The brain basis of a “consciousness monitor”
Imagine that you are anesthesized before a surgical operation. You would probably expect to “black out,” and then wake up some time afterward. What then if you suddenly found yourself conscious of the surgeon’s knife – during the operation? Even worse, you might not be able to let anyone know you are aware. In 1979 […]
Imagine that you are anesthesized before a surgical operation. You would probably expect to “black out,” and then wake up some time afterward. What then if you suddenly found yourself conscious of the surgeon’s knife – during the operation? Even worse, you might not be able to let anyone know you are aware.
In 1979 a “medically qualified lady” wrote an editorial called On Being Aware in the British Journal of Anesthesia, describing her own experience of waking up in the middle of a Caesarean section – an incision of her abdomen to facilitate giving birth. “I went unconscious very suddenly,” she wrote, “literally as though someone had switched the lights out. After a gap of uncertain time I gradualy became aware of a mental haze in front of me. I was profoundly confused. This relatively happy state was interrupted by a voice in the space above me (some remark about my bladder) and I instantly understood my predicament: that I was lying there, covered in green towels, my abdomen split open Immediately following this there came three rough stripes across my abdomen. Almost before the third stripe was finished, it was followed by the pain – as suddenly as though I had been stabbed. It was bad from the onset, and it increased in severity. The nearest comparison would be the pain of a tooth drilled without local anesthetic – when the drill hits a nerve. Multiply this pain and then pour a steady stream of molten lead into it. “
An estimated 40,000 to 200,000 mid-operative awakenings may occur each year in the United States alone. Medical science has been aware of the problem for many years, but it is maddeningly difficult to solve. Anesthetics are toxic at higher dosages; give too much, and patients may stop breathing; give too little, and they may feel pain. Anesthesiologists must walk a fine line, and so far there has been no objective way to measure “depth of unconsciousness” to give accurate guidance.
Now, in two upcoming articles, E.R. John and co-authors present the scientific basis of a “Consciousness Monitor,” designed to yield a single measure of depth of anesthesia. The US government has approved John’s Consciousness Monitor – called the Patient State Analyzer – for surgical use. The Consciousness Monitor is based on standard EEG (the brain electrical activity measured by electrodes on the scalp), which all hospitals use. It is built into a computer program that performs a quantitative analysis of the EEG from moment to moment. Early results are encouraging. If the Consciousness Monitor works, it may prevent thousands of cases of agonizing pain in the midst of surgery.
The fundamental science underlying the new technology is important. E.R. John and his team found a marked drop in the gamma-band activity (25-50 Hz) during anesthesia, a range of brain waves that is often associated with consciousness. As soon as patients lost consciousness they showed an increase in slower waveforms, a great increase in EEG power and synchrony over the front of the brain, and a loss of coordination between the major regions of the cortex. All the observed changes were reversed when the patients regained consciousness.
These findings are in line with long-standing observations about EEG in unconscious states like deep sleep and general anesthesia. Unconscious states generally show slow, high-voltage and coherent waveforms, very different from conscious states like waking and dreaming. Slow and coherent waves imply that little information is being passed between neurons; they are all singing the same song. In contrast, irregular electrical waves during waking and dreaming suggest continuous chattering of billions of neurons to each other. Information is then being passed back and forth at great speed and complexity.
Consciousness is often thought of as an integrative activity of the nervous system. Large-scale information transmission among groups of neurons is presumably required for integration, and it makes sense that a loss of consciousness will block this process.
In general, John et al suggests that anesthetics may work by inhibiting the cooperation between the thalamus – the way-station to the cortex – and the cortex itself. This idea is consistent with the widespread hypothesis that consciousness depends upon reentrant activity in the thalamocortical core of the brain.
What makes this work ingenious is the gathering of data into a coherent model, and the bold idea of a device that can measure patients’ conscious state. If the Consciousness Monitor should prove a success, it will have many uses. Patients going into surgery will have more confidence about being protected from pain awakenings; surgeons and anesthesiologists will benefit as well. And the Consciousness Monitor could have scientific uses as well – i.e. to study levels of consciousness.
The broad outlines of EEG in sleep and waking have been known for 70 years, ever since Hans Berger first noticed obvious differences in gross electrical brain activity when people woke up to consciousness and fell asleep. Today we may be coming closer to a real understanding of this fundamental fact.
© BJ Baars
Further reading
- Presentation at Physiometrix
- Presentation at Baxter Health.
- Allan Hobson: “Sleep”, at Amazon.com
- For another approach to brain monitoring of anesthetic unconsciousness, see Alaris’ A-line AEP Monitor
References
- Anonymous (1979) Editorial: On being aware. British Journal of Anesthesiology, 51, 711-712.
- Baars, B.J. (2001) Editorial: The Brain Basis of a “Consciousness Monitor”: Scientific and Medical Signifance. Consciousness and Cognition, 10, 2, 159-164.
- John, E.R., Easton, P. & Isenhart, R. (1997). Consciousness and cognition may be mediated by multiple independent coherent ensembles. Consciousness & Cognition, 6 (1), 3-39.
- Steriade, M., D.A. McCormick, R.J. Sejnowski (1993). Thalamocortical oscillations in the sleeping and aroused brain. Science, 262, 679-685.
- Tunstall, M.E. (1980) – On being aware by request. A mother’s unplanned request during the course of a Caesarean section under general anaesthesia. British Journal of Anaesthesia, Vol 52, Issue 10 1049-1051.
