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Module 10: Case Studies (ICETAP)

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ICETAP EEG

on 11 January 2015

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Transcript of Module 10: Case Studies (ICETAP)

Case Studies
In this module, we want to present to you cases and scenarios that you may encounter during your practice in the OR.
Before we start...
These explanations may be helpful for understanding case studies presented in this module
PSD = Power Spectral Density
Top row is EEG and PSD
before the event

Bottom row is EEG and PSD
after the event
Notice the gray PSD which is pre-event PSD (same as above) for better comparison
Notice in response to the incision, higher frequency patterns become more predominant in patient's EEG. This is easier to see on the PSD, which shows increase in power from approximately 10Hz and above.
Higher frequency power
has increased post-incision
Case 1 - What are the effects of surgical cautery?
Answer:
Sometimes almost no change...
But often an increase in high frequencies
Diathermy, or electrical cautery causes artifacts in the EEG. Notice the change in EEG after cautery is applied. It adds high frequency noise to the EEG. This is reflected in the PSD showing higher power in higher frequency range.
Increase in higher frequency waves
Case 3 - What are the effects of ketamine?
Answer:
Small doses often have little effect...
>20mg usually show an increase in high frequencies
Some explanations will be in
a small font below
Ketamine is an NMDA receptor antagonist. When ketamine is administered during GABAergic anesthesia,
it increases high frequency EEG waves. The change is reflected on the EEG but is easier to see on the PSD.
Note that this will be translated into a higher quantitative EEG number (BIS, entropy, etc) when in fact, the patient is
clinically in a deeper state of anesthesia.
Case 2 - Neuromuscular blocking agent
Here we see a comparison of EEG pre- and post- injection of rocuronium. The Pre-rocuronium EEG is contaminated with electromyography (EMG).
After delivery of rocuronium, EMG contamination is diminished. Post-rocuronium EEG shows mainly low frequency slow waves. PSD shows decrease in power in the high frequency range.
Take Home Point
A more "awake" looking
EEG is actually "deeper" anesthesia
Let's take a look at a typical
general anesthesia induction
55yr male for ankle fracture fixation
Anesthesia Plan
- Fentanyl 100ug iv
- Propofol 200mg iv
- LMA
- Desflurane in O2/Air
Changes in drug effect-site concentrations (upper)
and in the frequencies of the EEG (lower)
In the next slides we will look at the EEG at each of the four time points (A, B, C, and D)
(A) Pre-induction
Nervous patient
Shows blinks and EMG on the EEG trace
(B) Unresponsive after propofol
Fentanyl 100ug and Propofol 200mg iv
High amplitude delta waves commonly seen after high-ish dose propofol.
LMA inserted
(C) Waiting for the surgeon to gown…
Desflurane =7%, decreasing propofol, & no pain
BP = 85/45mmHg, HR=50/min
Consider giving vasopressors
Don’t decrease the anaesthesia… because it will change when the surgeon cuts…
(D) EEG response to skin incision
Desflurane still = 7%
Burst suppression replaced by classical delta-spindle EEG pattern
Balancing anesthesia vs surgical stimulus
See case 2 for more further learning concerning incision

The “depth of anesthesia” must always be assessed relative to the surgical stimulation.
In this case, cortical activity correlated with brain stem/CVS signs – but often cortical activity and brain stem/autonomic activity are not correlated
Case 1
Case 2
The effects of surgery on the EEG
Principles
Nociceptive input to the brain tends to release neuromodulators that depolarize the thalamo-cortical system.
Most commonly-used anesthetic and analgesic drugs encourage hyperpolarization of the CNS
The EEG effects will depend on the balance between the intensity of surgical stimulation, and the effectiveness anesthetic drug suppression of the CNS – i.e. these changes are an indicator of adequacy of general anaesthesia
The EEG effects of incision are:
1. No change (this suggests good levels of anaesthetic suppression
Example #1. No change
COMMENT: However a slight increase in the frequency of the spindle waves, hints at the presence of some unblocked nociceptive influences.
Example #3: Loss of Spindles
Example #2: Loss of (near) burst suppression
COMMENT: The pre-incision EEG shows episodic bursts over a slow low amplitude sub-delta oscillation (that has not quite achieved a classical burst suppression pattern). The frequency within these bursts is the same as that in the post-incision “spindles”, but the quiet (suppressed) periods have been lost. These suppressed periods are very good indicators of extreme neuronal hyperpolarisation and quiescence; prolonged so-called “DOWN” states.
Take-home message(s)
Changes in EEG pattern in response to surgery indicate incomplete anesthetic suppression of noxious input to the CNS.
It would seem reasonable to incorporate this information in our intra-operative titration of anesthetic and analgesic drugs.
However, at present there are no large-scale studies to determine if this approach significantly influences patient outcomes.
COMMENT: The very slight increase in the delta power,
hints at an element of paradoxical EEG arousal.
Take-home message(s)
2. Loss of the EEG signs of hyperpolarization:
Increased high frequencies (“Beta Arousal”)
Loss of delta/ spindles/ burst suppression patterns

