Table 1. Autonomic markers and their interpretation.

Autonomic marker Tool	Specific metrics	Interpretation
HRV	Time-domain
	SDNN	Standard deviation of NN intervals. It is highly correlated with ULF, VLF and LF. It is more accurate when calculated over 24h. An increase indicates a parasympathetic activation
	NN50	Numbers of consecutive NN intervals that differ more than 50ms. An increase indicates a parasympathetic activation
	RMSSD	Root mean square of consecutive RR intervals. It is considered one of the main measures to assess vagal activity. It is similar to the non-linear metric SD1. An increase indicates a parasympathetic activation
	pNN50	Percentage of NN50. It is correlated to RMSSD and HF power. An increase indicates a parasympathetic activation
	Frequency-domain
	ULF (Power)	Ultra-low frequency value. Non-consensus regarding the mechanisms underlying ULF power. Very solo-acting biological processes, such as circadian rhythms, are implicated
	VLF (Power)	Very low-frequency value. Related to the heart’s intrinsic nervous system, which generates VLF rhythm when afferent sensory cardiac neurons are stimulated. SNS activity due to physical and stress responses influences its oscillations’ amplitude and frequency
	LF (Peak, Power, normalised units)	Low-frequency value. Non-specific index that reflects baroreceptor activity, it contains contributions of both the sympathetic and parasympathetic influences. High values of LF indicate a sympathetic predominance
	HF (Peak, Power, normalised units)	High-frequency value. Expression of parasympathetic activity, it corresponds to the HR variations related to the respiratory cycle known as RSA. It changes according to vagal modulation but does not reflect vagal tone. High values of HF indicate a parasympathetic predominance
	LF/HF ratio	Used to estimate SNS and PNS balance, although LF does not purely represent SNS, and PNS and SNS interact in a complex non-linear manner. A reduction of the LF/HF ratio indicates a sympathovagal balance
	Non-linear
	SD1	Poincare plot standard deviation. It correlates with baroreflex sensitivity, defined as the change in IBI duration per unit of change of BP, and HF. An increase indicates a parasympathetic activation
	SD2	Poincare plot standard deviation. Correlates with LF power and baroreflex sensitivity. High values of LF indicate a sympathetic predominance
	SD1/SD2	Ratio between SD1 and SD2, it measures the unpredictability of the RR time series. It correlates with the LF/HF ratio. Values over 1 indicates a parasympathetic effect, whereas values below 1 show a sympathetic effect
	DFAα	Detrended fluctuation analysis describing short-term (α1) or long-term (β2) fluctuations. It is considered a sensitive parasympathetic index. A decrease indicates a parasympathetic activation
Heart Rate	Response to: breathing, Valsalva manoeuvre or postural change	The variability of heartbeat is used to assess cardiac sympathovagal function. An increase is considered a sympathetic effect, whereas a reduction shows parasympathetic activation
Blood Pressure	Response to: Valsalva manoeuvre, isometric exercise or postural change	Variation of blood pressure is utilized to assess adrenergic sympathetic function
Microneurography		An electrophysiological technique used for recording single or multi-unit nerve traffic directly from human peripheral nerves. It permits to elucidate and quantify the sympathetic nerve activity in muscle and skin
Spillover	urine or plasma	A neurochemical technique employed to assess SNS based on the plasma or urine noradrenaline concentration
Pupil Light Reflexes		A neurophysiological method assessing the dilation or restriction of the pupil. Parasympathetic action evokes pupil constriction whereas sympathetic noradrenergic activity produces pupil dilatation
Electrodermal activity	Galvanic skin response	A method utilized to measure neurally-mediated effects on sweat gland permeability—observed as changes in the resistance of the skin. It is considered a reliable measure of sympathetic cholinergic activation
Thermal InfraRed Imaging		A method employed to measure the variability of temperature within specific areas of the face. Increase or decrease of the detected temperature implies, respectively, a parasympathetic or sympathetic activity
Skin blood flow	Response to: hand grip, cold, heat, baseline variation	Different methods are implied. The most common is laser Doppler. It is mainly used to evaluate SNS. A decrease is interpreted as an increase of sympathetic outflow