TABLE 2.

Selected heart rate variability indices.

Variable	Unit	Description
RMSSD	ms	Root mean square of differences between successive NN intervals. Index of parasympathetic outflow.
pNN50	%	The percentage of successive NN intervals that differ more than 50 ms. Index of parasympathetic outflow.
LF	ms2	Low frequency power. Index of sympathetic and parasympathetic activity. Ranges from 0.04–0.15 Hz.
LFnu	nu	LF power in normalized units. LF/(Total Power − VLF) × 100.
HF	ms2	High frequency power. Index of parasympathetic activity. Represents the respiratory sinus arrhythmia. Ranges from 0.15–0.40 Hz.
HFnu	nu	HF power in normalized units. HF/(Total Power − VLF) × 100.
LF/HF ratio		Ratio between LF and HF band powers, which represents sympathovagal balance.
DFA1		In detrended fluctuation analysis, short-term fractal index. DFA1 reflects the sympathovagal balance: it increases as the vagal tone decreases.

NN, normal-to-normal beats, after removing ectopic beats; nu, normalized units.