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Module 3: Waveforms in wakefulness, sleep and anesthesia (ICETAP)

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ICETAP EEG

on 11 January 2015

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Transcript of Module 3: Waveforms in wakefulness, sleep and anesthesia (ICETAP)

Module 3
Waveforms in wakefulness, sleep, and anesthesia
Sleep
Sleep that knits up the ravelled sleave of care
The death of each day's life, sore labour's bath
Balm of hurt minds, great nature's second course, Chief nourisher in life's feast.
-William Shakespeare, MacBeth
This chap is about to fall asleep. Let’s look at characteristics of their EEG as they go through the different stages of sleep.
EEG will have high frequency, low amplitude, > 12 Hz Beta activity with a ‘fuzzy’ appearance, and high SEF (> 20)

Artifacts: blinks & eye movements. Electromyography (EMG) from ocular or extra-ocular facial muscles may also cause spurts of high frequency & higher amplitude waves

When relaxed with eyes closed (so that no blink artifacts are seen) there tends to be an increase in alpha frequencies.
Awake
- Somnolence or Drowsy Sleep
- Presence of Theta frequencies
- Loss of most conscious awareness of environment

Note that an arousing stimuli can lead to a paradoxical increase of alpha waves
- About half of total sleep is spent in Stage 2
- Characterized by sleep spindles and K complexes
There is synchronization of electrical firing between the thalamus and cortex
- Awareness of the external environment is apparently absent
- Slow Wave Sleep
- > 20% Delta waves
- Parasomnias can occur during this stage including:
Night terrors; Nocturnal enuresis; Sleep walking; Somniloquy
20 – 25% of sleep time
Difficult to distinguish from waking state
Rapid eye movements present
Heart Rate increases
Most dreaming occurs
Descending muscular atonia which might be preventative for muscular movements during dreaming
Stages of Sleep
With Deepening Anesthesia:
EEG Waveform – generally the waveforms decrease in frequency and increase in amplitude

SEF – generally decreases

EMG – BIS may be falsely elevated with excess muscle activity
GABAergic Anesthetics
GABAergic anesthetics frequently produce well described, characteristic changes in the EEG hallmarked by a transition from high-frequency, low-amplitude waves to low-frequency, high-amplitude waves

Some of these changes in the EEG look very similar to the patterns seen during Stage 3 non-REM sleep

Do take caution that not all frontal EEG traces of patients proceed through the following states in the same manner and note that variations exists. What are believed to be typical EEG trace patterns are presented here.
Propofol, Etomidate, and Volatiles
These GABAergic anesthetics can cause a decrease in cortical activity by:
i) direct action on the cortex and
ii) indirect action via inactivation of endogenous brain stem and hypothalamic systems that normally potentiate arousal. 2 The frontal EEG may be able to monitor this change in cortical activity.

Additionally, the frontal EEG might reflect GABAergic effects on the thalamus since neurons in the cortex are linked to the thalamus by cortico-thalamocortical connections 6
connections that when lost or dissociated from each other may be the cause of, or strongly correlated with, GABAergic induced loss of consciousness. 7
GABAergic anesthetics are able to have a large affect on the EEG because a small number of inhibitory interneurons control large numbers of excitatory pyramidal neurons, efficiently deactivating large regions of the brain 3.
Induction Sequence
Let’s give this chap some GABAergic anesthetics and look at the different stages they go through during an induction sequence
Awake
To review, high-frequency, low-amplitude Beta waves (15 – 40 Hz) are frequently seen on the EEG of an awake patient.

Beta waves are believed to indicate that cortical neurons are tonically depolarized. Also, when awake, thalamic neurons are in a depolarized, continuously firing state.
Drowsy
The patient is awake, but relaxed with eyes closed (blink and eye movement artifact are thus gone)

The higher frequency Alpha and Beta waves (8 – 15 Hz; 15 – 40 Hz) often remain present, which may appear as a “fuzziness” to the EEG trace.
Transition/Sedated EEG
Slower, larger waves can be seen that are < 8 Hz. Higher frequency waves may still be present.
Onset of sleep spindles – sporadic bursts of moderate amplitude, moderate frequency activity going up & down in spindle shape.
Paradoxical Excitation
As anesthesia is given the general trend of the EEG is from low amplitude, high-frequency waves to low-amplitude, high-frequency waves.
But at low doses of anesthesia, a paradoxical excitation in the EEG may be seen with an increase in higher frequency waveforms, particularly in the beta frequency range 2


A possible hypothesis for paradoxical excitation is that the globas pallidus interna provides tonic inhibition of the central thalamus (which is opposed by striatal inhibition on the pallidus). Binding of GABAa-ergic amnesics may suppress this tonic inhibition by the pallidus, thus leading to activation of thalamicocortical and thalamicostriatal connections.

