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Introduction

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The electroencephalogram (EEG) is used routinely in the intensive care unit (ICU) to evaluate patients for cortical electrical activity, and thereby to differentiate among various causes of coma, delirium, movement abnormalities, and identify brain death. In addition, EEG may be used to monitor depth of sedation. The EEG measures the summed voltage potentials (Figure 16-1) from a large number of neurons (not “brain waves”), and, depending on the number and placement of electrodes, can be used to localize certain pathological electrical processes in exactly the same way that an electrocardiogram (ECG) localizes injured cardiac muscle.

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Definitions and Terms

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  • ▪  Frequency: The measurement used to categorize several ranges of electrical activity found in the normal and abnormal brain:
    • —Delta: High-amplitude, low-frequency waves seen in sleep
    • —Theta: Low-frequency waves seen in younger children and meditation in adults—may be indicative of pathological activity, such as toxic encephalopathy
    • —Alpha: Higher frequency range seen in relaxation and certain forms of coma
    • —Beta: High frequencies seen during active thinking
  • ▪  Epileptiform discharges: Rhythmic discharges seen focally or diffusely in patients with epilepsy.
  • ▪  Diffuse slowing: Combination of delta and theta frequencies seen in many pathological states—when seen with preserved reactivity to external stimuli (ie, pain, sound); this pattern may have a better prognosis than when responsiveness is absent.
  • ▪  Intermittent rhythmic delta activity: Pathological pattern that can be seen with metabolic, toxic, hypoxic, or other diffuse intracranial diseases.
  • ▪  Burst suppression: High-voltage bursts of activity alternating with background suppression—seen in deep coma and pharmacologically induced coma.
  • ▪  Bispectral index (BIS): A limited form of EEG analysis in which brain electrical activity is indexed to a dimensionless number between 0 and 100, wherein a value of 0 represents the absence of EEG activity and 100 is typical for an awake person—values between 40 and 60 are found in well-anesthetized patients.

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Techniques

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  • ▪  EEG leads are placed circumferentially around the brain (Figures 16-2 and 16-3), as well as over the dome of the skull along coronal and saggital arcs—electrical activity is measured over specific areas of the brain (ie, frontal, temporal, cerebellar).
  • ▪  EEG may be used in the delirious patient to determine whether abnormal electrical activity in the brain may be responsible for the delirious state.
  • ▪  EEG is used as a diagnostic test in patients with diffuse or localized rhythmic movement to determine whether there is epileptic activity in the brain—the differential for rhythmic motor activity is long, but includes tremor, volitional movement, and shivering.
  • ▪  EEG is used to establish prognosis in the comatose patient following a hypoxic event or cardiac arrest.
  • ▪  EEG is used to guide pharmacological management of epileptic activity or intracranial hypertension.
  • ▪  EEG is used as a confirmatory test in the diagnosis of brain death.
  • ▪  ...

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