BIS, entropy and evoked potentials Flashcards
Measurement of depth of anaesthesia
Non specific, specific methods
Non-specific: secondary indication of depth of anaesthesia
* Clinical signs: BP, HR, sweating, tears. Note unreliable - may be influenced by inadequate analgesia, anti-cholinergics, beta-blockers
* End-expiratory volatile agent monitoring: MAC
* Estimated plasma concentration from TCI (based on anaesthetic delivery)
Specific methods
* Non-EEG: e.g. isolated forearm technique
* EEG: raw EEG, processed EEG
Specific, non-EEG methods for monitoring depth of anaesthesia (4)
- Isolated forearm technique (research)
- Lower oesophageal contractility (dual innervation by motor and autonomic nerves, proposed that contractility in lower oesophagus, predominantly autonomic innervetion, would reflect depth of anaesthesia. Evidence v limited, not used)
- Frontalis muscle activity
- Heart rate/ ECG variability (respiratory sinus arrythmia changes with depth of anaesthesia, and depressant effect of anaesthesia is reversed by surgical stimulation)
EEG changes with anaesthesia
- Awake: fast, low amplitude
- Administration of most anaesthetic drugs; increase in amplitude and decreasing frequency as doses are increased
- At higher doses of anaesthetic, EEG evolves into more isoelectric wave form, with increasingly less frequent bursts of activity (burst suppression)
BIS index
Definition, values at different clinical states
- Bispectral index (BIS) monitor analyses changes in EEG waveform in different dimentions and planes that occur with anaesthesia. Generates BIS number.
- BIS is a dimensionless number generated in real time by a BIS monitor that ranges 0-100 to denote depth of anaesthesia
100: Awake
>70: Light sedation
60-70: Deep sedation
60: GA
40-60: Moderate hypnotic state
40: Deep hypnotic state (burst suppression)
0: No cortical electrical activity
BIS monitor
Components, algorithm
- Front-temporal electrode connected to a microprocessor
- Algorithm to calculate bispectral index (BIS) uses mathematical techniques: fast fourier transformation, power spectrum, phase spectrum, spectral edge frequency, artifact detection
Limitations of BIS
Clinical conditions affecting values, ages, reliability
- May be affected by other factors: Ketamine elevates EEG activity and therefore BIS values. Influence of pre-existing neuropathology on BIS index is unknown
- Cannot be extrapolated to v young or v old. Derived on healthy adult volunteers. Note paediatric EEG approximates to the adult from ~5 years
- Considerable person to person variability
- Unable to predict patient movement under anaesthesia
Entropy monitor
Entropy definition, processing used, converstion to depth ‘scale’
EEG signal recorded using electrodes applied to forehead and side of head
Entropy:
* Entropy = measure of the degree of disorder in a system (in this case the EEG signal). Shannon entropy is measured on scale 0-1
* High entropy of EEG (more irregular/less predictable): awake. Lower levels correlate with deep unconsciousness
Mathematical processing:
* Fourier transformation used to ‘decompose’ EEG signal into separate functions based on the frequency, phase (time) and power of each constituent sine wave
* Frequencies of voltages for each given time sample (epoch) are calculated
* This is then converted into a normalised frequency spectrum by squaring the transformed components) for the selected frequency range
* Shannon entropy is applied to give spectral entropy
* Entropy values are normalised to 0 (total regularity) to 1 (total irregularity
‘Scale’ of depth of anaesthesia
* Commercially available M-entropy module converts entropy scale of 0-1 to scale of 0-100 (similar to BIS). Not exactly linear conversion to give greater resolution to range 0.5-1 (most important for depth of anaesthesia monitoring)
Entropy monitor: clinical use
Interpretation of values, state vs response entropy
Display shows two values
* Response entropy: ranges 100-0
* State entropy: ranges 91-0
In practice, 0 indicates very deep anaesthesia, ~100 corresponds to awake
Aim 40-60 - at this range, SE and RE indexes should be similar
As patient awakens, increase in difference between SE and RE values is sesn due to diminishing effect of drugs on CNS and increasing contribution from frontalis EMG
Entropy monitor: limitations of use
Increasing age, ketamine
- In cerebral atrophy (dementia, old age) the contribution from the EMG is proportionally increased over the normally dominant EEG signal. This produces a difference in SE and RE values which may not be due to a lightening of anaesthetic depth.
- Cannot be used in presence of ketamine
- Changes in difference between SE and RE in response to surgical stimulation can be due to inadequate analgesia, or inadequate anaesthesia - requires clinical context to interpret
Auditory evoked response monitor
Aka auditory evoked potential
aepEx
* Commercially available monitor incorporating the auditory evoked response (AER)
* Like BIS, produces index 0-100 (AAI)
Mechanism:
* Bipolar surface electrodes placed on centre of head and mastoid process (over temporal lobe)
* Auditory stimuli (usually clicks, frequency 2Hz) applied to patients ears. Stimulus often set 70dB above populations average threshold
* EEG signal following each click is recorded (digitalised and averaged so that EEG response corresponding to click emerges from background noise)
AER waveform
Wave groupings
Represents the passage of electrical activity from cochlea to cortex
Waves are grouped according to electrical activity in various parts of the auditory pathway
* Brainstem waves I-V
* Early cortical or middle latency waves ( No, Po, Na, Pa, Nb) - primary auditory cortex, medial geniculate
* Late cortical waves (P1, N1, P2, N2, P3) - frontal cortex and association areas
Can measure
* Interpeak amplitude of waves (mV)
* Latency in milliseconds from origin of response (the click) to peak of each wave
Key waves in depth of anaesthesia monitoring:
* Pa: first positive wave after brainstem response
* Nb: first negative wave after Pa
Changes in AER waveform with depth of anaesthesia
Which wave grouping is most relevant
Early cortical (or mid-latency) AER show graded changes with general anasethesia
* Increasing depth of anaesthesia -> amplitudes flatten, latencies lengthen
* Partially reversed by surgical stimulation
* Pa and Nb amplitudes with increasing desflurane shown in figure
Brainstem waves (prior to 30ms) appear stable to changes in level of arousal. Late cortical waves change dramatically during natural sleep and are not present during anaesthesia
Limitations to use of AER monitor
Not as widely used as other methods e.g. BIS
Problems with signal interference and a wider variability in the AAI, compared to BIS, between the awake and asleep state.
May be affected by age, temperature
Certain neurological factors can interfere with the AER: including conductive and sensorineural hearing disorders, tumours affecting the specific nerve tracts, brain ischaemia.
Does depth of anaesthesia monitoring prevent awareness?
- Large multi-centre studies have demonstrated a reduction in the incidence of awareness
- However, probably cannot prevent awareness in an individual patient
