Current controversies in intraoperative awareness: II

Monitoring (un)consciousness: the implications of a new definition of ‘anaesthesia’

“…The anaesthetic literature is surprisingly sparse on the properties of the ‘ideal’ depth of anaesthesia monitor(s), although Avidan and colleagues have cogently approached the problem, focusing on EEG‐based monitoring . Whyte and Booker suggested some ideal properties in an educational publication , as did Gelb in unpublished material accessible on the internet .…”

Using neuroimaging to uncover awareness in brain-injured and anesthetized patients

“…However, gas monitors have several limitations. First, the predefined MAC concentration threshold can fluctuate widely based on the anesthetic, as well as factors not accurately captured by standard threshold modelling algorithms, such as patient age (19), genetic background, intensity of the surgical stimulus, or the use of other drugs in the anesthetic cocktail (22). Second, anesthetic agents with relatively high solubility (e.g., halothane, isoflurane) are more easily absorbed in blood vessel rich tissue groups (e.g., heart, brain, liver, kidneys), which can result in an equilibration delay between endtidal levels and effect-site concentration (23).…”

“…A significant shortcoming of processed-EEG monitors like the BIS is that they presume uniform changes in neural responses in all patients based on an arbitrarily-defined level of awareness (i.e., fully awake, light anesthesia and deep anesthesia/ unconsciousness), regardless of the type of anesthetic agent used. However, the EEG signal can be affected by several factors, including drugs (e.g., beta-blockers), neurological conditions, (e.g., encephalopathy, dementia, stroke), muscle paralysis, and patient age (22,25). For example, if a monitor is calibrated to a specific set of variables (e.g., a younger and healthy population), its values cannot reliably be extrapolated to a different population (e.g., older patients with dementia, or pregnant women).…”

“…These studies suggest that processed-EEG monitors can discriminate between conscious and unconscious patients with at most 90% accuracy. In other words, 10% or more of patients who remain aware, may be mistakenly identified as unconscious (22,35).…”