Consciousness monitors have two jobs. The first is to prevent awareness for patients under general anesthesia. The second is to titrate anesthetics to "just-right" levels, enough to sedate the patient while also allowing for a quick emergence. As they say, less is sometimes more when it comes to anesthesia. Despite sound bites from the evening news, there's still no support for the notion that monitors prevent intraoperative awareness.1 Monitors can, however, enhance clinical signs of consciousness and help with depth of anesthesia trends that ensure safe outcomes with the quickest possible recovery.

The sensation deprivation spectrum
You must define consciousness before you can measure it. The three main subcategories of anesthesia practice (local, regional and general) emphasize a spectrum of sensation deprivation. Local anesthesia sits on the low end of the spectrum, usually in combination with intravenous sedation. Sensory deprivation here should be restricted to the analgesic numbing provided by the local anesthetic at the site of the procedure; sedation should be limited to mitigating the patient's nervousness about the OR environment.

The sensation spectrum then progresses to regional anesthesia, which renders a larger portion of the patient's body insensate; this engenders more patient anxiety with concerns about loss of a body part. A higher level of anxiety could require more hypnosis in the intravenous sedation component of the anesthesia.

General anesthesia requires the patient's entire body be insensate to the noxious stimuli of the procedure and the environment; this requires unconsciousness - the inability to process thoughts, feelings or impressions.

While we can't directly measure the desired state of unconsciousness, providers still use Guedel's classic description of clinical signs to assess anesthetic depth.2 His four stages were based on his assessment of physical eye signs, breathing patterns and muscle tone. We generally condense Guedel's stages of physical signs into the following.

• First stage (analgesia): conscious patient with eyelid reflex present.
• Second stage (delirium): patient excited and unconscious with uninhibited activity and increased risk of vomiting or laryngospasm.
• Third stage (surgical anesthesia): subdivided into four planes with progressive increase in eye and abdominal muscle paralysis and irregular respiratory pattern.
• Fourth stage (respiratory paralysis): cardiovascular and respiratory arrest.

Standard clinical signs often used by today's practitioners include eyelid response, tachycardia, hypertension, ability to follow commands, tearing, swallowing, movement in response to stimulus, and papillary size and position. But because of the many drugs anesthesia providers use and the many prescription and over-the-counter drugs patients take routinely, clinical signs are considered unreliable tools for consciousness monitoring. 3,4

Drugs that mitigate the patient's response to stimuli are among the reasons clinical signs can be unreliable. These include muscle relaxants, beta-blockers and dopamine receptor antagonists that can abolish eyelash reflexes. Medical illnesses, like Parkinson's disease or psychiatric disorders, may also prevent patients from responding as commanded. Still, clinical signs should not be ignored. Here are two clinical tools to help measure consciousness levels.

• Observer's Assessment of Alertness/Sedation Scale (OAA/S). Developed by Chernik, this scale provides a standardized assessment of sedation.4 The scale's scores range from 1 (the patient responds only after a painful trapezius squeeze) to 5 (patient responds readily to name spoken in normal tone). Later modified, the scale (MOAA/S) now includes a score of 0, occurring when the patient doesn't respond after a painful trapezius squeeze.
• Drug concentrations. Maintaining a target plasma concentration (Cp) for intravenous agents or a minimum alveolar concentration (MAC) for inhalational agents could be used as a tool for monitoring consciousness.5 The dynamic situation of surgery makes this tool very dependent on clinical signs, requiring frequent adjustments in the drug concentration levels.

Mind readers
It is well known that anesthetic drugs affect the frequency and amplitude of electroencephalographic waveforms.6 Achieving a universal correlation between a certain EEG pattern and depth of anesthesia has not been realized for all anesthetic techniques. Middle-latency auditory evoked potentials (MLAEP) and spontaneous EEG have been proposed as monitors for depth of anesthesia because both provide information about the hypnotic state.1

The raw waveforms of the spontaneous EEG, however, are difficult to interpret. Recently developed monitors can transform the raw information into a numerical value used to assess consciousness. The following are some tools currently available.3,7 They may help offset the inherent weakness of relying on clinical signs alone.

• Bispectral Index (Aspect Medical). Sensors perform a bispectral analysis of the EEG waveform and determine a numerical BIS index.2 This index ranges from 0 (isoelectric EEG) to 100 (awake). A BIS value between 40 and 60 is the desired range for the hypnotic effect of general anesthesia.
• SNAP index (Stryker). According to the ASA, the SNAP index is calculated from a single channel of electroencephalograph. The index calculation is based on a spectral analysis of electroencephalographic activity in the 0 to 18Hz and 80Hz to 420Hz frequency ranges and a burst suppression algorithm.
• Patient State Analyzer (Physiometrix Hospira). This index is based on a multivariate algorithm that varies as a function of the hypnotic state. The derivation of the PSI is based on the observation that there are reversible spatial changes in power distribution of quantitative electroencephalograph at loss and return of consciousness. The PSI has a range of 0 to 100, with decreasing values indicating decreasing levels of consciousness or increasing levels of sedation.

Anesthesia providers don't need a great deal of anesthetics to suppress pain and reflexes, and monitors allow for the titration of anesthetics to individual patients. Theoretically, this "just right" objective might result in increased incidence of awareness, although this has not been shown to occur.7

As documented in a multi-center study, the anesthesia awareness phenomenon is infrequent, occurring at an overall rate of less than 1 percent.8 Although infrequent, intraoperative awareness continues to be a major concern to patients, resulting in periodic high-profile media reports. It also causes significant perioperative distress and has been reported to have as high as a 56.3 percent incidence of developing post-traumatic stress disorder.9

Walking the line
Anesthesia demands a blending of the science of drugs with the art of dosing appropriately for positive outcomes. Accurately measuring states of consciousness would help, especially in outpatient facilities where getting patients to pre-procedure status immediately after surgery is paramount. Ensuring that a patient is comfortable and physiologically stable throughout a procedure is the ultimate challenge during each and every case.