Table 4.

HRV domain groups and an abbreviated list of derived variable examples

Domain	Comments	Variable examples
Statistical	Describe statistical features of time-series data; assumes the state of subsequent data is determined independent of prior data	SDNN, RMSSD, NN50, pNN50, IQRNN
Frequency	Deconstructs R-R interval sequences into their spectral components to construct the power distribution of the time series	Total power, ULF, VLF, LF, HF, LF/HF
Geometric	Identifies and creates a “shape” from the histogram representation of some specified property in an R-R interval series (see indices column).	NN interval length distribution, Poincare plot, Differential index, TINN, HTI
Nonlinear methods	Describes properties that demonstrate fractality, and other characteristics that do not vary in time and space	SampEn, ApEn, Shannon entropy, DFA, Lyapunov exponents, Dispersion analysis

HRV=heart rate variability; NN= The interval between two normal R-waves (i.e., from non-ectopic beats); RMSSD=Squared root of the mean squared differences of successive; NN50=number of interval differences of successive NN intervals >50ms; pNN50=proportion derived by diving NN50 by the total NN intervals; IQRNN=Interquartile range of NN; SDSD=Standard deviation of the first derivative of the time series; ULH= “ultralow” frequency (</=0.003 Hz); VLF= very low range (0.003–0.04 Hz); LF= low frequency (0.04–0.15 Hz); HF=high frequency (0.15–0.4 Hz); TINN= Triangular interpolation of NN interval histogram; HTI= HRV triangular index; SampEn= sample entropy; ApEn= approximate entropy; DFA=Detrended fluctuation analysis; FDDA= fractal dimension by dispersion analysis; FDCL= fractal dimension of the signal