Table 2.

Major Heart Rate Variability Indices.

Indices	References	Description	Functional Significance
			Sympathetic Nervous System	Parasympathetic Nervous System	Overall Autonomic Nervous System	Others
Time-Domain
AVNN	[60]	Average of all normalised R–R intervals				Equivalent to heart rate (inversed)
SDNN	[61]	Standard deviation of all normalised R–R intervals			✓	All cyclic components
RMSSD	[65]	Root mean square of the differences between each successive normalised R-R interval		✓		Respiratory activity
pNN50	[65]	Proportion of normalised R-R intervals that are more than 50 ms from preceding interval		✓		Respiratory activity
Frequency-Domain
Very-low-frequency power	[66,67,68]	Spectral power within the very-low-frequency range of 0.003–0.04 Hz		✓		Thermoregulation; renin-angiotensin-aldosterone activity
Low-frequency power	[65,69,70,71,72]	Spectral power within the low-frequency range of 0.04–0.15 Hz	✓	✓		Baroreflex activity
High-frequency power	[65,70,71]	Spectral power within the high-frequency range of 0.15 to 0.4 Hz		✓		Respiratory activity
Low-frequency/High-frequency power	[70,73]	Ratio of low- to high- frequency spectral power	✓	✓		Sympathovagal balance
Total spectral power	[61]	Spectral power ≤ 0.4 Hz			✓	All cyclic components
Non-Linear
SD1	[74,75,76]	Standard deviation—Poincaré plot (Perpendicular)		✓		Short term changes in heart rate variability
SD2	[75,76,77]	Standard deviation—Poincaré plot (Parallel)	✓	✓		Long term changes in heart rate variability
D2	[62,78]	Correlation dimension				Complexity of the system
CVI	[79]	log10(longitudinal axis × transverse axis)—Lorenz plot		✓
CSI	[79]	longitudinal axis/transverse axis—Lorenz plot	✓

Note. The ticks represent the putative components of the autonomic nervous system each heart rate variability index is thought to reflect. CVI = Cardiac vagal index. CSI = Cardiac sympathetic index.