Monitoring depth of anesthesia

MONITORING DEPTH OF
ANESTHESIA
- Dr. Richa Kumar

• One of the objectives of modern anaesthesia is to ensure adequate depth of
anaesthesia
• To prevent awareness without inadvertently overloading the patients with potent
drugs.
• Various methods have been described to measure the depth of anaesthesia from
time to time.

• In 1957, Woodbridge defined anaesthesia as having four components; sensory
blockade, motor blockade, blockade of autonomic reflexes and loss of
consciousness.
John Snow in 1847,
Described five degrees of
narcotism for ether
anaesthesia.
These were later refined by Guedel into
four stages on the basis of somatic muscle
tone, respiratory parameters and ocular
signs.
In 1954, Artusio divided
Guedel’s stage I into 3
planes.

• According to Prys-Roberts common feature of general
anaesthesia is suppression of conscious perception of
noxious stimuli.
• He divided the noxious stimuli into somatic and autonomic components,
which were further, divided into sensory, motor and respiratory,
haemodynamic , pseudomotor and hormonal
• Analgesia, autonomic stability and muscle relaxation are desirable but not actual
components of anaesthesia.

• General anaesthesia is a state of drug-induced, reversible loss of consciousness.
• It includes the time of induction of anaesthesia to emergence during which
patients will not be conscious of their surgery or their surroundings.
• Unintended awareness (‘awareness’) occurs when general anaesthesia has failed.
• Awareness is unpleasant, feared by patients, and is a cause of psychological harm
to both the patient and the anaesthetist

RECALL.
• It is the patient’s ability to retrieve stored memories
• Assessed by a patient’s report of previous events, in particular, events that occurred
during general anaesthesia.
• Recall can be either explicit or implicit.
• Explicit memory: It refers to intentional or conscious recollection of prior experiences as
assessed so called direct memory test.
• Implicit memory (perception without conscious recall): The patient denies recall, but may
remember “something” under hypnosis.
DEFINITIONS

AWARENESS
• Generally considered to have occurred when, after completion of anaesthesia, an
individual has explicit recall of intraoperative events with or without pain.
• Detection of awareness is necessarily retrospective and dependent on patient
recall;
• There may be confusion as to when in the perioperative period the event being
recalled took place.

• Only 50% of affected patients report awareness immediately after operation, and
recall may be reported up to a month after anaesthesia.
• Particular difficulties arise in differentiating memories from dreams.
• Detection can be facilitated by a structured interview, and the one most
commonly used in both research and clinical practice is based on that by Brice

• The problem of awareness first became clinically important during the 1940s after the
introduction of curare, and the practice of ‘balanced’ anaesthesia.
• With administration of neuromuscular blocking agents, it was possible for patients to be
conscious but unable to respond to environmental stimuli
• Despite continued advances in anaesthesia delivery and monitoring over 50 years,
neuromuscular blocking (NMB) drugs remain the single most frequently implicated risk
factor for awareness

AMNESIA.
• Amnesia is the absence of recall
• Many anaesthetic drugs produce amnesia at concentrations well below those necessary
for suppression of consciousness
• Anterograde amnesia is intended when a drug with amnestic properties is administered
before induction of anaesthesia
• Retrograde amnesia is intended when a drug such as a benzodiazepine is administered
after an event that may have caused or been associated with intraoperative
consciousness in the hope that it will suppress memory formation and “rescue” from
recall.

DEPTH OF ANAESTHESIA
• Depth of anaesthesia or depth of hypnosis refers to a continuum of progressive
central nervous system depression and decreased responsiveness to stimulation.

MONITORING DEPTH OF ANESTHESIA
METHODS OF MONITORING:
• Clinical techniques and conventional monitoring
• Pharamcological principles
• Brain electrical activity monitoring
• Spontaneous EEG activity monitoring
• Evoked brain electrical activity monitoring

A. CLINICAL TECHNIQUES AND
CONVENTIONAL MONITORING

I. Clinical signs:
• The most commonly used scoring system incorporates the PRST or Evan’s score
• This assesses autonomic activity related to P (systolic blood Pressure), R (heart
Rate), S (Sweating) and T (Tears).
• Advantage of being simple and not requiring any specialized equipment
• D/A is parameters are not specific for the effects of anaesthesia and the values
can vary widely among individuals.
• The scores range from 0 to 8 but the midpoint is seldom exceeded, reflecting the
inadequacy of this scoring system.

