depth of anaesthesia monitoring...

Question 1

why is depth of anaesthesia monitored?

Answer 1

1. TIVA - to monitor depth when MAC not available to reduce awareness
2. Elderly population when a MAC of <1 or minimal anaesthetic agent is desired due to instability.

Question 2

what are guedels stages of anaesthesia?

Answer 2

guedel classified stages of anaesthesia based on clinical signs and loss of reflexes noticed ..

stage 1 = loss of consciousness. amnesia and analgesia

stage 2 = excitement and delirium. airway reflexes intact and infact hyperstimulated. risk of laryngospasm

stage 3 = surgical anaesthesia stage. loss of eye movement and various reflexes along the way and patterns of breathing
- plane 1 = fixed gaze, regular breathing
- plane 2 = pauses in breathing, loss of corneal reflex
- plane 3 = regular shallow breathing, loss of pupil reflex, fixed dilated pupil. true surgical anaesthesia. complete muscle relaxation
- plane 4 = apnoea

stage 4 = brainstem anaesthesia and death.

Question 3

what are the different method for monitoring depth of anaesthesia?

Answer 3

**can be classified into measured and clinical observations.. **

clinical observations are useful to be aware of however are confounded by many other drugs used and patient status. e.g. HR, BP, dilated pupils, tears, sweating, RR, eyelash reflex. Guedels stages of anaesthia

more specific form of clinical observation is the isolated forarm technique. tourniquet around patient arm to block off arterial supply before NMBA given so can move hand and communicate if they are awake.

Specific methods include electroencephalographic based methods and electromyographical methids.

EEG based methods = BIS, E-entropy monitors, evoked auditory potentials, raw EEG data, power spectral analysis

electromyography methods include oesophageal contractions = balloon infalated into lower half of oesophagus and will measure oesophageal contractions. these correlate to stress and wakefullness.

Question 4

how is raw EEG data obtained?

Answer 4

16 - 20 scalp electrodes placed around patients head

various combination of the potential differences measured can create a number of axis across the brain.

creates many different waveforms
often with characteristic waves indicated certain brain states.

Question 5

Describe the different patterns of EEG you know…

Answer 5

Delta = deep anaesthesia/sleep 0-4Hz
Theta = moderate anaesthesia/ sleep 4-8Hz
Alpha = eyes shut / light sedation 8-12Hz
Beta = awake 12-25
Gamma = high brain activity e.g. problem solving ?40Hz

donalds theives ate all 12 burgers.

Question 6

what happens to characteristic of EEG waves as depth of anaesthesia increases?

Answer 6

As a patient moves from wakefullness to deep anaesthesia the waves typically become less frequent, higher amplitude and more regular

Question 7

what is the burst supression?

Answer 7

an EEG pattern associated with deep anaesthesia..

consists of delta waves (high amplitude, low frequency) with intermittent periods of supression where activity is almost isoelectric for a few seconds.

this is an undesirable state and indicates the patient is too deep

Question 8

how is raw EEG data analysed?

Answer 8

Waveform analysis using complex mathematical models.

e.g. compressed spectral assay, bicoherance analysis, fouriers analysis

break down the waveform into smaller chunks for analysis

Question 9

what is BIS?

Answer 9

BISPECTRAL INDEX ANALYSIS
is a type of EEG monitoring used in anaesthesia
simplified form of normal EEG
uses only 3-4 electrodes placed on patients forehead

uses mathematical models including fouriers analsysis, complex spectral array (CSA), biocoherance analysis to produce a single waveform for easier interpretation by the user and a single number to indicate the depth of anaesthesia

this BIS number is a dimensionless number ranging from 0 to 100, where 0 is no brain activity and 100 is full wakefullness.

Question 10

what do the different BIS numbers correspond to?

Answer 10

0-30 = v. deep anaesthesia, associated with burst supression
30-40 = deep hypnotic state
40-60 = surgical anaesthesia
60-80 = light sedation, amnesia
80-100= awake.

Question 11

what is the signal quality index given by BIS? (SQI)

Answer 11

This index tells you how reliable the BIS number is by comparing the signal to the amount of EMG activity and hence how much is disrupted by this.

0 - 100. 100 is very good quality signal

Question 12

why does a BIS also record EMG?

Answer 12

The BIS also record EMG of the frontalis muscle to differentiate the signals from that of the EEG

Question 13

what is the SR ratio?

Answer 13

SR ratio is the supression ratio.
looks at level of burst supression over the last 60 seconds.

Question 14

what are the components of a BIS monitor?

Answer 14

electrodes
cables
procesing unit - amplifier, filters, algorithms
display

Question 15

what factors can cause artifacts in BIS?

