Variable (units)	Definition	Physiological Significance
Time Domain
SDNN (ms)	Standard deviation of intervals between heartbeats	Global index of ANS function
RMSSD (ms)	Root mean square of successive differences; calculated through squaring the intervals between heartbeats	Global index of ANS function
pNN50 (%)	Proportion of heartbeat intervals that differ by more than 50 ms	Indicative of parasympathetic activity
NN50	A count variable;	Indicative of parasympathetic activity
number of pairs of adjacent NN intervals differing by more than 50 ms
STD HR (s)	Standard deviation of instantaneous heart rate values	Global index of ANS function
Geometric Methods
RR Triangular Index	Total number of all NN intervals divided by the height of the histogram of all NN intervals	Global index of ANS function
measured on a discrete scale (i.e. the number of all NN intervals divided by the maximum of the density distribution)
TINN (ms)	Baseline width of the distribution measured as a base of a triangle, approximating the NN interval distribution	Global index of ANS function
Frequency Domain
HF (ms2)	Power (magnitude) in high frequency range, 0.15-0.4 Hz	Index of parasympathetic activity on heart based on rhythmic respiration cycles
HFnu (%)	HF power in normalized units, as a ratio of the total power; [HF/(HF+LF)] x 100	Proportion of parasympathetic activity
LF (ms2)*	Power (magnitude) in low frequency range, 0.04-0.15 Hz	Measure of sympathetic and/or parasympathetic activity
LFnu (%)*	LF power in normalized units, as a ratio of the total power; [LF/(HF+LF)] x 100	Measure of sympathetic and/or parasympathetic activity
LF/HF (ms2)*	Ratio of low frequency power to high frequency power	Measure of sympathetic and/or parasympathetic activity
Total power (ms2)	Variance of all RR intervals	Overall magnitude of variability within ANS; ability of ANS system to be flexible and adaptable