Table 1.

Description and explanation of abbreviations of the heart rate variability (HRV) parameters applied in this study. Hereafter, NN refers to the time elapsed between consecutive ’normal-to-normal’ beats of the heart, equivalent to R-R, that is, peak-to-peak period between the R-waves on the ECG.

HRV Index	Description
SDNN	Standard deviation (SD) of the NN (R-R) inter-beat intervals
SDANN	Standard deviation of averaged over 5-minute periods NN (R-R) intervals
SDNNIDX	Mean value index (IDX) of SDNN
pNN50	Proportion of the adjacent (successive) NN (R-R) intervals greater than 50 ms
SDSD	Standard deviation of the successive differences between the adjacent NN (RR) intervals
rMSSD	Root mean square differences between the successive NN (R-R) intervals
IRRR	Length of the interval between the first and the third quantile of the ΔRR time series
MADRR	Median of the absolute values of the successive differences between the adjacent NN (R-R) intervals
TINN	Triangular interpolation of the NN (R-R) interval histogram.
HRVi	Index reflecting the slowing down of the heart
SD1	Dispersion of the points along the minor axis of the Pointcare plot (SD of the short-term R-R interval variability)
SD2	Dispersion of the points along the major axis of the Pointcare plot (SD of the long-term R-R interval variability)
$D_2$, ScalExp	Correlation dimension and scaling exponent alpha, non-linear dynamics measures of time series.