CiNii Books - Neurophysiological monitoring during intensive care and surgery

The principal aim of neurophysiological monitoring is to prevent damage to the nervous system. Achieving this aim places great demands on both personnel and equipment. Traditionally the need for monitoring has been determined by clinicians (surgeons, anaesthetists, intensivists), and it has been provided by clinical neurophysiologists acting as visiting consultants in the operating theatre and intensive care unit. While there can be no doubt that neurophysiological monitoring is vital for the patient at risk of permanent neurological damage, monitoring presents challenging practical problems for anaesthetist and neurophysiologist. On the one hand, the neurophysiologist finds that the level of electrical interference from patient-connected anaesthetic and surgical equipment is an obstacle to recording low amplitude signals from the nervous system; on the other hand, the anaesthetist may be asked to avoid neuromuscular blocking drugs and powerful anaesthetic agents, so that the task of keeping the patient asleep and immobile is more difficult. These competing requirements should be approached in a spirit of cooperative teamwork and compromise. In the intensive care unit neurophysiological recordings can give diagnostic and prognostic information, and sometimes influence treatment. Prolonged monitoring of brain function allows clinicians to recognize spontaneous fluctuations of activity in comatose patients, the arousal responses of inadequately sedated patients, and the previously unrecognized incidence of epileptiform activity, particularly after head injury or neurosurgical procedures. For these reasons neurophysiological monitoring is being used more often in general as well as neuro-intensive care units. During surgical operations monitoring has been used at the request of the surgeon to detect impending damage to neural pathways, the onset of ischaemia, and the development of epileptiform discharges. In addition, the possibility of unintentional awareness has now become an important legal issue for anaesthetists, and any means of avoiding it is warmly welcomed. As a result of the great increase in power of readily-available computers in the 1990s, monitoring devices began to appear which extracted certain characteristics of the EEG or an EP, and in some cases compressed them into a single number thought to indicate clinically important states of the nervous system. Enthusiastic marketing ensured that anaesthetists and surgeons became interested in brain waves; now they had a box to tell the anaesthetist how close the patient was to awareness, or at least that was what the makers claimed it could do. As often happens following the introduction of new technology, clinicians were divided into the enthusiasts, the sceptics, and those waiting for compelling evidence before deciding one way or the other. According to the enthusiasts, EEG-monitored anaesthesia reduces the incidence of unintentional awareness, shortens recovery time, and makes anaesthesia a pleasanter experience for the patient, who is more alert and less nauseous postoperatively. The enthusiasts therefore advocate monitoring every anaesthetized patient with an EEG or EP index. Sceptics, on the other hand, point out that some anaesthetic agents do not affect the EEG or EPs in the expected manner, and so render the indices unreliable. Poor electrode application, external electrical interference, and muscle and movement artefacts produce misleading results. Because the indices are validated in normal subjects, they are less reliable, and in some cases useless, in the presence of epileptiform discharges or the EEG patterns produced by encephalopathies. The reluctance of some manufacturers to publish the algorithms underlying their indices has only served to heighten scepticism. The situation is not helped by the fact that few anaesthetists or surgeons are expert in neurophysiology, and merely follow the behaviour of an index number, without understanding the EEG or EP signal from which it is derived; some have even attempted to detect cerebral ischaemia with monitoring devices designed for prevention of awareness. Editors of journals therefore have a particularly difficult task in ensuring that published reports on surgery and anaesthesia monitored by EEG or EPs are of the same scientific standard as those on other subjects. While EEG and EP monitors undoubtedly have their place in the operating theatre, their precise contribution needs to be clarified. They must not be regarded as replacing conventional EEG or EPs, but merely as indicating the effect upon the brain of clearly defined and rigidly specified anaesthetic and surgical techniques. The inappropriate perioperative use of neurophysiological methods will only be avoided by educating anaesthetists and surgeons. They should not be expected to become neurophysiologists, but they should understand the physiology of neuronal activity, and the basics of signal acquisition and interpretation. In particular, anaesthetists should be encouraged to demand a high quality display of the raw EEG or EP along with a depth-of-anaesthesia index, just This book is derived from the two volumes entitled Clinical Neurophysiology (Binnie et al, 2003, 2004). We have compiled it using material extracted, rearranged and expanded from these parent volumes to cater for the particular needs of those working in neurophysiological monitoring. Such monitoring in the intensive care unit and the operating theatre has developed into a rapidly expanding subspecialty of clinical neurophysiology, to which many neurophysiologists and their technologists now devote a substantial part of their professional lives. At the same time, it was appreciated that colleagues in other specialties needed to understand what could - and, more important, what could not - be expected from monitoring. Also, in many hospitals, monitoring was being carried out by anaesthetists, surgeons, intensivists and others who lacked a firm grounding in the basics of clinical neurophysiology. There are several important differences between monitoring and other branches of clinical neurophysiology. Firstly, the principal aim of monitoring is to warn of deterioration, and thereby to avoid catastrophe, rather than to provide diagnostic information. Secondly, the population norm is of much less importance because the patient usually serves as his or her own control. Thirdly, monitoring has found applications in many apparently disparate branches of surgery and intensive care, and no one can claim to be expert, or even reasonably competent, in all of them. We have tried to keep these points in mind in preparing this book, and to provide sufficient explanation of the neurophysiological background for those entering the field from other specialties, including biomedical engineers and medico-legal experts as well as clinicians and clinical scientists. Nevertheless, in many cases the reader is referred to one or other of the parent volumes for an account of the basics; more detailed information on neurophysiological technology is given in Cooper, Binnie and Billings (2005), which provides a convenient companion to this present volume. We are acutely aware that neurophysiological monitoring is still developing rapidly, so that parts of the present volume may soon be out of date, if they are not already. Faced with such an actively developing subject, a printed book is perhaps not the best way of presenting information; an Internet version would provide greater possibilities of regular updating if the demand arises. Finally, we must reiterate the injunction with which the Editors of the parent volumes concluded their Preface. Although the information in this book is correct to the best of our knowledge, no one should attempt to carry out any of the techniques described without having undertaken adequate, recognized training, and without consulting up-to-date local, national and international safety regulations, and local, national and international recommendations on good practice.