• Measurement of heart rate and blood pressure & regularly assessing pupil size, and the
presence of sweating and lacrimation provides useful information regarding the
adequacy of analgesia and depth of anaesthesia
• Tachycardia secondary to anticholinergic drugs such as atropine make the heart rate
uninterpretable
• Beta -adrenergic blocking drugs , opiates and regional anaesthetic techniques will obtund
the sympathetic nervous system response to pain
• Conventional monitoring systems (e.g., electrocardiogram, blood pressure, HR, end-tidal
anaesthetic analyzer, capnography) are valuable and should be used to help assess
intraoperative consciousness

II. Skin conductance:
• is the phenomenon that the skin momentarily becomes a better conductor of
electricity when either external or internal stimuli occur that are physiologically
arousing.
• It is a quantification of the clinical sign of sweat production.
• Skin conductance is initially low and is increased as anaesthetic depth increases,
reducing again with surgical incision.
• Other factors affecting sweating (e.g. atropine, autonomic neuropathy) can
reduce the accuracy of this monitoring.

III. Isolated forearm technique:
• A tourniquet is applied to the patient’s upper arm, inflated above systolic blood pressure before
the administration of muscle relaxants.
• Movement of the arm either spontaneously or to command indicated wakefulness, although not
necessarily explicit awareness.
• It has been used previously as a means of detecting awareness during caesarean section under
general anaesthesia and during clinical trials assessing rates of awareness.
• Not all patients responding have recall.
• One limitation of this technique is the limited time available before patients are unable to move
their arm due to tourniquet induced ischaemia

IV. SPONTANEOUS SURFACE ELECTROMYOGRAM (SEMG)
• In patients who are not completely paralyzed, spontaneous surface electromyogram
(SEMG) can be recorded from various muscle groups, especially facial, abdominal and
neck muscles.
• Frontalis muscle is innervated by a branch of the facial nerve and is less affected by the
neuromuscular blockade.
• A stick on electrode positioned over the frontalis muscle can record the frontalis
electromyogram (FEMG).
• The level of FEMG has been observed to fall during anaesthesia and to rise to pre-
anaesthetic levels just before awakening

V. Lower oesophageal contractility (LOC)
The non-striated muscles in the lower half of oesophagus retain their potential activity even after
full skeletal muscle paralysis by neuromuscular blocking agents.
SPONTANEOUS LOC PROVOKED LOC
It arises spontaneously and can be
detected by a pressure transducer..
These result from sudden distension of the
oesophagus, as if due to the arrival of a
food bolus.
It can be induced by emotion and stress
in the awake individual.
PLOC are induced by the rapid inflation of a
balloon catheter in the lower oesophagus.
This causes smooth muscle contraction and
is detected by a more distally placed
pressure transducer.
It is believed to be in control of a central
oesophageal motility centre
The dose-response curve for PLOC is
shallower than that of SLOC.
the activity of which is influenced by
higher centres
Reduce in latency and amplitude during
general anaesthesia.
reduce in latency and amplitude during
general anaesthesia.

VI. HEART RATE VARIABILITY
• Anaesthetic agents either directly or indirectly first acts on the brain stem and
then (probably) inhibit the cerebral cortex via ascending efferent projections from
the midbrain.
• Therefore, objective measurement of brain stem-mediated autonomic tone that
is not affected by any factor other than anaesthetic depth may be a good
indicator of depth of anaesthesia.
• .

• The special analysis of HRV revealed 3 components:
1. Low frequency fluctuations; believed to be circadian.
2. Medium frequency fluctuations; attributed to baroreceptor reflex.
3. High frequency fluctuations HRV coincides with the frequency of ventilation, in which
heart rate increases during inspiration and decreases during expiration, through a
predominantly parasympathetic reflex connecting stretch receptors in the lungs and aorta
to vagal motor neurons innervating the heart.
• This is called as respiratory sinus arrhythmia (RSA). It is typically characterized by
greater than 10% variation in the ECG P-wave interval over 5 minutes

• Surgical stimulation during light anaesthesia elicits a greater increase on RSA than seen
during lightening anaesthesia alone.
• Some monitors use HRV at respiratory frequency or respiratory sinus arrhythmia (RSA) as
a method of assessing anaesthetic depth.
• This is useful, but depends on an intact autonomic nervous system and healthy
myocardial conducting system.
• Beta-blockers, conduction abnormalities, autonomic neuropathy and sepsis all cause
problems.
• The ‘Fathom’ (Amtec Medical Limited) is based on the use of HRV, and does not use
cortical activity directly but depends on the influence of respiration on the brain stem
and the resulting change in heart rate.

B. PHARMACOLOGICAL PRINCIPLES
OF MEASURING DEPTH OF
ANAESTHESIA

• Depends primarily on the following factors:
1. The equilibration of the drug’s concentration in plasma with the
concentration of the drug at its site of action and with the measured drug
effect.
2. The relationship between drug concentration and drug effect
3. The influence of noxious stimuli.

