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Direct and indirect inhibitory effects of anaesthetic agents on cortical activity are reflected in the electroencephalogram (EEG) as: (i) a shift from low-amplitude, high-frequency EEG, to high-amplitude, low-frequency activity (indicative of cortical depowering) and; (ii) the appearance of spindles and K-complexes (indicative of thalamocortical hyperpolarisation and sensory blockade). Existing EEG monitors use cortical activity as a proxy measure for consciousness. However the state of the cortex at any given moment does not accurately predict the state that it will enter in response to a noxious stimulus, and EEG monitors do not differentiate well between different levels of rousability. Also the literature reveals many instances where the EEG pattern is dissociated from conscious state (e.g. an awake-looking EEG, but an unresponsive patient; or a slow-wave EEG in an awake patient). Fortunately, a slow-wave EEG (even in the presence of a responsive patient) usually indicates profound amnesia for explicit memory.

Identifying the central nervous system sites of action of anaesthetics is important for understanding the link between their molecular actions and clinical effects. The aim of the present pilot study was to compare the anaesthetic effect of bilateral microinjections of propofol and thiopentone (both 200 microg/microl, in Intralipid and 0.9% saline respectively) into a recently discovered anaesthetic-sensitive region in the rat brainstem, the "mesopontine tegmental anaesthetic area" (MPTA). Microinjections (1 microl per side) were made into the MPTA of fifteen male Sprague-Dawley rats. The effect of each agent on spontaneous behaviour, postural control and nociceptive responsiveness was subjectively assessed according to established criteria. The main finding was that thiopentone induced an "anaesthesia-like" state, including complete atonia and loss of righting ability, in 20% of the subjects. Overall, thiopentone significantly reduced postural control and had a moderate antinociceptive effect compared to saline microinjections (P < 0.01 and 0.05, respectively, Wilcoxon test). In contrast, propofol did not induce "anaesthesia" in any animal tested, although a similar antinociceptive effect to that of thiopentone was observed (P < 0.05, Wilcoxon test). In summary, propofol and thiopentone have different effects when microinjected into the MPTA. While both agents reduced reflex withdrawal to a nociceptive stimulus, only thiopentone induced an "anaesthesia-like" state.

Background Monitoring the effect of anesthetic drugs on the neural system is a major ongoing challenge for anesthetists. During the past few years, several electroencephalogram ( EEG)-based methods such as the response entropy ( RE) as implemented in the Datex- Ohmeda M- Entropy Module have been proposed. In this paper, sample entropy is used to quantify the predictability of EEG series, which could provide an index to show the effect of sevoflurane anesthesia. The dose-response relation of sample entropy is compared with that of RE. Methods EEG data from 21 subjects is collected during the induction of general anesthesia with sevoflurane. The sample entropy is applied to the EEG recording. Pharmacokinetic-pharmacodynamic modeling and prediction probability statistic are used to evaluate the efficiency of sample entropy in comparison with RE. Results Both methods track the gross changes in EEG, especially the occurrence of burst-suppression pattern at high doses of anesthetics. However, our method produces faster reaction to transients in EEG during the induction of anesthesia as indicated from the pharmacokinetic and pharmacodynamic modeled parameters and analysis around the point of loss of consciousness. Also, sample entropy correlated more closely with effect-site sevoflurane concentration than the RE. In addition, our proposed method exhibits greater resistance to noise in the EEG signals. Conclusion The results demonstrate that sample entropy can estimate the sevoflurane drug effect on the EEG more effectively than the commercial RE index with a stronger noise resistance. [ABSTRACT FROM AUTHOR]

The article discusses a study on the effect of noxious stimuli on episodic electroencephalogram (EEG) waveforms and the non-episodic delta waves in healthy patients undergoing general anaesthesia. The study involved 30 patients undergoing elective surgery at Waikato Hospital in Hamilton, New Zealand. Laryngoscopy/intubation (ETT) and surgical incision are the two varied events of noxious stimulation. Results show that quantifying various episodic patterns in the EEG under general anaesthesia is possible.

The interaction of many poorly defined, physiological, pharmacological, and pathological factors make titration of general anaesthesia in the elderly difficult. There may be a potential clinical benefit using the processed electroencephalogram (EEG) to monitor hypnotic level in this population. We prospectively studied 16 patients aged over 65 years having hip fractures repaired under general anaesthesia by experienced anaesthetists blinded to Bispectral Index (BISXP) and Entropy values. Pre-induction EEG indices did not correlate with age or mini-mental state examination (MMSE). During maintenance of anaesthesia, BISXP and Response Entropy (RE) values were within the recommended range of 40–60, 45% and 32% of the total time, respectively. BISXP and Response Entropy (RE) values were above 60 for 11% and 13% of the total time, respectively, and below 40 for 44% and 55% of the total time, respectively. BISXP correlated well with RE in 12 patients, but in the other four patients there was a difference of more than 20 points between BISXP and RE. [ABSTRACT FROM AUTHOR]

Abstract: Entropy is a theoretical framework rather than a single concept or value. Two families of EEG entropies are typified by the ‘approximate entropy’ and the ‘spectral entropy’. Both are measures of uncertainty, or regularity, of a signal. They have maximal values when the signal is Gaussian random noise; and minimal values when the signal is completely predictable (e.g. single-frequency sine wave). EEG entropies correlate closely with cortical activity—a drug (or disease state) that depresses cortical activity will correlate with a change in EEG entropy. For example, GABAergic anaesthetic agents cause the EEG to become more regular, more sinusoidal, and the EEG entropy to decrease in proportion to anaesthetic effect and level of consciousness. Conversely, many adjunctive anaesthetic drugs (nitrous oxide and opioids) have little effect on cortical activity and consequently no significant effect on the entropy of the frontal EEG. In such cases, the entropy of the EEG will tend to give falsely high values in relation to the level of consciousness. Correct interpretation of entropy in the setting of clinical anaesthesia requires an understanding of these limitations. [Copyright &y& Elsevier]