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.
- ▪ 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
- ▪ Burst suppression: High-voltage bursts of activity alternating
with background suppression—seen in deep coma and pharmacologically
- ▪ 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.
- ▪ 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,
- ▪ 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
- ▪ BIS ...