Abstract
It is a well established clinical observation that, at low concentrations, most anesthetic agents produce a surge in brain activity that occurs around the time of loss of consciousness. At higher concentrations, brain activity slows, and eventually tends towards electrical silence. A second surge in EEG power occurs during the return to consciousness. These induction and recovery biphasic power surges were first explained in terms of a first-order switching transition between distinct active and quiescent neural states, but subsequent modeling by other researchers has demonstrated that biphasic surges can also be generated by a smooth, graduated transition between normal and suppressed levels of cortical activity. In this chapter we examine the contrasting predictions of the switching model versus the continuous model for anesthetic induction. If the continuous non-switching picture is correct, then the return path to recovery will retrace the trajectory for induction, so the biphasic peaks should occur at identical drug concentrations. In contrast, the switching model predicts that there must be a hysteresis separation between the entry and recovery EEG power maxima, and that the patient will awaken at a lower drug concentration than that required to put her to sleep.
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Notes
- 1.
This is analogous to the noise immunity provided by the positive-feedback hysteresis engineered into an electronic Schmitt trigger for clean binary switching in digital circuits.
- 2.
The convolve2 function is written by David Young, and available for download from www.mathworks.com/matlabcentral/fileexchange/22619-fast-2-d-convolution.
- 3.
G. Ludbrook, C. Grant, and R. Upton, from the Department of Anaesthesia and Intensive Care, University of Adelaide, Adelaide, South Australia, Australia. Experimental methods were approved by the Animal Ethics Committee of University of Adelaide.
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Steyn-Ross, D.A., Steyn-Ross, M.L., Sleigh, J.W., Wilson, M.T. (2011). Progress in Modeling EEG Effects of General Anesthesia: Biphasic Response and Hysteresis. In: Hutt, A. (eds) Sleep and Anesthesia. Springer Series in Computational Neuroscience, vol 15. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-0173-5_8
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