5. Depth of Anaesthesia Monitoring

Question 1

Methods of Determining Depth of Anaesthesia

Answer 1

Clinical signs

Sympathetic stimulation

Evoked potentials (EPs):

Compressed spectral array

Spectral edge

Median frequency

Respiratory sinus arrhythmia and R–R interval variation

Isolated forearm technique

EEG:

Cerebral function monitor (CFM

Cerebral function analyzing monitor (CFAM

Oesophageal contractility

Frontalis (scalp) electromyogram (EMG):

Question 2

Clinical signs

Answer 2

Clinical signs:

In the spontaneously breathing patient who is not paralyzed,

awareness may be manifest by purposeful movement.

Movement is a reliable indicator of light anaesthesia,

although a patient may have no recall.

Question 3

Sympathetic stimulation:

Answer 3

the main clinical signs are
tachycardia,
hypertension,
diaphoresis
lachrymation.

Attempts have been made to quantify these objectively
by using the PRST scoring system (blood Pressure, heart Rate, Sweating, Tear
formation),
but without any real evidence of its benefit.

In the absence of other causes,
sympathetic signs may be reliable if present,
but the main difficulty is that
their absence does not exclude awareness.

Question 4

Evoked potentials (EPs)

Answer 4

Evoked potentials (EPs):

visual, somatosensory and auditory EPs
have been investigated
as indicators of the depth of anaesthesia.

The few microvolts that are generated by each potential
have to be separated from the

overall electrical noise that is produced
by the brain as a whole.

Auditory EPs appear to be the most effective because they are
the last to disappear and are the best indicators of anaesthetic depth.

The patient’s auditory system is stimulated by repetitive clicks at around 6–10 Hz.

The electroencephalogram (EEG) is recorded immediately after each stimulus and is amplified,

before the auditory EPs are extracted by taking the average of a large number of responses.

This is covered in greater detail in ‘Evoked Potentials’ in the next section.

Question 5

Compressed spectral array

Answer 5

Compressed spectral array:

this is a method of simplifying the EEG in which the signals are subjected to Fourier analysis.

Fourier transformation is the mathematical technique
whereby complex waveforms are analyzed into their simpler sine wave
components.

Spectral analysis calculates the total power contained within the different
frequencies of cerebral activity over a period of time (known as an epoch).

The graph of power against frequency forms a spectral array.

As an anaesthetic continues
or deepens, each linear plot obtained during successive epochs can be superimposed
to give the typical peak and trough, or ‘hill and valley’, display.

This compressed display is what constitutes compressed spectral array. In an anaesthetized patient, power shifts to the lower frequencies

Question 6

Respiratory sinus arrhythmia and R–R interval variation:

Answer 6

Respiratory sinus arrhythmia and R–R interval variation:

this method does have promise, although
it is only useful in the presence of an intact autonomic nervous system
and healthy myocardial conducting system.

Its value is greatly restricted in patients,
for example, who are being treated with β-adrenoceptor blockers, who have
autonomic neuropathy or dysfunction (common in the elderly), sepsis or who have
cardiac conduction abnormalities.

It provides a measure of brain stem function,
which decreases with increasing depth of anaesthesia.

Question 7

EEG

Answer 7

EEG: the formal EEG is a highly complex monitor,

usually with multiple channels,

which is generally regarded as producing too much data
to be of any practical use in theatre.

The raw EEG demonstrates differing patterns in response to different
anaesthetic agents and changes in response to events such as hypoxia and hypercarbia.

It also processes a lot of information from the cerebral cortex, which may not in
fact be the area most appropriate for examining depth of anaesthesia

Question 8

Oesophageal contractility:

Answer 8

Oesophageal contractility: the amplitude and frequency of contractions of lower
oesophageal smooth muscle reduce with increasing depth of anaesthesia. The technique
is of limited value because of the high rate of false positive and false negative results.

Question 9

Frontalis (scalp) electromyogram (EMG

Answer 9

Frontalis (scalp) electromyogram (EMG): this technique measures the amplitude of
the EMG, which decreases with increasing depth of anaesthesia. It is of very
restricted benefit if for no other reason than it cannot be used in paralyzed patients.

Question 10

NICE Recommendations

Answer 10

(NICE Diagnostics Guidance [DG6]. Depth of anaesthesia monitors. 2012).

‘the use of EEG-based depth of anaesthesia
monitors is recommended as an option during any type of general anaesthesia in
patients considered at higher risk of adverse outcomes. This includes patients at
higher risk of unintended awareness and patients at higher risk of excessively deep
anaesthesia. The Bispectral Index (BIS) depth of anaesthesia monitor is therefore
recommended as an option in these patients.

‘although there is greater uncertainty of clinical benefit for the E-Entropy and
Narcotrend-Compact M monitors than for the BIS monitor . . . they are broadly
equivalent.’ The report also recommended depth of anaesthesia monitoring in all
patients receiving total intravenous anaesthesia (TIVA).

Question 11

Bispectral analysis and bispectral index (BIS):

Answer 11

modification of the EEG, in which there is analysis of the phase and power relationships between the numerous frequencies.

is a number generated from these phased and power frequencies that are the components
of the EEG, and in essence compares frequency harmonics in the frontal EEG.

dimensionless scale of 0 (cortical electrical silence) to 100 (normal cortical
electrical activity) has been derived from EEG recordings in volunteers and patients
undergoing transitions between consciousness and unconsciousness

A bispectral
index score of between 40 and 60 suggests surgical anaesthesia. The details of the
calculation algorithm remain a commercial secret. The device has a variable
response time, with a lag that is usually around 25 seconds, but which can extend
for as long as 4 minutes

The BIS
number will reduce if a patient is given muscle relaxants. This is usually attributed
to their effect on the EMG, but it is possible that deafferentation may also contribute,
with some loss of proprioceptive and muscle input through the reticular
activating system. BIS has no predictive value for any particular individual’s
threshold for loss of consciousness.

