Further studies have questioned the B-Aware conclusion. To add insult to injury, a recent study showed that patients receiving muscle relaxant had lower BIS values for the same anaesthetic depth, a state in which anaesthetic depth monitoring is needed most.

Dear Dr. Felsby, thank you very much for your important comments. Perhaps it is better to look directly at the EEG curves rather than relying on the numerical BIS data. In addition, The end-tidal concentrations of volatile anesthetics have proven to be similarily protective against awareness during total intravenous anesthesia than the numerical BIS data. However, data in this context are scarce.

End-tidal volatile anesthetic concentration measurement has proven to be equally protective against intraoperative awareness than BIS during total intravenous anesthesia.

I agree with Dr. Felsby that the use of neuromuscular blocking agents, a risk factor for awareness per se, may influence the BIS reading. EMG still plays a role in the BIS algorithm. The heretical results of the initial case series published by Messner et al. (The bispectral index declines during neuromuscular block in fully awake persons. Anesth Analg. 2003 Aug;97(2):488-91) have recently been confirmed by Schuller et al. (Response of bispectral index to neuromuscular block in awake volunteers. Br J Anaesth. 2015 Jul;115 Suppl 1:i95-i103).

This is an important issue, indeed! Let me contribute as a surgeon rather than from the anesthetist' perspective. After having a patient of mine recount essentially ALL of the conversation had during an operation under GA with NO signs of awareness, I have steadfastly held the position that ALL my patients have some awareness. They don't likely but it is always worth erring in this direction. NO ONE should ever make a comment in an OR that they would not want the patient to hear if awake and in your office. That should be a minimum patient respect and care policy. Beyond this, the anxiety of awareness under GA with neuromuscular blockade, is another issue. The fear of this happening is made extreme if the patient is not only aware but feeling pain. When ever I am operating and can pair physical reactions to tissue contact, I alert my anesthesia team that "the patient is feeling this". This can and commonly does occur with out any other outward signs of awareness like tachycardia or hypertension. This is the surgeons responsibility to provide this feedback to you, our invaluable anesthesia colleagues. 

Dear Paul

Thank you for a very empathic and thoughtful answer. This is worth considering. Of course, you always have the problem of differentiating between a conscious response and twitching due to direct nerve stimulation from the diathermy electrode.

Dear Dr. Castellanos, thank you for your thoughts. It is indeed of great importance to get immediate feedback from our surgical colleagues so the neurophysiological condition can be estimated more precisely. I also agree with Dr. Felsby that muscle twitching in non-relaxated or insufficiently relaxated patients shoul not be misinterpreted as awareness. If muscle activity alone is over interpreted as awareness, the patient is at great risk of being anesthezied too deeply.

On the other hand, I agree with Dr. Castellanos that conversations held during an operation should never be directed towards a direction that could be discomforting the patient, as all our measures aiming to detect awareness are unsafe and providing limited information regarding the neurophysiological and neurocognitive states of our patients. I therefore recommend close clinical observation and additional interpretation of the electroencephalogram (EEG) (although only frontal EEG  traces are available from routine bispectral index (BIS) monitoring) in addition to the monitoring of the numerical BIS, end-tidal concentrations of the volatile anesthetics and the variability of vital sings. However, there are no data regarding the safety of such multimodal monitoring regarding the prevention of awareness and studies in this context are urgently needed.

I use BIS for cardiac surgery on a regular basis and also for and some head and neck cases. I have used it for volatile as well as propofol anaesthesia. One of the most important issues is to ensure good signal quality. I do not give importance to BIS readings unless the SQ is >90% particularly if the BIS reading is low.  

I fully agree with Dr. Sutter about the great limitations of bispectral index as well as its translation to patient's level of awareness, since this method assesses only  the electrical activity of frontal regions, especially the slower bands of frequency. Anecdotally, I saw a patient having their scalp incised when their BIS index was 40, and jumping nearly half a meter from the surgical table. Indeed, a more detailed study of methods of awareness evaluation in anesthetized patients is much needed.