Best practice & research. Clinical anaesthesiology
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Best Pract Res Clin Anaesthesiol · Mar 2006
ReviewAltered states of consciousness: processed EEG in mental disease.
Due to increasing life expectancy and a rising elderly population in Europe, the incidence of mild cognitive impairment which may predict diseases like Alzheimer's Disease or Vascular Dementia, is rising. Neurophysiological techniques are simple and inexpensive tools for early diagnosis and provide useful and objective correlates of cognitive activity both in normal subjects and patients suffering from the above conditions. Cognitive impairment due to different mental disease is characterized by decreased power and coherence in the alpha/beta band, which suggests functional disconnection among cortical areas, whereas both power and coherence in the delta and theta bands increase as a sign of cortical deafferentation from subcortical structures. Quantification of power and phase relationship by bispectral analysis suggests the Bispectral Index could be a useful but simple tool for early diagnosis of mental disease.
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This chapter will focus on the two auditory evoked potentials (AEP) most commonly used to assess the effects of general anesthetics on the brain, the auditory middle latency response (AMLR) and the 40 Hz auditory steady-state response (40 Hz-ASSR). We will review their physiological basis, the recording methodology, the effects of general anesthetics, their ability to track changes in level of consciousness and their clinical applications. Because of space constraints, this review will be limited to human studies.
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Best Pract Res Clin Anaesthesiol · Mar 2006
ReviewAnaesthesia defined (gentlemen, this is no humbug).
Our charge was to define anaesthesia as produced by inhaled anaesthetics. A definition may be useful to an understanding of the anaesthetic state, and it may guide studies of the mechanisms by which anaesthesia is produced. ⋯ Some conditions are unmeasurable (unconsciousness), not present for all inhaled anaesthetics (relaxation), or are not present at anaesthetizing concentrations (suppression of autonomic reflexes.) One (analgesia) is unmeasurable (the anaesthetized patient cannot tell an investigator that he/she hurts or does not hurt), and surrogate measures (increases in breathing, blood pressure, and heart rate with surgery) suggest that some pain is perceived. These and myriad other changes produced by inhaled anaesthetics are side effects; they do not define anaesthesia; only immobility and amnesia supply such a definition.
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Since 1997, bispectral index (BIS; Aspect Medical Systems Inc., Natick, MA) has been in clinical practice and a wealth of experimental research has accumulated on its use. Originally, the device was approved only for monitoring hypnosis and has now received an indication for reducing the incidence of intraoperative awareness during anesthesia. Numerous studies have documented the ability of BIS to reduce intermediate outcomes such as hypnotic drug administration, extubation time, postoperative nausea and shortened recovery room discharge. ⋯ Some limitations exist to the use of BIS and it is not useful for some individual hypnotic agents (ketamine, dexmedetomidine, nitrous oxide, xenon, opioids). BIS technology is moving out of the operating room and into diverse environments where conscious and deep sedation are provided. Anesthesiologists need to be actively involved in promoting patient safety and helping transition this technology into broader use.
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Best Pract Res Clin Anaesthesiol · Mar 2006
ReviewClassic electroencephalographic parameters: median frequency, spectral edge frequency etc.
Even today many anaesthesiologists rely on parameters of the autonomic nervous system, such as blood pressure and heart rate to decide if a patient is adequately anaesthetized. It is thought that the electroencephalogram (EEG) may provide more information on the state of anaesthesia. Because full EEG analysis is not possible in the operating room, processed EEG parameters have been developed comprising complex information into a single value. ⋯ This biphasic response makes it difficult to clearly distinguish the exact anaesthetic state of a patient. Median frequency and spectral edge frequency have been studied in numerous studies. However, no sole indicator has been derived from the EEG that could serve as a descriptor of anaesthetic depth.