Question 12

B-Aware

Answer 12

No authoritative body has yet endorsed the routine use of the processed EEG
(pEEG), probably because the evidence base is not sufficiently robust. The B-Aware
randomized controlled trial in 2,463 patients reported an 82% reduced risk of
awareness in a population identified as being at high risk

Question 13

B-unaware

Answer 13

B-unaware study of 2,000 patients showed no
difference between subjects monitored with BIS and with those managed according
to a MAC-based protocol.

Question 14

BAG RECALL

Answer 14

The same primary investigator headed up the
BAG-RECALL trial which compared BIS of 40–60 to age-adjusted MAC values of
0.7–1.3 and in the 6,041 patients found no difference

Question 15

Entropy

Answer 15

E-entropy. In the context of depth of anaesthesia monitoring, entropy is defined as a
measure of irregularity in a signal. In the awake subject the EEG shows very irregular
patterns which become more ordered as anaesthesia deepens. The E-entropy device
is another form of processed EEG, but it also incorporates signals from the frontalis
muscle EMG

Question 16

AAGA

Answer 16

Awareness, or accidental awareness during general anaesthesia (AAGA) to give it its full
title, is an enduring concern both of anaesthetists and patients.

The fifth National Audit project of the Royal College of Anaesthetists and Association of Anaesthetists of Great Britain and Ireland (NAP 5) reported

an overall incidence of awareness of

1 in 19,000 but a much higher incidence in selected groups;

1 in 8,000 patients given neuromuscular blockers and a disconcertingly high
1 in 670 patients undergoing caesarean section.
It therefore remains a problem.

Question 17

Definitions:

Answer 17

Definitions: awareness can be ‘explicit’ or ‘implicit’.

Explicit awareness is defined by spontaneous or prompted recall of intraoperative events,

which may or may not include pain.

Implicit awareness is a less specific concept and is defined by some
degree of cortical perception but without conscious recall

Question 18

Causes of AAGA

Answer 18

Causes: its causes lie in equipment and its (mis)use, in pharmacology and its
application and, very rarely, in the physiology of patients

Question 19

Equipment and apparatus:

Answer 19

Equipment and apparatus:
awareness may result from a failure of the apparatus to
deliver adequate concentrations of anaesthetic agent.

The anaesthetic machine must deliver an accurate fresh gas flow via an appropriate breathing system using a vaporizer.

For patients receiving TIVA, an accurate syringe driver is required,
together with a reliable system of infusion tubing.

Awareness may result if there are failures in any part of these systems.
Such failures include leaks, faulty or empty
vaporizers, a misconnected or disconnected breathing system, inaccurate pumps,
misplaced venous cannulae and occluded infusion tubing.

Question 20

Use of equipment and apparatus:

Answer 20

awareness may result from a failure of the
anaesthetist to use the equipment properly. Circle systems can present a particular
difficulty.

Question 21

Monitoring

Answer 21

: failure to monitor the concentrations of inspired and expired volatile
agents may result in inadequate anaesthetic agent being delivered.

TIVA is more difficult to monitor in this respect because the effect site concentrations are calculated rather than measured directly

Question 22

Pharmacology

Answer 22

: awareness, by definition, results from inadequate anaesthesia. The dose
of induction agent may have been inadequate, as may be the alveolar concentration (it
is important to remember that the MAC value that is quoted is only the MAC50) or the
computed blood concentration in target-controlled infusion (TCI). Awareness is not
prevented by hyperventilation, by the use of nitrous oxide and oxygen alone, nor by the
use of opioids. Muscle relaxant drugs are not anaesthetics, and anaesthesia should not
be discontinued until their effects have been reversed fully.

Question 23

Physiology

Answer 23

: very rarely, a patient may be ‘resistant’ to anaesthetic agents. Alcohol
and other drugs of abuse are convenient scapegoats, but the evidence is unconvincing.
Similarly, high anxiety is frequently cited as the reason that some patients may
need larger than normal induction doses. In any of these situations the anaesthetist should be alert to the clinical signs indicative of inadequate anaesthesia. On occasion,
a patient may be so moribund (or so inadequately resuscitated) that adequate
anaesthesia may be incompatible with maintaining cardiac function.

Question 24

Airway problems and bronchoscopy:

Answer 24

Airway problems and bronchoscopy: during a difficult intubation, the effects of the
induction agent may wear off before those of the muscle relaxant. Awareness is also a
potential problem during some anaesthetic techniques for rigid bronchoscopy (such
as those using insufflation or injectors). Cardiac bypass is another technique in which
the likely incidence of awareness is higher.

Question 25

Sequelae:

Answer 25

Sequelae: it is very unusual to cause physical morbidity as a result of cardiovascular
stresses provoked by being aware, although it is a theoretical possibility. Much more
common are manifestations of a post-traumatic stress disorder (PTSD), whose
typical features may include nightmares, insomnia, panic attacks and agoraphobia.
Patients with PTSD will need considerable support, explanation and counselling.