Quantitative Sleep Monitoring - Quantitative EEG Analysis Methods and Clinical Applications - page 261

10.4 Introduction to EEG Terminology and Monitoring

EEG activity is characterized by the frequency in cycles per second or hertz (Hz),
amplitude (voltage), and the direction of major deflection (polarity). The classically
described frequency ranges are delta (<4 Hz), theta (4 to 7 Hz), alpha (8 to 13 Hz),
and beta (>13 Hz). Alpha waves (8 to 13 Hz) are commonly noted when the patient
is in an awake but relaxed state with the eyes closed (Figure 10.2). They are best
recorded over the occiput and are attenuated when the eyes are open. Bursts of
alpha waves also are seen during brief awakenings from sleep—called arousals .
Alpha activity can also be seen during REM sleep. Alpha activity is prominent dur-
ing drowsy eyes-closed wakefulness. This activity decreases with the onset of stage 1
sleep. Near the transition from stage 1 to stage 2 sleep, vertex sharp
waves —high-amplitude negative waves (upward deflection on EEG tracings) with a
short duration—occur. They are more prominent in central than in occipital EEG
tracings. A sharp wave is defined as deflection of 70 to 200 ms in duration.
Sleep spindles are oscillations of 12 to 14 Hz with a duration of 0.5 to 1.5 sec-
onds. They are characteristic of stage 2 sleep. They may persist into stages 3 and 4
but usually do not occur in stage REM. The K complex is a high-amplitude,
biphasic wave of at least 0.5-second duration. As classically defined, a K complex
consists of an initial sharp, negative voltage (by convention, an upward deflection)
followed by a positive-deflection (down) slow wave. Spindles frequently are super-
imposed on K complexes. Sharp waves differ from K complexes in that they are nar-
rower, not biphasic, and usually of lower amplitude.
As sleep deepens, slow (delta) waves appear. These are high-amplitude, broad
waves. Whereas delta EEG activity is usually defined as <4 Hz, in human sleep scor-
ing the slow-wave activity used for staging is defined as EEG activity slower than 2
Hz (longer than 0.5-second duration) with a peak-to-peak amplitude of V.

The amount of slow-wave activity as measured in the central EEG tracings is used
to determine if stage 3/4 is present [1]. Because a K complex resembles slow-wave
activity, differentiating the two is sometimes difficult. However, by definition, a K
complex should stand out (be distinct) from the lower-amplitude, background EEG
activity. Therefore, a continuous series of high-voltage slow waves would not be
considered to be a series of K complexes.
Sawtooth waves (Figure 10.2) are notched-jagged waves of frequency in the
theta range (3 to 7 Hz) that may be present during REM sleep. Although they are
not part of the criteria for scoring REM sleep, their presence is a clue that REM
sleep is present.

> 75 μ

Figure 10.2 Stage 2 sleep is shown. The EEG shows a K complex. (Courtesy of James Geyer and Paul Carney.)