variability

\({C}_{HRV}=\frac{SD_{NN}}{NN}x100\)

LF-HF ratio [11, 23, 59, 60, 63, 64, 73,74,75, 78, 83, 86]

Ratio between the total energy in low frequency and the total energy in the high frequency.

\(\frac{LF}{HF}=\frac{\frac{1}{T}\sum_{t=1}^T{x}_{LF}^2(t)}{\frac{1}{T}\sum_{t=1}^T{x}_{HF}^2(t)}\)