Hypothesis summary: GABAa-ergic inhibition of the globas pallidus inhibitor leads to activation of the cortex.
Deep Sedation
With higher doses of GABAa-ergic anesthesia Theta and Delta waves (4 – 8 Hz; 0.5 – 4 Hz) begin to predominate as the amplitude continues to increase.
The transition to lower frequency waves may reflect thalamicocortical hyperpolarization and synchronization of neuronal burst activity.

Also with higher doses of GABAA-ergic anesthesia Spindles and K complexes begin to immerge.

The Spindles and K complexes might indicate that:
1. There is thalamocortical sensory blockade
2. Cortical neurons can no longer generate transient high firing rates that are usually associated with cognitive activity.
Surgical Anesthesia
The surgical anesthesia state is frequently characterized by predominant Delta wave (0.5 – 4 Hz) activity.
In fact, predominance of Delta waves tends to be a good indication of deep anesthesia (as well as slow wave sleep).
Delta waves are believed to represent slow, travelling waves of synchronous hyperpolarization (alternating with depolarization), initiated predominantly in the prefrontal cortex.

But do take caution and keep surgical context in mind. Delta waves seen while surgery is already underway can be greatly reassuring, while delta waves seen during the onset of surgery may not indicate a sufficiently deep anesthetic state since, as an example, nociceptive input of incision can change the EEG, reflecting cortical response to nociceptive afferents.
Burst Suppression
Burst Suppression is characterized by alternating periods of spindle-like, or fast spiky bursts, followed by periods of nearly flat-line suppression.

Burst Suppresion is NOT normally see during sleep and can be used to distinguish anesthesia from sleep (in contrast to Delta waves that are seen during both anesthesia and sleep)
Burst Suppression may be indicative of a deeper anesthetic state since burst suppression is typically only seen after administration of higher doses of anesthesia

But do take caution with maintaining a patient in a burst suppression state.
Burst suppression was associated with increased mortality in medical ICU patients receiving IV sedation.
Dosing to the level of burst suppression will usually result in hypotension, slow recovery times, and excessive consumption of general anesthetic drugs.
Continuous Suppression
Often referred to as “isoelectric” (a misnomer since true isoelectricity is not achieved in the living), continuous suppression is characterized by a nearly flat line with no deflections.

Continuous Suppression may represent an even deeper state of anesthesia than burst suppression because continuous suppression is typically only seen after administration of even higher doses of anesthesia

Continuous Suppression may indicate a complete absence of neuronal communication resulting either from high dosages of anesthesia or from a severe physiological aberration.
*Zoom into each stage for description
Continuous Suppression
Waking up - Deep Sedation
Surgical Anesthesia - multiple sweep speeds
Awake - with blink artifacts
Brown, Lydic, and Schiff. General Anesthesia, Sleep, and Coma. N Engl J Med. 2010 ; 363:2638-50.
Anesthesia vs Sleep
Awake
Drowsy
Paradoxical Excitation
Deep Sedation
Surgical Anesthesia
Burst Suppression
Continuous Suppression
(Isoelectricity)
But before we look at sleep, let's see what happens during an awake state
EEG Recordings
Burst Suppression
Drowsy - Stage 1/2
Deep Sleep - Stage 2/3
Sleep
Anesthesia
VS
Now let's look at what happens during anesthesia!
REM Sleep
Summary of Stages of Anesthesia
Notice the bovie artifacts later in the movie
Notice the transition from stage 3 sleep to REM sleep
Dog with Somnambulism
Bottros MM et al. Estimation of the bispectral index by anesthesiologists: an inverse turing test. Anesthesiology. 2011; 114: 1093-101.
Content by: Paul McNair and Andrew Park
Prezi by: Andrew Park
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