22. Depth of Anaesthesia Monitoring Flashcards
Intro Piece
Depth of anaesthesia (DOA) monitoring can be used to reduce the risk of accidental awareness during general
anaesthesia and to titrate the dose of anaesthetic agent used in order to minimise the adverse effects of excessively deep anaesthesia (hypotension, impaired cardiac function, increased nausea and vomiting, and delayed recovery),
which would also have cost-saving benefits. NICE in 2012 recommended the use of electroencephalography (EEG)-
based DOA monitoring (Bispectral Index (BIS), E-Entropy or Narcotrend-Compact M) as an ‘option’ during any type
of general anaesthesia where patients were deemed to be at ‘higher risk’ of awareness or excessive DOA and in
patients receiving total intravenous anaesthesia. The National Audit Project 5 published in 2014 found that DOA
monitoring was only used in 2.83% of general anaesthetics administered in the UK.
How can the depth of anaesthesia
be monitored?
Electroencephalography-based monitors:
EEG-based DOA monitors use
electrodes on the forehead to measure the EEG
activity,
which is then processed using various
algorithms that are currently a commercial secret.
Propofol, thiopentone and volatile anaesthetic agents all produce a similar pattern of EEG changes with increasing brain concentrations and corresponding DOA
(↑ high-frequency EEG components –
↑ low-frequency EEG components –
↑ waveform amplitude –
↑ regularity of EEG signal –
burst suppression with deep anaesthesia –
isoelectric ‘flat line’ EEG with very deep
anaesthesia).
Ketamine, nitrous oxide and xenon do not produce this same pattern of EEG changes and therefore the use of EEG-derived indices to guide anaesthetic administration becomes less useful if these agents are among the anaesthetic drugs being used.
> BIS monitor
> BIS monitor –
this is currently the most commonly used technique.
A disposable four-electrode sensor is
placed on the patient’s forehead
and cortical EEG activity is recorded.
A proprietary algorithm processes
this data and produces a
dimensionless number between 0 and 100,
which provides a measure of the DOA.
When the patient is awake,
cerebrocortical activity is increased
with more higher-frequency signals generated.
This leads to the generation of a higher number:
• 0–40 Burst suppression
(0 = ‘flat-line’ or electrical silence)
• 40–60 Surgical anaesthesia where auditory processing and reflex movement are still possible but memory is less likely – low probability of explicit recall
• 60–85 I ncreasing sedation,
impaired memory processing but
arousable with stimulation
• 85–100 Awake and capable of explicit recall.
The B-Aware trial published in 2004
compared BIS-guided anaesthesia with
standard care in 2463 adult patients with neuromuscular blockade who were at increased risk of awareness.
The results indicated a lower incidence
of accidental awareness in the BIS group compared with the standard care group.
E -Entropy monitors –
E -Entropy monitors –
this measures the irregularity
in spontaneous brain and facial muscular activity
using a disposable three-electrode
sensor placed on the patient’s forehead.
uses a proprietary algorithm to process
EEG and frontal electromyography data
to produce two values that indicate
the DOA-response entropy (RE)
and state entropy (SE).
Highly irregular signals with
variation of wavelength and amplitude
over time produce
high entropy values and
may indicate that the patient
is awake or aware.
More ordered signals with less variation
produce low or zero entropy values,
indicating suppression of brain electrical activity.
The RE scale ranges from
0 (no brain activity) to 100 (fully awake)
and
the SE scale ranges from 0 to 91.
The target range is 40–60.
RE and SE values near 40 indicate a
low probability of awareness with explicit recall.
> Power spectral analysis (PSA
this is also based on EEG analysis
The raw EEG data undergo
Fourier’s analysis to break it
down into its constituent sine waves.
These are then processed and
displayed graphically.
The ‘power’ wave amplitude
drops as the DOA increases.
There are problems with inter-patient variability.
> Auditory-evoked potentials (AEP)
> Auditory-evoked potentials (AEP) –
this has evolved on the basis
that the auditory sense
disappears last when undergoing anaesthesia.
A series of clicks is delivered into the patient’s
ear as they are anaesthetised.
The resulting EEG is monitored and analysed
to give an AEP index
(AEP > 80 is awake and AEP < 50 is asleep).
Unlike BIS monitoring, the system exhibits
much less hysteresis and
there are defined ‘awake’ and ‘asleep’ points.
It is now being marketed with an
algorithm giving arbitrary values of 0–100.
> Raw EEG
> Raw EEG –
this can be accurate but
needs expert interpretation.
Clinical methods:
> Clinical signs –
in the spontaneously ventilating patient,
movement and the depth and frequency of
respiration are useful indicators of anaesthetic adequacy.
In the paralysed patient these features are
lost and instead
signs of sympathetic stimulation are often used.
The PRST (or Evan’s) scoring system (pressure, rate, sweating and tears)
was developed to provide an objective assessment of sympathetic stimulation.
However, indirect autonomic or
involuntary responses
have all proved to be unreliable signs of consciousness.
These scoring systems are not specific to the
effects of anaesthesia and absence
of sympathetic activity does
not exclude awareness
(in fact, there is good evidence from
large case series that autonomic responses
are uncommon in cases of reported accidental awareness under anaesthesia).
Isolated forearm technique
Isolated forearm technique –
a tourniquet is applied to the
arm and inflated to above
arterial blood pressure before a
neuromuscular blocking
drug is administered through a
vein elsewhere in the body.
Therefore, the muscle relaxant
does not reach the muscles of the arm distal
to the tourniquet and
movement of the hand is preserved.
If the patient is aware,
they will be able to move their
hand to alert the anaesthetist.
Tunstall, an obstetric anaesthetist,
originally described the technique in
1977, but very few anaesthetists actually use this technique today.
> Lower oesophageal contractility
> Lower oesophageal contractility –
a balloon catheter with a distal
pressure transducer is
put in the lower oesophagus.
Provoked oesophageal contractions
are triggered by inflation of the balloon
while spontaneous oesophageal contractions
are triggered by stress
and emotion in an awake patient.
These contractions are recorded by the
pressure transducer, and using an algorithm, the device generates the oesophageal contractility index for that patient.
> End-tidal anaesthetic gas monitoring:
National Audit Project
> End-tidal anaesthetic gas monitoring: National Audit Project (NAP) 5 identified that end-tidal anaesthetic gas monitoring used with
audible alarms appropriately set
was a reliable way of ensuring a
desired concentration of inhalational
agent was given to a patient.
The B-Unaware trial published in 2008
and the BAG-RECALL trial in 2011
compared BIS-guided anaesthesia
with a protocol in which alarms were
used to alert the anaesthetist to keep end-tidal anaesthetic gases at
an age-adjusted MAC >0.7.
These trials found no difference in risk of
awareness between the two groups.
What are some of the limitations
in EEG-based monitoring of
depth of anaesthesia?
There is considerable heterogeneity
and uncertainty between
various studies using EEG-based monitors to determine DOA.
This is mainly due to the individual response to anaesthesia,
diverse case mix and the variation in
administering anaesthesia in clinical practice.
It is also not fully established
how EEG DOA monitors perform
when drugs such as ketamine or nitrous
oxide are used in conjunction with propofol or inhalation anaesthetic agents.
In what other settings might a BIS monitor be used?
On intensive care units, BIS can be used to monitor burst suppression, which is a technique used to reduce cerebral metabolic oxygen requirements in patients with head injuries and raised intracranial pressures or in status epilepticus.
It can also be used to reduce awareness in those paralysed for long time.
