Sedation Assessment with Objective Methods

Within several objective methods to sedation assessment proposed, none of them have fully yielded satisfying results. For example, bispectral index (BIS) monitor is a four-channel electroencephalographic (EEG) monitor which generates a single number that correlates with depth of consciousness during general anesthesia. The poor correlation between BIS and validated ICU sedation scales is related with BIS values variability at the awake/agitated levels and of the electromyography (EMG) interference [49]. Based on the analysis of EEG signal irregularity, the entropy monitor also utilizes the EMG signal, which may provide information useful for assessing whether a patient is responding to an external, painful stimulus, but without adding useful information to sedation assessment. In this context, some advantages could be offered by the responsiveness index [50]. Auditory evoked potentials (AEP) are electrophysiological responses of the nervous system to standard sensory stimulation transmitted through headphones. These methods may have a role in monitoring sedation levels only in patients needing deep sedation, or receiving neuromuscular blocking agents, as in this circumstance sedation scales cannot be used [49].

Monitoring blood drug values is useful only when there is a correlation between plasmatic concentration and pharmacological effect [51]. However, ARDS patients may be affected by renal and hepatic dysfunctions that impair their ability to metabolize and excrete drugs. Moreover, hypoxia, inflammatory mediators, and abnormal diets are common, too, and all affect enzymatic function. Thus, this method cannot be recommended for sedation monitoring [52, 53]. Spontaneous, non-propulsive lower esophageal contractility (LOC) is definitely stress related and increases in frequency as the dose of anesthetic is reduced. Deepening of anesthesia resulted in progressive suppression of LOC. However, LOC has great inter-variability and is affected by drugs such as atropine. The electromyogram responsiveness is not sufficiently sensitive to monitor sedation in ICU patients [51].

Actigraphy provides a continuous measure of body movements and was initially developed to measure sleep-wake cycles. This small electronic device containing an accelerometer continuously senses and records minimal movements, summarizing such data in numerical form. Wrist actigraphy provides useful nonspecific observations in ICU patients. Even if it does not discriminate the lack (or the excess) of analgesics and sedatives from other acute neurological dysfunctions, preliminary observations suggest that the measurement of body movements could provide a timely indication of acute changes in neurological status generating motor agitation or hypoactive behavior [54]. This objective method is relatively new in this context. It presents interesting properties, worthy of future investigation [55].