Moments with Morgan and Mickhail - Chapter 6, Part 2

Continue your journey to mastering anaesthesia—one chapter at a time. In this episode, Dr. J.R. Decker reads and discusses Chapter 6 (Part 2) of Morgan & Mikhail’s Clinical Anesthesiology (7th Edition). Follow as you read along to strengthen your foundations in anaesthesia, one clear and engaging session at a time. Perfect for trainees, revision, or daily listening.

Episode Transcript

(00:01):
Morgan and Michael's Clinical Anesthesiology, 7th edition,
Chapter 6, Part 2. Neurological system monitors
electroencephalography indications and

(00:25):
contraindications. The electroencephalogram EEG is
occasionally used during cerebrovascular surgery to
confirm the adequacy of cerebraloxygenation or during
cardiovascular surgery to ensurethat burst suppression or an

(00:52):
isoelectric signal has been obtained before circulatory
arrest. A full 16 lead 8 channel EEG is
not necessary for these tasks, and simpler systems are

(01:15):
available. There are no contraindications,
techniques and complications. The EEG is a recording of
electrical potentials generated by cells in the cerebral cortex.

(01:42):
Although standard EEG electrodescan be used, silver discs
containing a conductive gel are preferred.
Platinum or stainless steel needle electrodes traumatise the
scalp and have high impedance, that is resistance.

(02:07):
However, they can be sterilised and placed in a surgical field.
Electrode position that is montage is governed by the
international 1020 system. Electric potential differences

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between combinations of electrodes are filtered,
amplified and displayed by an oscilloscope or pen recorder.
EEG activity occurs mostly at frequencies between 1 and 30

(02:55):
cycles per second, that is Hertz.
Alpha waves have a frequency of 8 to 13 Hertz and are often
found in a resting adult with the eyes closed.

(03:16):
Beta waves at 8 to 13 Hertz are found in concentrating
individuals and at times in individuals under anaesthesia.
Delta waves have a frequency of 0.5 to 4 Hertz and are found in

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brain injury, seizure disorders,deep sleep and anaesthesia.
Theta waves, that is 4 to 7 Hertz, are also found in
sleeping individuals and during anaesthesia.

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EEG waves are also characterisedby their amplitude, which is
related to their potential, thatis, high amplitude greater than
50 microvolts, medium amplitude 20 to 50 microvolts and low

(04:30):
amplitude less than 20 microvolts.
Lastly, the EEG is examined for symmetry between the left and
right hemispheres. Next we go to Figure 6-5

(04:55):
describing the International 1020 system.
Kindly pause this recording and go through Figure 6.6 hyphen 5.
Examination of a multi channel EEG is at times performed

(05:17):
intraoperatively to detect areasof cerebral ischemia, such as
during carotid end arteryptomy. Likewise, it can be used to
detect EEG isoelectricity and maximal cerebral protection

(05:39):
during hypothermic arrest. The strip chart EEG is
cumbersome in the operating room, and often the EEG is
processed using power spectral analysis.

(06:02):
Frequency analysis divides the EEG into a series of sine waves
at different frequencies and then plots the power of the
signal at each frequency, allowing for representation of
EEG activity in a more easily interpreted form than the raw

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EEG. Next we go to Figure 6-6,
explaining patient states, candidate depth of anaesthesia
devices or approaches, key features of different monitoring

(06:49):
approaches, and possible readings at different depths of
anaesthesia. Kindly pause this recording and
take a while to go through this figure.
As inhalational anaesthesia progressively deepens, initial

(07:14):
beta activation is followed by slowing burst suppression and
isoelectricity. Intravenous agents, depending on
those and drug used, can producea variety of EEG patterns.

(07:37):
Awareness during general anaesthesia remains a vexing
concern for anaesthesia practitioners.
Devices have been developed thatprocess 2 channel EEG signals
and display a dimensionless variable to indicate the level

(08:00):
of wakefulness. The bispectral index BIS is most
commonly used in this regard. BIS monitors examine 4
components within the EEG that are associated with the

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anaesthetic state. 1 Low frequency as found during deep
anaesthesia. 2 High frequency beta activation found during

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light anaesthesia. 3 Suppressed EEG waves and four burst
suppression. Other devices attempt to include

(09:04):
measures of spontaneous muscle activity as influenced by the
activity of subcortical structures not contributing to
the EEG. To provide an assessment of
anaesthetic depth. Various devices, each with its

(09:25):
own algorithm to process the EEGor incorporate other variables
to ascertain patient wakefulnessmay become available in the
future. Next, we move to Table 6-1,
talking about the characteristics of the