Table of Contents

Contributors, v
Foreword, vii
Preface, ix
Dedication, x
Acknowledgements, x
Abbreviations, xi
1 The why and the how of neurophysiological monitoring in the ICU and surgery, 1
Introduction, 1
Applications of clinical neurophysiology in the ICU, 1
Monitoring during surgical operations, 3
Assessment of results, 7
Statistical significance of changes, 7
Clinical significance of changes, 8
Confidence intervals, 10
Determining the optimal cut-off point, 11
Comparing two methods of monitoring, 11
The effect of corrective action, 12
Is the risk eliminated by monitoring?, 13
References, 14
2 Neurophysiological instrumentation, connection with patients and recording methods, 17
Introduction, 17
Recording the electrical activities of the nervous system, 18
Electrodes for neurophysiological recording, 18
Electrode placement systems, 24
Connecting electrodes to amplifiers and the recording convention, 29
Amplifiers, 32
Signal bandwidth and filters, 35
Digital systems, 37
EEG recording equipment, 38
General features of EEG machines, 38
Input circuits for EEG recording, 39
Amplifiers for EEG recording, 39
Filters for EEG recording, 40
Analogue EEG systems, 40
Digital EEG systems, 42
Evoked potential recording systems, 48
Electrodes for EP recording, 48
Amplifiers for EP recording, 48
Filters for EP recording, 48
Averaging, 48
EP display, 49
Calibration of EP recorders, 49
EP stimulators, 49
EMG and nerve conduction studies, 53
Electrodes for EMG and nerve conduction studies, 53
Amplifiers for EMG and nerve conduction, 54
Displaying EMG and nerve conduction data, 55
Stimulation for nerve conduction studies, 55
Practical aspects of intraoperative and ICU neurophysiological recording, 55
Safety during neurophysiological recording in the ICU and operating theatres, 55
Intraoperative and ICU EEG recording, 59
Intraoperative and ICU EP recording, 65
EMG and nerve conduction studies, 68
References, 68
3 Introduction to methods for continuous EEG and evoked potential monitoring, 73
Introduction, 73
EEG monitoring, 73
Historical development of EEG monitors, 73
Technical requirements for EEG monitoring in the ICU and during surgical operations, 85
Evoked potential monitoring, 94
Technical requirements for EP monitoring, 94
Comparisons between currently used methods, 100
Conclusions, 102
References, 103. 4 Normal and pathological phenomena in EEG, evoked potential, EMG and nerve conduction studies, 109
General introduction, 109
EEG WAVEFORMS AND INTERPRETATION, 109
Introduction, 109
Describing EEG phenomena, 109
Wave shape (morphology), 110
Rhythmicity, 110
Frequency of repetition, 110
Amplitude, 110
Transients, 112
Spatial distribution, 113
Spatiotemporal patterns, 113
Symmetry and synchrony, 113
Inherent variability of the EEG and other biological rhythms, 113
Reactivity, 114
General categories of abnormality in the EEG of importance for ICU and intraoperative monitoring, 114
Change in frequency content, 114
Amplitude reduction, 115
Localized and lateralized abnormalities, 116
Rhythms at a distance (projected rhythms), 117
Altered reactivity, 118
Epileptiform activity, 119
Periodicity, 120
Burst suppression pattern, 123
Electrocerebral inactivity - electrocerebral silence (ECS) - the isoelectric EEG, 124
EVOKED POTENTIAL WAVEFORMS AND INTERPRETATION, 125
General definition - limitations - clinical utility of evoked potentials, 125
Responses and EP components, 126
The electroretinogram and electrocochleogram, 126
Relation between neuronal responses and surface evoked potentials, 127
Action potentials, 127
Postsynaptic potentials, 128
Near-field versus far-field evoked potentials, 128
How to localize evoked potential sources from surface recordings, 130
Normal findings by modality, 131
Flash VEPs, 131
Auditory evoked potentials: BAEPs and MLAEPs, 132
Somatosensory evoked potentials, 134
EMG FINDINGS AND INTERPRETATION, 145
Features of motor units recorded by needle electrodes, 145
Other normal EEG phenomena, 145
Insertion activity, 145
