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Criticare - A special feature on Anaesthesiology
Monitoring depth of anaesthesia
Dr N Batra
Anaesthesia can be defined as a lack of response and recall to noxious stimuli.
It includes the triad: i) Paralysis ii) Unconscious iii) Analysis. Different
surgical operations require different proportions of the three components. In
the early days of anaesthesia, when single agents were used, all the components
of the triad had to be obtained from a single drug. The proportions of the different
components were very difficult to titrate and hence incidences of postoperative
complications were high. Modern anaesthetic agents allow the proportion of the
three components of the triad to be more easily adjusted to individual requirements
with a corresponding improvement in the patients operative and post-operative
well-being.
Accuracy in anaesthesia depends on:
- Equilibration of drug concentration is plasma with
the effective site.
- Influence of noxious stimuli. Attempts have been
made to measure the depth of awareness and reduce them. Five stages of awareness
are recognised as:
- Conscious awareness with spontaneous or prompted
recall (Explicit recall)
- Conscious awareness with amnesia
- Dreaming
- Unconscious awareness with amnesia
- No awareness or recall.
Awareness during anaesthesia is one of the anaesthetic complication patient
fears the most. Although there is no evidence that monitors the depth of unconsciousness
prevents awareness. It is conceivable that by maintaining the sufficient depth
of anaesthesia, this can be achieved.
Methods of monitoring depth of anaesthesia
No single method has been found to measure depth of anaesthesia reliable for
all anaesthetic agents. Subjective methods depend on the interpretation of autonomic
response to noxious stimuli and depend on the opinion and expertise of the anaesthetist.
Objective methods depend on the sensibility of the monitoring device.
Techniques
1. Autonomic response (historic methods) n To monitor heart rate
- To measure blood pressure n Measure sweating/ lacrimation
- Pupils. However, it has several limitations as drug
may interfere with the signs.
2. Skin conductance to change in sweat production.
3. Isolated forearm technique to left arm without paralysis by applying technique.
4. Evoked potential processed EEG. The EEG detects voltages of 1-500. It comprises
L, B, Q & S waves with using depths of anaesthesia. There is a progressive
increase in signal amplitude and a reduced frequency (burst suppression). Overall,
anaesthesia results in a shift in the principal frequencies of the EEG waves
to the lower end of the spectrum.
Changes are not consistent for all the anaesthetic agents. a) Bispectral Index
is a statistically based, especially derived complex parameter composed of the
complication of time domain, frequency domain and high order spectral sets parameters.
It is presented as a numerical index, ranging from 100 to 0. A number less than
60 implies that the patient is almost certainly unconscious.
b) Somato Sensory Evoked Potentials (SSEP) and auditory Evoked potentials (AEP)
progressively using the concentration of anaesthetic drugs results in delay
in the transmission of the waveforms and a decrease in amplitude.
c) Waves Oesophageal contractility: This is unaffected by neuro muscular blockage
as it consists of smooth muscle. Peristasis is associated with primary and secondary
muscle activity, but tertiary activity has an unknown function and increase
with stress. Anaesthetic agent suppresses this activity in a dose related manner.
But there is a wide inter-subject variability and responses differ with the
type of anaesthetic agent. This technique is obsolete.
The writer is head of department and senior consultant anaesthetist
Escorts Hospital & Research Centre, Faridabad
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