Table 1.

Overview of pain and nociception monitors.

	Device	Score/unit	Measurement principle	Monitoring nociception	Monitoring pain	Comments
Single-parameter scores	Analgesia nociception index	0–100; dimensionless	Cardiac parasympathetic tone	Possibly negative predictive value (ANI <43 at end of surgery) for absence of significant postoperative pain33; value for pre-emption of haemodynamic events controversially discussed29, 30; may ‘react’ to noxious stimuli somewhat stronger than standard haemodynamics, but controversial17, 27, 28	Possibly some negative predictive value for exclusion of significant pain, but controversially discussed19, 20, 21, 22, 26	Well published; may be best for exclusion of significant nociception/pain
	Skin conductance	Number of fluctuations in skin conductance per second (NFSC) (n)	Peripheral (skin) sympathetic tone	NFSC >0.2 may correlate with severe nociception and, at the end of surgery, significant postoperative pain7, 37	Influence by many confounders (i.e. arousal) may limit its use in awake patients37, 38, 39, 40	Easy to use, but NFSC tends to react predominantly with higher levels of stress/nociception (‘smoke detector’)
	Pupillometry	Absolute or changes in pupillar width (mm %−1)	Sympathetic tone (pupillar innervation)	Pupillometry-guided analgesia may reduce opioid consumption and persistent pain (but only n=55 included subjects)44; may predict responsiveness to postoperative opioids43; influenced by depth of anaesthesia52	May predict response to noxious stimulus in the ICU50	Promising results, but somewhat awkward method (requiring repeated access to open eye)
	Nociceptive flexion reflex threshold	mA	RIII reflex modification by level of analgesia	May aid the prediction of a response to a noxious stimulus54; limited predictive value for postoperative pain when measured at the end of surgery9	Little published research in the postoperative setting	NFTS is a reasonably well-known instrument in pain research, but its perioperative use in clinical routine is not yet well researched
Two-parameter scores	Surgical pleth index (SPI)	0–100; dimensionless	Peripheral vascular and cardiac sympathetic tone	Meta-analysis (six publications and 463 patients) states that SPI-guided analgesia resulted in lower opioid consumption and shorter times to tracheal extubation61; conflicting evidence regarding the prediction of acute postoperative pain57, 63; ‘ideal’ score during surgery may be affected by age and might be closer to 30 (vs frequently quoted <50)57, 62	Manufacturer does not recommend use in conscious patients; little or no use in awake subjects7	Well researched, but guidelines for ideal range of SPI overall not well validated; only monitor which does not require consumables (but GE monitor)
	qNOX	0–99; dimensionless	Electromyographic/electroencephalographic patterns associated with nociception	May help to predict probability of sudden movement in response to stimulation8	No published data to evaluate the score in this setting	To date, insufficient evidence to allow firm conclusions about perioperative use and its benefits
Multi-parameter scores	Nociception level (NOL) index	0–100; dimensionless	Four parameters: skin galvanic response (sympathetic tone), plethysmographic pulse wave (sympathetic vascular tone), temperature, and accelerometry (association with nociception unclear)	May discriminate noxious stimuli slightly better than standard haemodynamics65, 67; ‘ideal’ intraoperative NOL may be 10–2510	No published data	Multiple parameters may increase accuracy, but, to date, little evidence for clinical relevance