depth of anaesthesia monitoring... Flashcards
why is depth of anaesthesia monitored?
- TIVA - to monitor depth when MAC not available to reduce awareness
- Elderly population when a MAC of <1 or minimal anaesthetic agent is desired due to instability.
what are guedels stages of anaesthesia?
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.
what are the different method for monitoring depth of anaesthesia?
**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.
how is raw EEG data obtained?
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.
Describe the different patterns of EEG you know…
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.
what happens to characteristic of EEG waves as depth of anaesthesia increases?
As a patient moves from wakefullness to deep anaesthesia the waves typically become less frequent, higher amplitude and more regular
what is the burst supression?
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
how is raw EEG data analysed?
Waveform analysis using complex mathematical models.
e.g. compressed spectral array, bicoherance analysis, fouriers analysis
break down the waveform into smaller chunks for analysis
what is BIS?
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.
what do the different BIS numbers correspond to?
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.
what is the signal quality index given by BIS? (SQI)
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
why does a BIS also record EMG?
The BIS also record EMG of the frontalis muscle to differentiate the signals from that of the EEG
what is the SR ratio?
SR ratio is the supression ratio.
looks at level of burst supression over the last 60 seconds.
what are the components of a BIS monitor?
electrodes
cables
procesing unit - amplifier, filters, algorithms
display
what factors can cause artifacts in BIS?
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.
what are the advantages of BIS?
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
what are the limitations of BIS?
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.
other than theatre, where else may BIS be used?
ITU in certain disease states you want to induce burst supression to reduce brain O2 consumption e.g. after head injury/ status epilepticus.
what are E - entropy monitors?
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
what are the limitations of E entropy monitors?
In cerebral atrophy the EMG component becomes more significant, and thus less validated
cant be used with ketamine
how do auditory evoked potentials monitor depth of anaesthesia?
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.
what are the limitations of auditory evoked potentials?
hearing loss
what is meant by awareness under anaesthesia..
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.
what is the difference between implicit and explicit awareness?
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