(09:47):
commercially available monitors of an aesthetic depth.
It's a big table. Take your time, pause this
recording, and go through this table.
So far, the medical literature provides very limited support

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for the efficacy of these devices in preventing awareness.
Some studies have demonstrated Areduced incidence of awareness
when these devices were used, whereas other studies have
failed to reveal any advantage over the use of end tidal

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inhalational gas measurements toensure an adequate concentration
of volatile anaesthetic because individual EEG responsiveness to
anaesthetic agents and level of surgical stimulus are variable.
EEG monitoring to assess anaesthesia depth or titrate

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anaesthetic delivery may not always ensure the absence of
wakefulness. Moreover, many monitors have a
delay, which might only indicatea risk for the patient being
aware after he or she had already become conscious.

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Next, we move to Table 6-2, talking about the checklist for
preventing awareness. We'll go through them 1 by 1.

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Check all equipment, drugs and dosages.
Ensure that drugs are clearly labelled and that infusions are
running into veins next. Consider administering an
amnesic pre medication next. Avoid or minimise the

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administration of muscle relaxants.
Use a peripheral nerve stimulator to guide minimal
required dose next. Consider using the isolated
forearm technique if intense paralysis is indicated next.

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Choose potent inhalation agents rather than total intravenous
anaesthesia if possible. Next, administer at least 0.5 to
0.7 minimum alveolar concentration that is mark of
the inhalation agent. Next, set an alarm for a low

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anaesthetic gas concentration. Next, monitor anaesthetic gas
concentration during cardio pulmonary bypass from the bypass
machine. Next, consider alternative
treatments for hypotension otherthan decreasing anaesthetic

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concentration. Next, if it is thought that
sufficient anaesthesia cannot beadministered because of concern
about hemodynamic compromise, consider the administration of
benzodiazepines or scopolamine for amnesia.

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Next, supplement hypnotic agentswith analgesic agents such as
opioids or local anaesthetics, which may help decrease the
experience of pain in the event of awareness.
Next, consider using a brain monitor such as a raw or

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processed Electro encephalogram,but do not try to minimise the
anaesthetic dose based on the brain monitor because there is
currently because there currently is insufficient
evidence to support this practise.
Next, monitor the brain routinely if using total

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intravenous anaesthesia. Next, evaluate known risk
factors for awareness and if specific risk factors are
identified, consider increasing the administered anaesthetic
concentration. And finally, redose intravenous

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intravenous anaesthesia when delivery of inhalation
anaesthesia is difficult, such as during a long intubation
attempt or during rigid bronchoscopy.
Clinical considerations. To perform a bispectral

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analysis, data measured by EEG are taken through a number of
steps of steps to calculate a single number that correlates
with the depth of anaesthesia, stroke, hypnosis.

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Next, we move to Figure 6-67, explaining the calculation of
the bispectral index. Kindly pause this recording to
go through this figure. BIS values of 65 to 85 have been

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advocated as a measure of sedation, whereas values 40 to
65 have been recommended for general anaesthesia.
Next we move to Figure 6-8, explaining that the bispectral

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index, that is, BIS versions 3 point O and higher, is a
dimensionless scale from zero, that is complete cortical
electroencephalographic suppression 200, that is a week.
Kindly pause this recording and take your time to go through

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Figure 6-8. Many of the initial studies of
BIS were not prospective randomised controlled trials but
were observational, underpoweredand not masked.

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The monitor costs several $1000 and the single use electrodes
cost approximately $10 to $15 per anaesthetic.
Unfortunately, some patients with awareness have had Abis of

(17:57):
less than 65, calling into question the value of this
measurement. Detection of awareness can often
minimise its consequences. Questions during post operative
visits can identify a potential awareness event.

(18:26):
Ask patients to recall the following What do you remember
before going to sleep? What do you remember right when
awakening? Do you remember anything in
between going to sleep and awakening?

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And did you have any dreams while asleep?
Close follow up and involvement of mental health experts may
avoid the traumatic stress that can be associated with
intraoperative awareness events.Increasingly, patients are

(19:15):
managed with regional anaesthesia and propofol
sedation. Patients undergoing such
anaesthetics should be made aware that they might recall
perioperative events. Clarification of the techniques

(19:36):
used may prevent patients so managed from the belief that
they were awake during anaesthesia.
Those BIS associates with outcomes.
Some investigators have suggested that hospital stay and

(19:57):
mortality are increased in patients experiencing.
The so-called triple low of low mean arterial blood pressure,
low BIS score and low minimum alveolar concentration of
volatile anaesthetist. Other investigations have failed

(20:23):
to identify such an association.Evoked potentials indications
Indications for intraoperative monitoring of evoked potentials,

