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Current depth of anaesthesia monitors: jacks of all trades and masters of none?

Current electroencephalographic (EEG)-based monitors of anaesthetic depth are being subjected to a myriad of applications, as evidenced by a number of papers this issue of Anaesthesia and Intensive Care (1-4). These applications include: 1) titrating anaesthesia using closed-loop control to increase stability of anaesthetic depth and decrease interventions by the anaesthetist (1); 2) titrating anaesthesia to decrease anaesthetic consumption and improve haemodynamic control (3); 3) titrating sedation in the intensive care unit (ICU)2 and 4) monitoring for and preventing adverse events (4). However, the limitations of current technologies mean that not all of these attempts will be completely successful.

Current EEG monitors are usually able to discriminate between the extremes of alertness and deep anaesthesia most of the time, which is how they help us to avoid excessive anaesthesia (3) and awareness (5). However, there is a 'twilight zone' in which their discriminatory power is much less impressive (6), as evidenced by the ICU sedation study (2) and the closed-loop anaesthesia study (1) in this issue. In these studies, there was considerable oscillation in bispectral index (BIS) values during apparently stable hypnotic administration. Puri et al reported on their closed-loop control system that targeted a relatively high BIS value of 50 (1). The system made changes to propofol administration if BIS varied by [+ or -]5. The median BIS value was close to the target (median prediction error [approximately equal to] 1%) but the BIS was within 10 units of the target for only 87 minutes (standard deviation 9 min) of the 97 minutes (inter-quartile range 41-298 min) of closed-loop control. Similarly, Sackey et al reported that the BIS fluctuated by 7.5-16.5% per minute in their ICU patients (2). These oscillations likely result from varying stimulation (noxious and otherwise), varying depth of analgesia and deficiencies in the way current monitors analyse the EEG.

Monitors based on the spontaneous EEG, such as the bispectral index (BIS), M-Entropy and Narcotrend, document the current state of the cerebral cortex. They 'tell it how it is' but not 'why it is', because they analyse the raw scalp EEG using a series of statistically-derived heuristic criteria rather than an understanding of the underlying processes responsible for the genesis of the EEG. However, changes in brain state can result from altered cortical function (due to hypnotic drug administration, sleep, hypercarbia [as in the case report in this issue'] and other cerebral depressants) and from altered input to the cerebral cortex.

Drugs such as nitrous oxide' and the opioids (8) may alter cortical input by affecting various subcortical structures. At low concentrations, opioids induce analgesia but have little amnesic effect (9). As drug concentration increases, patients become sedated and at high doses they may not respond to command or noxious stimulation (although opioids lack 100% efficacy in this respect). These effects likely occur at different parts of the central nervous system. Opioids as sole agents at analgesic doses have no effect on BIS or other available EEG-based monitors (10,11). Propofol concentrations required to prevent response to command are decreased in the presence of opioids, but BIS values are increased (indicating that the BIS only reflects the effect of propofol on the cortical EEG and not the contribution of the opioid to anaesthetic depth) (12,13).

The state of analgesia of the patient therefore, may have impressive effects on the clinical state of sedation of the patient at any moment, the oscillation in their depth of sedation, their response to noxious stimulation and whether an EEG-based monitor can be used to titrate anaesthesia or sedation, or predict response to command. The current EEG monitors do not give us a complete picture of the sedation-analgesia state of our patients.

REFERENCES

(1.) Puri GD, Kumar B, Aveek J. Closed-loop anaesthesia delivery system (CLADS[TM]) using bispectral index: a performance assessment study. Anaesth Intensive Care 2007; 35:357-362.

(2.) Sackey PV, Radell PJ, Granath F, Martling CR. Bispectral index as a predictor of sedation depth during isoflurane or midazolam sedation in ICU patients. Anaesth Intensive Care 2007; 35:348-356.

(3.) Chiu CL, Ong G, Majid AA. Impact of bispectral index monitoring on propofol administration in patients undergoing cardiopulmonary bypass. Anaesth Intensive Care 2007; 35:342-347.

(4.) Lee Y-W, Chang C-C. The bispectral index in a patient with carbon dioxide narcosis. Anaesth Intensive Care 2007; 35:453-454.

(5.) Myles P, Leslie K, McNeil J, Forbes A, Chan MT A randomised controlled trial of BIS monitoring to prevent awareness during anaesthesia: The B-Aware Trial. Lancet 2004; 363:1757-1763.

(6.) Voss L, Sleigh J. Monitoring consciousness: the current status of EEG-based depth of anaesthesia monitors. Best Pract Res Clin Anaesthesiol 2007; (in press).

(7.) Rampil I, Kim J, Lenhardt R, Negishi C, Sessler DI. Bispectral EEG Index during nitrous oxide administration. Anesthesiology 1998;89:671-677.

(8.) Glass PS, Bloom M, Kearse L, Rosow C, Sebel P, Manberg P Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane and alfentanil in healthy volunteers. Anesthesiology 1997; 86:836-847.

(9.) Veselis R, Reinsel R, Feshchenko V, Wronski M. The comparative amnestic effects of midazolam, propofol, thiopental and fentanyl at equisedative concentrations. Anesthesiology 1997; 87:749-764.

(10.) Guignard B, Menigaux C Dupont X, Fletcher D, Chauvin M. The effect of remifentanil on the bispectral index change and hemodynamic responses after orotracheal intubation. Anesth Analg 2000; 90:161-167.

(11.) Schmidt GN, Bischoff P, Standl T, Voigt M, Papavero L, Schulte am Esch J. Narcotrend, bispectral index and classical electroencephalographic variables during emergence from propofol/ remifentanil anesthesia. Anesth Analg 2002; 95:1324-1330.

(12.) Struys MM, Vereecke H, Moerman A, Jensen EW, Verhaeghen D, De Neve N, Dumortier FJ, Mortier EP Ability of the bispectral index, autoregressive modelling with exogenous input-derived auditory evoked potentials and predicted propofol concentrations to measure patient responsiveness during anesthesia with propofol and remifentanil. Anesthesiology 2003; 99:802-812.

(13.) Mi WD, Sakai T, Singh H, Kudo T, Kudo M, Matsuki A. Hypnotic endpoints vs. the bispectral index, 95% spectral edge frequency and median frequency during propofol infusion with or without fentanyl. Eur J Anaesthesiol 1999; 16:47-52.

K. LESLIE

Department of Anaesthesia and Pain Management, Royal Melbourne Hospital and Department of Pharmacology, University of

Melbourne, Melbourne, Victoria, Australia
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No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Leslie, K.
Publication:Anaesthesia and Intensive Care
Article Type:Editorial
Geographic Code:8AUST
Date:Jun 1, 2007
Words:1027
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