Answer 15

equiptment factors:
* incorrect placement of electrodes
* poor contact of electrodes = sweating, hair
* interference with other monitoring

patient factors
* other drugs - e.g. ketamine increases BIS, N20 has no effect, other psychoactive drugs may interfer
* muscular activity can interfer.

Question 16

what are the advantages of BIS?

Answer 16

gives a simple / user friendly way to analyse the EEG activity and monitor level of anaesthesia in certain situations.

agent non-specific

B aware RCT found it reduced awareness

Question 17

what are the limitations of BIS?

Answer 17

Cant be used with certain agents e.g. ketamine increase, N20 and xenon has little effect on BIS. yet all of these cause anaesthesia

the algorithms / monitor has been calibrated against young healthy individuals. Data is then being extrapolated to elderly population

The BIS number has variabiltiy amoungst individuals e.g. in one person 80 might be awake, another asleep.

unknown influence of **neuropathology **

interference from EMG - in a study patients paralysed and not anaesthetised and BIS went down.

Question 18

other than theatre, where else may BIS be used?

Answer 18

ITU in certain disease states you want to induce burst supression to reduce brain O2 consumption e.g. after head injury/ status epilepticus.

Question 19

what are E - entropy monitors?

Answer 19

E entropy monitors work similarly to BIS but look at the level of disorder in EEG activity. 3-4 electrodes placed on the head and mathematical models used to look at the disorder of activity i.e. entropy of the system

2 signals given
response signal = 0 to 100 where 100 is more entropy and correlates with wakefullness. uses both EMG and EEG.

state signal = 0 to 91 = only looks at EEG activy

in both aiming for value of 40

Question 20

what are the limitations of E entropy monitors?

Answer 20

In cerebral atrophy the EMG component becomes more significant, and thus less validated

cant be used with ketamine

Question 21

how do auditory evoked potentials monitor depth of anaesthesia?

Answer 21

EEG activity is evoked by stimulation and then record.
headphones placed in patients ear and series of clicks played at 2Hz throughout anaesthesia

The electrical activity is then recorded along the auditory pathway by placing electrodes on the head- the evoked electrical potentials

the pattern of these changes and correlates with depth of anaesthesia

less response and slower response the more anaesthetised.

Question 22

what are the limitations of auditory evoked potentials?

Answer 22

hearing loss

Question 23

what is meant by awareness under anaesthesia..

Answer 23

ability to recall events occuring during anaesthesia. may be due to accidental lack of anaesthetic agent or individuals resistance to agents.

can be classified as implicit and explicit awareness.

Question 24

what is the difference between implicit and explicit awareness?

Answer 24

implicit = memories without conscious recall but can lead to a change in behaviour after the event

explicit = conscious recall of the event either spontaneously or when questioned. can be associated with or without pain

Question 25

what are the risk factors for accidental awareness.

Answer 25

can be divided into patient factors, anesthetic factors and surgical factors

patient factors
- frail/elderly - aiming to reduce amount of agent given
- female, young , anxious, obesity = lack of agent given, more tolerant
- difficult aiway

anaesthetic
- use of muscle relaxants
- TIVA - risk of cannula tissuing
- RSI
- mistake
- junior anaesthetist

surgery
- emergency surgery
- obstetric

Question 26

what is the problem with the isolated forearm technique?

Answer 26

actually poor correlation between moving arm and awareness post surgery

after 30 mins - ischaemia and unable to move anyway

Question 27

what other evoked potentials do you know?

Answer 27

visual
auditory
somatosensory

Question 28

what are the AABGI guidelines for depth of anaesthesia monitoring

Answer 28

BIS recommended when using TIVA and NMBA

Question 29

what is meant by surgical depth of anaesthesia?

Answer 29

the optimal depth of anaesthesia for surgery. Can be defined by guedels stages of anaesthesia as stage 3 plane 3 whereby there is muscle relaxation.

any less deep results in risk of awareness
more deep results in haemodynamic consequences

Question 30

do you know any scoring systems for the depth of anaesthesia?

Answer 30

PRST score
Pressure
Pulse rate
Sweating
Tears

max score of 8 - higher the more likely to be aware

Question 31

how does the regularity of EEG waves alter with wakefullness?

Answer 31

less order, less regularity when patient awake i.e. higher entropy

Question 32

what is the Narcotrend?

Answer 32

type of EEG monitoring
uses raw EEG data and analyses through spectral analysis

Question 33

what did the NAP5 find about rate of awareness?

Answer 33

around 1 in 20,000

much higher in obstetric patients, when NMBA used and in cardiothoracics.

Question 34

what is the most common time for awareness during op?

Answer 34

induction and emergence
between emergence and getting to operating table.