3. So-called “Paradoxical” or “Delta Arousal” pattern
Increased delta waves
Wait, there's more...
Here's a EEG spectrogram, showing the spectral density of EEG signal over time. Horizontal axis is time, vertical axis is frequency, and color represents the amplitude of a given frequency occurring at a given time point
Red = high amplitude
Yellow = medium amplitude
light blue = low amplitude
Dark Blue = very low amplitude
Mini Cases
Content by: Jamie Sleigh
Prezi by: Andrew Park

More cases to come soon!
2 seconds
Slow waves
(Delta)
Faster waves
(Theta & Alpha)
Pure Propofol Anesthesia
25 mm/sec
50 mm/sec
12.5 mm/sec
6.25 mm/sec
4 sec.
8 sec.
16 sec.
32 sec.
EEG from BIS electrodes (single frontal channel): 4 sweep speeds; filters off
4 sec.
8 sec.
50 mcv
50 mm/sec.
25 mm/sec.
25 mm/sec
25 mm/sec
4 sec
4 sec
12.5 mm/sec
16 sec
Sweep speed = 12.5 mm/sec
8 waves in 16 seconds => ½ wave per second
The underlying waveform is slow delta at a frequency of 0.5 Hz.
What other waves are there?
9 waves in 2 seconds; i.e., theta and delta
(Perhaps some waves ‘riding’ on these waves
0 Hz.
30 Hz.
Return of the Beta
12.5 mm/sec (16 sec)
Information to Glean from the Frontal EEG
Andrew Park
Michael Avidan
Mostly Delta
Case 3
Mostly Delta
Return of the Beta
The End
EEG during an abnormal induction
Mark Willingham
Jaime Sleigh
Case 4
35 yo Female for Wrist Fracture Plating
Preoperative anxiety
Induction:
Fentanyl 100μg IV
Propofol 200mg IV (rapidly)
Result:
Loss of consciousness
Increased muscle tone & abnormal jerking movements in arms/legs
Settled slowly with increased volatile anesthetic
LMA placed
Remainder of surgery was uneventful
EEG Findings to Notice
1. EMG falsely elevates the EEG depth of anesthesia indices (State and Response Entropy, & Suppression Ratio).
A: Awake with EMG
Spectrogram, EEG parameters, & Raw EEG
Take-home message(s)
You can occasionally get extreme propofol muscle rigidity even in the presence of “deep” anesthesia
Sometimes the muscle rigidity and limb movements are not just brain-stem/spinal effects - but can be associated with epileptiform spikes in the cortex.
In the presence of increased EMG or electrical interference, the burst suppression algorithm of the EEG monitor may fail to correctly detect the depth of anesthesia.
3. The difference between EMG and electrical interference on the spectrogram.
2. Epileptiform patterns may be related to propofol-induced muscle rigidity.
B & C: Loss of Consciousness (with EMG)
D: Electrical Interference
In the next slides we will look at the EEG at each of the four time points (A, B, C, and D)
~30 Minutes Later
Everything is fine!
EEG Frequencies
Response Entropy
State Entropy
Hypnotic Effect Site Concentration
Burst-Suppression Ratio
Heart Rate
Case Parameters
Raw EEG
Notice the rough spiky activity
Most likely EMG artifact as patient was tense/nervous
Could be electrical interference if electrode impedances were abnormal
Increased EMG indicates episodes of muscle rigidity
During minimal EMG (3-8s), notice EEG patterns of deep anesthesia
Near-suppression
Delta waves (with some accompanying higher frequencies riding on them)
High amplitude (>150μV) epileptiform spikes at 10s
Indicate burst of high frequency EMG activity & muscle rigidity
AC power oscillates at 60hz in USA (50hz in many other countries, including NZ)
May contaminate EEG through surrounding electrical fields & imperfect electrode impedance
Look at D above. Notice the 50hz orange at the top, compared to the broad spectrum (orange/red) EMG interference seen at time B.
Filtering reveals the underlying EEG signal
Notice the underlying burst suppression
Notice episodes of EMG & epileptiform spikes on underlying burst suppression pattern.
Indicates Deep anethesia
Unlikely that more hypnotic will improve relaxation


Sevoflurane was used to try to deepen anesthesia before LMA placement.
In view of epileptiform activity on EEG, sevoflurane was exchanged for desflurane.
Notice the broad spectrum (orange/red) EEG interference caused by epileptiform discharges
Persistent at time B
Intermittent at time C
A: EMG artifacts
D: Interference from ambient electrical fields (low-amplitude)
B & C: Epileptiform discharges related to propofol-induced muscle rigidity
(Amplitude ~ 150 μV)
Summary Discussion
Full transcript