End-plate noise, 145
Fibrillations at single sites, 146
Fasciculations, 146
Nerve conduction, 147
Effects of limb temperature on nerve conduction, 147
References, 148
5 Neurophysiological monitoring during sedation and anaesthesia, 155
Introduction, 155
Sedation: assessment with EEG and evoked potentials, 155
Effects of sedative drugs on the EEG, 155
Effects of sedative drugs on EPs, 157
Assessment of sedation, 159
Anaesthesia: assessment with EEG and evoked potentials, 161
Effect of anaesthetic agents on the EEG, 161
Effect of anaesthetic agents on EPs, 172
Combined EEG and evoked potential measures, 176
Awareness during anaesthesia, 176
Neurophysiological features useful in prediction of possible awareness during anaesthesia, 177
Medico-legal aspects of awareness during anaesthesia, 180
References, 182
6 Neurophysiological work in the ICU, 189
Introduction, 189
Neurophysiological parameters to be monitored and procedures, 189
Sleep in the ICU, 189
CLINICAL CONDITIONS AFFECTING THE CENTRAL NERVOUS SYSTEM ENCOUNTERED IN ICU, 189
Coma and related states, 189
EEG recording and interpretation in coma and related states, 189
EPs in comatose patients, 199
EPs combined with other variables, 215
Cardiac arrest and hypoxic - ischaemic encephalopathies, 216
Other metabolic and toxic encephalopathies and multiple organ failure, 218
Encephalitis, 218
Epileptiform discharges and status epilepticus, 218
Head injury and other neurosurgical applications, 221. Vegetative states and brainstem death (brain death), 225
Vegetative states, 225
Brainstem death (brain death), 228
CLINICAL CONDITIONS AFFECTING THE PERIPHERAL NERVOUS SYSTEM AND MUSCLES ENCOUNTERED IN THE ICU, 231
Neuromuscular syndromes of critical illness (critical illness neuropathy), 231
Acute onset neuropathies, 234
Guillain - Barre syndrome (GBS), 234
Acute intermittent porphyria, 237
Other causes of acute or subacute peripheral neuropathy, 242
Acute weakness due to disorders of neuromuscular transmission, 242
Botulism, 242
Myasthenia gravis, 242
Familial periodic paralysis and the channelopathies, 242
Non-peripheral causes of acute onset generalized weakness, 243
References, 243
7 Neurophysiological monitoring during surgical operations, 253
Introduction, 253
Intracranial surgery, 253
EEG monitoring in intracranial surgery, 253
Visual evoked potential monitoring, 256
SEP monitoring in intracranial surgery, 257
Posterior fossa surgery, 260
Spinal cord function monitoring, 268
Experimental studies and mechanisms of spinal cord damage, 268
Methods of monitoring cord function, 269
Practical aspects of spinal cord monitoring, 272
Clinical applications of spinal cord monitoring, 274
Spinal root surgery and peripheral nerve surgery, 278
Cerebral ischaemia during non-intracranial surgery, 279
Cardiac surgery, 279
Carotid endarterectomy, 285
References, 292
8 Further signal analysis, 309
Introduction, 309
Data acquisition, 311
Technology, 311
Safety issues, 315
Improving signal quality, 317
Filtering methods, 318
Artefact detection and rejection methods, 327
Signal processing and interpretation, 327
Common processing tasks, 329
Processing methods, 332
Integration of features into the clinical context - pattern classification, 346
Statistical process control, 354
Use of the tools, 358
Decision assistance, 358
Development and uptake of methods, 359
Performance assessment, 359
Display of results, 360
Use of IT facilities, 363
References, 365
9 Legal implications of neurophysiological monitoring, 371
Safety of patients and staff, 371
Electromedical equipment, 372
Infection control, 373
Drugs, 373
Identification of patients and consent to procedures, 373
Inherent risk of procedures, 374
Responsibilities in training and supervising staff, 374
Responsibilities in respect of reports on investigations, 375
References, 375
Index, 377.

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Neurophysiological monitoring during intensive care and surgery

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