INHALATIONAL AGENTS
• The purposeful movement of any part of the body in response to noxious perioperative stimuli is
one of the most useful clinical sign of depth of anaesthesia.
• Minimum alveolar concentration (MAC) of inhaled anaesthetics as the concentration required to
prevent 50% of subjects from responding to painful stimuli.
• MAC-intubation: that would inhibit movement and coughing during endotracheal
intubation.
• MAC-incision: To prevent movement during initial surgical incision.
• MAC-BAR: To prevent adrenergic response to skin incision, as measured by the venous
concentration of catecholamine.
• MAC-awake: that would allow opening of the eyes on verbal command during emergence
from anaesthesia.

• The MAC curves (representing the relationship between the concentration of anaesthetic agent
and the probability of response) are located from left to right in the order: MAC-awake < MAC-
incision < MACintubation < MAC-BAR
• In contrast to somatic reflexes, haemodynamic responses to noxious stimuli do not correlate well
with end-tidal drug concentration. Consequently the relationship between somatic (movement)
and autonomic (haemodynamic) responses is poor during inhalational anaesthesia

C. BRAIN ELECTRICAL
ACTIVITY MONITORING

• Brain electrical activity for the purpose of assessing anaesthetic effect is monitored by EEG
activity from electrodes placed on the forehead.
• Systems can be subdivided into those that process spontaneous EEG and EMG activity
and those that acquire evoked responses to auditory stimuli i.e. auditory evoked potential
(AEPs).
• From raw EEG or AEP analog signal is amplified and converted into digital domain 
various alogorithms are applied to the frequency, amplitude, latency, and/or phase
relationship data and a single number is generated referred to as an “index,” typically
scaled between 0 and 100.
• This index represents the progression of clinical states of consciousness (“awake,”
“sedated,” “light anaesthesia,” “deep anaesthesia”)

• Value of 100 is associated with the awake state and values of 0 occurring with an
isoelectric EEG (or absent middle latency AEP).
• EMG activity from scalp muscles can be considered an artifact from the viewpoint
of pure EEG analysis, it may be an important source of clinically relevant
information.
• Sudden appearance of frontal (forehead) EMG activity suggests somatic response
to noxious stimulation resulting from inadequate analgesia and may give warning
of impending arousal.
• For this reason, some monitors separately provide information on the level of
EMG activity.

1. Spontaneous EEG activity monitors:

(i) EEG:
• An EEG can be obtained using the standard 19-electrode method
• Time-consuming and impractical and requires expert interpretation.
• In its unprocessed form, it is not a practical tool for monitoring depth of anaesthesia.
• Increasingly sophisticated, automated analysis of various EEG components has generated several
potential quantitative descriptors of anaesthetic depth.
• There are two generic problems with processed EEG technologies:
1. Dissimilar anaesthetic agents generate different EEG patterns or signatures and
2. Various pathophysiological events also affect the EEG (e.g. hypotension, hypoxia,
hypercarbia).
• Such events may modify both the patient’s level of consciousness and the expected EEG signature
that any given anaesthetic agent generates, thus confounding interpretation

(ii) Compressed spectral analysis:
• The compressed spectral array (CSA) is obtained by superimposing linear plots of
successive epochs of time on each other
• Generates a three-dimensional ‘hill and valley’ display of the power amplitude
vertically (y-axis), frequency horizontally (x-axis) and time (z-axis).
• Anaesthesia causes a reduction in high-frequency and an increase in low-
frequency amplitudes, which is easier to interpret than raw EEG.

• However the problems of patient- and agent variability and the confounding
effects of other pathophysiological processes such as hypoxia, hypotension and
hypercarbia remain
• It is nota reliable monitor of the depth of anaesthesia, but can provide a trend for
use in conjunction with clinical observations.

(iii) Cerebral function monitor (CFM):
• This device is modified from the conventional EEG
• It uses a single biparietal or bitemporal lead (three wires) to obtain an EEG
signal.
• This signal is filtered, semi-logarithmically compressed, and rectified.
• The output is displayed at a very slow chart speed, 1 mm/minute, giving a
trace

• As a result of this processing, the output is presentation of the overall
electrocortical background activity of the brain.
• A high reading on the chart indicates a high level of activity. A low
value indicates low activity. It has been used in cardiac, neuro- and
vascular surgery, where trends in activity may reflect changes in
cerebral perfusion.

(iv) Bispectral Index:
• BIS is a proprietary algorithm that converts a single channel of frontal EEG into an
index of hypnotic level (BIS).
• It is a statistically based, empirically derived complex parameter
• Composed of a combination of time domain, frequency domain and high order
spectral sub parameters.
• It is unique as it integrates several disparate descriptors of EEG into a single
variable, based on a large volume of clinical data to synthesize a combination that
correlates behavioral assessments of sedation

• Insensitive to the specific anaesthetic or sedative agent chosen.
• It is a numerical index, ranging from 100 (awake) to 0 (isoelectric EEG).
• Correlates well with the level of the responsiveness and provide an excellent prediction
of the level of consciousness with propofol, midazolam and isoflurane anaesthesia.
• Also correlates with the haemodynamic response to intubation, patient’s response to skin
incision and verbal command during inhalational as well as total intravenous anaesthesia.

