Table 1. Summary of different modalities available for monitoring the nociception-antinociception balance.

Measures of nociception- antinociception (NAN) balance	Equipment required	Continuous measurement	Differentiates noxious from non-stimuli	Greater sensitivity than HR and BP to noxious stimuli			Correlated with immediate post-operative pain			NAN-guided analgesia reduces total opioid consumption			NAN-guided analgesia reduces immediate post-operative pain		Known confounders
				Positive studies	Negative studies		Positive studies	Negative studies		Positive studies	Negative studies		Positive studies	Negative studies
ANI	ECG leads, Monitor unit	✓	✓	Jeanne et al., 2012			Boselli et al. 2013	Ledowski et al., 2013		Dundar et al., 2018			Upton et al., 2017		Arrhythmias/pacemakers; α2 or β1-adrenergic agonists; Antimuscarinics
				Grünewald et al., 2013
				Funcke et al., 2017
NoL	Finger probe, Monitor unit	✓	✓	Martini et al., 2015 Edry et al., 2016			—	—		—	—		—	—	β1-adrenergic antagonists
SPI	Finger clip, Monitor unit	✓	✓	Struys et al., 2007	Bonhomme et al., 2011		Thee et al., 2015			Chen et al., 2011	Grünewald et al., 2014			Bergmann et al., 2013	Intravascular volume status; Antihypertensives; Atropine; Posture; Level of consciousness; Hypothermia; Anxiety in awake patients
							Ledowski et al., 2016			Bergmann et al., 2013	Colombo et al., 2015			Grünewald et al., 2014
							Ledowski et al., 2017			Won et al., 2016				Colombo et al., 2015
Pupillometry	Handheld pupillometer	✗	✓	Larson et al., 1993			Aissou et al., 2012	Kantor et al., 2014		Sabourdin et al., 2017					Diseases affecting the eye or the oculomotor nerve
				Constant et al., 2006

NAN, nociception-antinociception; HR, heart rate; BP, blood pressure (systolic or mean arterial).