Monitoring the Responses to Nociceptive Stimuli During General Anesthesia Based on Electroencephalographic Signals, an Observational Study

Monitoring the anti-nociceptive drug effect is useful because a sudden and strong nociceptive stimulus may result in untoward autonomic responses and muscular reflex movements. Unopposed stimulation may also 'overrule' a state of stable unconsciousness, with resultant awakening and awareness. The traditional clinical use of systolic or mean blood pressure is actually still one of the methods in everyday use for this monitoring purpose. Another cornerstone is the experience of which drugs and doses are effective in attenuating nociception. Alpiger and colleagues found that simple end-tidal monitoring of sevoflurane was a better predictor of nociceptive response than Auditory Evoked Potential.

Thus, monitoring the state of anti-nociception with objective, non-clinical methods is still in a state of testing and development, without well-documented and proven methods for consistent 'no-fuss' clinical daily use. Some methods, like those using systolic blood pressure, are based on the reduced sympathetic response from the Central Nervous System (CNS) when in a state of drug-induced anti-nociception during concomitant surgical stress. These include the pulse plethysmogram amplitude, heart rate variability and/or amplitude, pupillometry, muscle tonus and skin conductance.They all have limitations in interpretation, as the state of sympathetic tone is strongly influenced by numerous factors, including hypovolemia, vasopressors, atropine and patient positioning. In addition, sympathetic tone is very unspecific in the awake or lightly sedated patient, as mood and subjective feelings have a strong impact.

Attempts are also been made on using the EEG for monitoring of anti-nociception. This approach has been challenged as difficult, as most of the antinociceptive drugs effects are in the periphery, the medullary cord or deeper cerebral layers, far from the EEG signals derived from the frontal cortex. However, EEG is a 'mirror' of what is going on in other parts of the CNS and peripheral nervous system. One problem is to elucidate how the EEG signals may be used in a sensitive and specific way to reflect anti-nociception. Concepts such as response-entropy, Composite Variability Index and BIS variability score have been tested and launched.

Quantium Medical has an EEG-based algorithm with two outputs: the qCON for unconsciousness and the qNOX for anti-nociception. This means that calculates and displays two indices. One, the qCON, is designed to provide information about the depth of the hypnotic state, similar to that provided by the BIS™ and Sedline™ monitor (Masimo, Irvine CA). The second index, the qNOX, is designed to provide information about the depth of the antinociceptive state. The qCON has shown a comparable performance with BIS, and qNOX has proved correlation with rough clinical signs of insufficient antinociception, such as movements during LMA insertion, laryngoscopy and tracheal intubation