• It reduces the cost by saving anaesthetic use and stay in PACU and provides a
useful guide for titration of anaesthetic agents in cardiac surgery, elderly and
paediatric patients
• Senile dementia may be a confounding factor in interpretation of BIS value. In
some instances BIS has been observed to increase with the use of N2O and
ketamine
• Routine intraoperative events (e.g., administration of depolarizing muscle
relaxants, activation of electromagnetic equipment or devices, patient warming or
planned hypothermia) may interfere with BIS functioning and patient experiencing
awareness despite monitored values indicating adequate depth of anaesthesia

(v)Entropy:
• Based on acquisition and processing of raw EEG and FEMG signals by using the
Entropy algorithm.
• Entropy describes the irregularity, complexity, or unpredictability characteristics
of a signal.
• Entropy of the signal drops when a patient falls asleep and increase again when
the patient wakes up.
• FEMG quiets down as the deeper parts of the brain are increasingly saturated
with anaesthetics.

• Deeper the level of consciousness the patterns are more regular and
ordered
• Correlates with level of consciousness
• Paradoxical high readings with N2O or Ketamine

• Entropy measurement is an objective monitoring and is of two types-
Response Entropy (RE) and State Entropy (SE) indicating analgesic and
hypnotic levels during general anaesthesia.

(vi) Narcotrend®:
• The Narcotrend is an EEG monitor designed to measure the depth of anaesthesia.
• The newest Narcotrend software version includes index from 100 (awake) to 0
(electrical silence).
• Two commercially available electrodes are placed on the forehead of the patient;
a third electrode serves as a reference
• After artifact analysis a multivariate statistical algorithm transforms the raw EEG
data finally resulting in a 6-letter classification of the depth of anaesthesia.

• A(awake), B(sedated), C(light anaesthesia), D (general anaesthesia),E (general
anaesthesia with deep hypnosis),F (general anaesthesia with increasing burst
suppression).
• Sub-classifications resulting in a total 14possible sub-stages :A, B0–2,C0–2, D0– 2,
E0–1, and F0–131. In the most recent version

(vii) Cerebral State Monitor/Cerebral State Index (CSI):
• It is a handheld device that analyses a single channel EEG and presents a CSI scaled from
0to100.
• It also provides EEG suppression percentage and a measure of EMG activity (75–85 Hz).
• The EEG waveform is derived from the signal recorded between the frontal and mastoid
electrodes.
• The frequency content is 2-35Hz.
• The performance of the CSI is based on the analysis of the frequency content of the EEG
signal.
• The energy of the EEG is evaluated in specific frequency band called alpha and beta

• The monitor also on-line evaluates the amount of instantaneous burst
suppression(BS) in each thirty-second period of the EEG.
• The CSI is a unit-less scale from 0 to 100, where 0 indicates flat EEG and 100
indicates EEG activity corresponding to the awake state.
• The range of adequate anaesthesia is designed to be between 40 and 60.
• All values in the table are approximate values based on the mean values of the
patient behaviour.

2. EVOKED BRAIN ELECTRICAL
ACTIVITY MONITORING

I. SSEP
• A supramaximal stimulus is applied to peripheral nerves while a recording scalp
electrode is placed over the appropriate sensory area.
• Most anaesthetic agents increase the latency and decrease the amplitude in
dose-dependent manner.
• Etomidate consistently increases the amplitude.

II. Visual evoked potentials (VEP):
• Light-emitting diodes are incorporated into specialized goggles and the optic
nerve is stimulated at 2 Hz.
• EEG electrodes take recordings from the occiput
• Most anaesthetic agents increase the latency and decrease the amplitude in
dose-dependent manner.
• VEP are considered less reliable than AEP
• They have been used to monitor function during surgery for lesions involving the
pituitary gland, optic nerve and chiasma

III Auditory evoked potential(AEP):
• Defined as the passage of electrical activity from the cochlea to the cortex,
which produces a waveform consisting of 15 waves
• The waveform can be divided into three parts:
1. Brainstem Auditory Evoked Potential (BAEP)
2. Middle Latency Auditory Evoked Potential (MLAEP) and
3. Long Latency Auditory Evoked Potential (LLAEP).
• These parts indicate the sites in the brain from which the various waves are
thought to originate

• The brainstem response is relatively insensitive to anaesthetics
• Early cortical responses (MLAEPs) change predictably with increasing
concentrations of both volatile and intravenous anaesthetics.
• There is increased latency and decreased amplitude of the various waveform
components with anesthetic agents in dose dependent manner

Monitoring depth of anesthesia

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