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that is, EPS include surgical procedures associated with
possible neurological injury, spinal fusion with
instrumentation, spine and spinal cord tumour resection,
brachial plexus repair, thoracoabdominal aortic aneurysm

(21:11):
repair, epilepsy surgery, and insome cases, cerebral tumour
resection. Ischemia in the spinal cord or
cerebral cortex can be detected by EPS Auditory EPS have also

(21:36):
been used to assess the effects of general anaesthesia on the
brain. The middle latency auditory EP
may be a more sensitive indicator than BIS in regard to
anaesthetic depth. The amplitude and latency of

(22:00):
this signal following an auditory stimulus are influenced
by anaesthetics Contra indications.
Although there are no specific contraindications for
somatosensory evoked potentials,that is, SEPS, this modality is

(22:28):
severely limited by the availability of monitoring
sites, equipment and trained personnel.
Sensitivity to anaesthetic agents can also be a limiting
factor, particularly in children.

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Moto evoked potentials, that is,MEPs are contraindicated in
patients with retained intracranial metal skull defects
or implantable devices, as well as after seizures and any major

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cerebral insult. Brain injury secondary to
repetitive stimulation of the cortex and inducement of
seizures is a concern with MEPs techniques and complications.

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EP monitoring noninvasively assesses neural function by
measuring electrophysiological responses to sensory or motor
pathway stimulation. Commonly monitored EPS are brain

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stem auditory evoked responses, that is, BAERSSEPS and
increasingly MEPs. Next, we move to Figure 6-9,
talking about neuroanatomic pathways of somatosensory evoked

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potential and Moto evoked potentials.
Kindly pause this recording to go through Figure 6-9.
For Seps, A brief electrical current is delivered to a

(24:45):
sensory or mixed peripheral nerve by a pair of electrodes.
If the intervening pathway is intact, a nerve action potential
will be transmitted to the contralateral sensory cortex to
produce an EP. This potential can be measured

(25:11):
by cortical surface electrodes, but is usually measured by scalp
electrodes. Multiple responses are averaged
and background noise is eliminated to distinguish the
cortical response to a specific stimulus.

(25:34):
E PS are represented by a plot of voltage versus time.
The resulting waveforms are analysed for their post stimulus
latency, that is, the time between stimulation and
potential detection and peak amplitude.

(25:58):
These are compared with baselinetracings.
Technical and physiological causes of a change in an EP must
be distinguished from changes due to neural damage.
Complications of EP monitoring are rare but include skin

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irritation and pressure ischemiaat the sites of electrode
application. Clinical Considerations EPS are
altered by many variables other than neural damage.

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The effect of his anaesthetics is complex and not easily
summarised. In general.
Intravenous anaesthetic techniques with or without
nitrous oxide can cause minimal changes, whereas volatile

(27:13):
agents, that is, cerval, fluorine, desfluorine and
isofluorine are best avoided or used at a constant low
concentration. Early occurring, that is
specific EPS are less affected by anaesthetics than a late

(27:37):
occurring that is nonspecific responses.
Changes in BAERS may provide a measure of the depth of
anaesthesia physiological. For example, blood pressure,

(27:59):
temperature and oxygen saturation and pharmacological
factors should be kept as constant as possible.
Persistent obliteration of EPS is predictive of post operative

(28:21):
neurological deficit. Although SEPS usually identify
spinal cord damage because of their different anatomic
pathways, sensory that is dorsalspinal cord EP preservation does

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not guarantee normal motor that is ventral spinal cord function.
This is a false negative. Furthermore, Seps elicited from
posterior tibial nerve stimulation cannot distinguish

(29:05):
between peripheral and central ischemia.
That is, false positive Techniques that elicit MEPs by
using transcranial, magnetic or electrical stimulation of the
cortex allow the detection of action potentials in the muscles

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if the neural pathway is intact.The advantage of using MEPs as
opposed to Seps for spinal cord monitoring is that MEPs monitor
the ventral spinal cord and if sensitive and specific enough

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they can be used to indicate which patients might develop a
post operative motor deficit. MEPs are more sensitive to
spinal cord ischemia than Seps. The same considerations for Seps

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are applicable to MEPs in that they are reduced in amplitude by
volatile inhalational agents, high dose benzodiazepines, and
moderate hypothermia, that is, temperatures below 32°C.

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MEPs require monitoring of the level of neuromuscular blockade.
Close communication with a new neurophysiologist or monitoring
technician is essential before the start of any case where

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these monitors are used. These are cases in which inhaled
anaesthetics, if used at all, must be maintained at a constant
end tidal concentration to ensure monitoring reliability.