2. INTRODUCTION
■ 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.
■ General anaesthesia is a state of drug-induced, reversible loss of
consciousness.
■ Depth of anaesthesia or depth of hypnosis refers to a continuum of
progressive central nervous system depression and decreased
responsiveness to stimulation.
3. CLINICAL SIGNS:
Signs of light anaesthesia are –
1. Tachycardia, Hypertension
2. Lacrimation, Perspiration
3. Movement response to painful stimuli
4. Tachypnea, Breath-holding, Coughing, Laryngospasm, Bronchospasm
5. Eye movements
4. MONITORS:
ELECTOENCEPHALOGRAPHY (EEG) –
■ An EEG can be obtained using the standard 19-electrode method.
■ Time consuming and impractical and requires expert interpretation.
■ It is unprocessed form, it is not a practical tool for monitoring depth of
anaesthesia.
5. ■ There are two generic problems with processed EEG technologies:
1. Dissimilar anaesthetic agents generate different EEG patterns.
2. Various pathophysiological events also affect the EEG
(eg: hypotension, hypoxia, hypercarbia).
■ Due to technical infeasibility, difficult interpretation, artifacts, EEG
cannot be employed routinely to monitor the depth of anaesthesia.
However, there are EEG based monitors which can be utilized to
monitor the depth of anaesthesia.
6. EEG–BASED MONITORS
BISPECTRAL INDEX (BIS) MONITOR:
■ It was the first scientifically validated and commercially available
monitor to monitor the depth of anaesthesia.
■ It analyzes multiple facets of real-time EEG to generate a score by set
algorithm.
■ It exhibits a score of 100 for fully awake state and 0 for completely
silent brain.
■ BIS score of 45 – 60 indicates adequate depth.
7.
8. ENTROPY:
■ Entropy describes the irregularity, complexity, or unpredictability
characteristics of a signal.
■ Entropy of a signal drops when a patient falls asleep and increases
again when the patient wakes up.
■ Deeper the level of consciousness the patterns are more regular and
ordered.
As ketamine, nitrous oxide and dexmedetomidine do not causes
depression of EEG therefore EEG-based monitors cannot be utilized
to monitor the depth of anaesthesia with these agents.
9. ■ 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.
10. PATIENT STATE INDEX (PSI) :
■ It is same like BIS index , however a score of 25 -50 indicates adequate
depth.
■ The patient state index (PSI) is a clinically validated measure of the
effect of anaesthesia and sedation.
■ The PSI is calculated via a proprietary algorithm by a high-resolution 4-
channel electroencephalograph (EEG) monitor after advanced artifact
rejection.
■ The PSI has been designed specifically for intra-operative and intensive
care use to monitor patient sedation and drug effect.
11. NARCOTREND:
■ The narcotrend is an EEG monitor designed to
measure the depth of anaesthesia.
■ It analyzes EEG to give 6 stages, from A to F .
A - represents awake state
B - sedated
C - light anaesthesia
D - general anaesthesia
E - general anaesthesia with deep hypnosis
F - represents general anaesthesia with
increasing burst suppression
12. MONITORING NOCICEPTIN:
Till date, we do not have a monitor which can measure the intensity of
pain during general anaesthesia and we have to rely on clinical signs
such as hypertension and tachycardia.
AUDITORY-EVOKED RESPONSE:
Although difficult to monitor but it is considered as reliable as BIS index
to monitor the depth of anaesthesia.
