Table 3. Non-linear metrics for HRV analysis.

Non-Linear HRV analysis	Parameters	Units	Description
Poincaré analysis	S	ms	Area of the ellipse, representing total variability
	SD1	ms	Standard deviation perpendicular to the line of identity
	SD2	ms	Standard deviation along the line of identity
	SD1/SD2	%	Ratio of SD1 to SD2
Entropy analysis	ApEn	n/a	Approximate entropy, measures the regularity and complexity associated with a time series
	SampEn	n/a	Sample entropy, measures the regularity and complexity associated with a time series
Detrended fluctuation analysis	DFA α1	n/a	Describes short-term fluctuations (less than 11 beats)
	DFA α2	n/a	Describes long-term fluctuations (more than 10 beats)
	D2	n/a	Estimates the minimum number of variables required to model system dynamics