|Anaesthesia can be defined as a lack of response and recall to noxious stimuli. It includes the triad of paralysis, unconsciousness and analgesia. Depth of anaesthesia is difficult to measure accurately; it depends on:
Attempts have been made to measure the depth of anaesthesia and reduce the incidence of all causes of awareness. Griffith and Jones recognise five stages of awareness:
- Equilibration of drug concentrations in the plasma with drug concentrations at the effect site, and hence the measured drug effect;
- The relationship between drug concentration and drug effects;
- The influence of noxious stimuli.
Awareness during anaesthesia is one of the anaesthetic complications that patients fear the most. The incidence of awareness with explicit recall in non-cardiac and non-obstetric surgery is 0.07–0.2%, but in emergency Caesarean sections, under general anaesthesia, the incidence increases to 0.4% and for cardiac surgery the incidence is 1.1-1.5%. Although there is no evidence that monitoring the depth of unconsciousness prevents awareness, it is conceivable that by maintaining a sufficient depth of anaesthesia, this will be achieved.
- Conscious awareness with spontaneous or prompted recall (explicit recall);
- Conscious awareness with amnesia;
- Unconscious awareness with amnesia (implicit recall);
- No awareness or recall.
Methods for monitoring depth of anaesthesia
No single method has been found to measure the depth of anaesthesia reliably for all anaesthetic agents. There are subjective and objective methods of assessing the depth of anaesthesia (Figure 1). Subjective methods depend on the interpretation of autonomic responses to noxious stimuli and depend on the opinion and experience of the anaesthetist. Objective methods depend on the sensitivity of the monitoring device.
Clinical assessment uses autonomic signs such as pulse, blood pressure, sweating and lacrimation to predict anaesthetic depth. However, autonomic functions are not affected by depth of anaesthesia alone, and patients with autonomic neuropathy will not react as predicted. Clinical assessment is the most common method of assessing anaesthetic depth. However, it has several limitations: it is subjective and operator dependent; other drugs may interfere with the signs (e.g. atropine causes tachycardia and pupillary dilatation); signs of light anaesthesia are relatively common but correlation with awareness is poor.
Skin conductance: measurement of skin conductance quantifies sweat production. This is a modified method of measuring autonomic responses. This technique is of historical interest only.
Lower oesophageal contractility: the lower portion of the oesophagus consists of smooth muscle and is unaffected by neuromuscular blockers. Peristalsis is associated with primary and secondary muscle activity, but tertiary activity has an unknown function and increases with stress. Anaesthetic agents suppress this activity in a dose-related manner. Spontaneous activity disappears at about 2 minimum alveolar concentrations (MAC) for volatile agents, but there is wide inter-subject variability and the response differs with the type of anaesthetic agent, limiting its usefulness as a monitor. This technique is obsolete.
Isolated forearm technique: placing a cuff round the upper arm and inflating it before neuromuscular blockers are administered leaves the arm unparalysed, allowing the patient to communicate, via arm movements, if they are aware. This technique has many limitations. Up to 75% of patients may respond to command but have no recall. Up to 50% of patients may move an unparalysed arm randomly and this movement may coincide with a command from the experimenter. The technique is limited to 20 minutes of surgical time and the tourniquet must be deflated at regular intervals to prevent ischaemic damage or nerve injury.
EEG, evoked potentials and heart rate variability
The EEG detects voltages of 1–500 µV. It comprises a, ß, ?, and d waves. With increasing depth of anaesthesia, there is a progressive increase in signal amplitude and a reduced frequency (burst suppression). The raw EEG is of little importance in determining the depth of anaesthesia because of difficulty in interpreting the information obtained. Processed EEG is more useful. Fourier analysis is used to separate the raw EEG into a number of component sine waves. The derived parameters, spectral edge frequency and median frequency, describe the entire EEG as a single value. Spectral edge frequency is a single value representing the frequency below which 95% of the total power is present. The median frequency is the point at which 50% of the power lies above and below this value. Median frequency has been shown to control closed loop feedback of intravenous drug administration, but no studies have demonstrated the usefulness of spectral edge frequency. General 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 anaesthetic agents.
Bispectral index (BIS) 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 a proprietary signal-processing technique combining EEG, electromyography (EMG) and a previously collected database of patterns corresponding to the movement of recovery of response to verbal command. It is presented as a numerical index, ranging from 100 (awake) to 0 (isoelectric EEG). A number less than 60 implies that the patient is almost certainly unconscious. Most of the studies have been conducted using propofol or one of the halogenated ethers. These agents probably act via GABA-ergic effects. When nitrous oxide, ketamine or xenon is used, there is little change in BIS, even when the patient is anaesthetised. Measured plasma concentrations of propofol or etomidate are poorly correlated with BIS.
BIS monitors (Figure 2) are used to guide the titration of sedatives, analgesics and anaesthetic agents and have been shown to reduce anaesthetic agent doses, costs and recovery time, although they have little effect on hospital discharge time. There is no evidence that use of the monitor reduces the incidence of awareness. The incidence of awareness is 1/600 anaesthetics, and prospective studies to address this question would be large and costly. Interpretation of the BIS value is difficult if the patient has dementia.
Evoked potentials: somatosensory evoked potentials (SSEP) and auditory evoked potentials (AEP) have been used to monitor the depth of anaesthesia. Progressively increasing the concentration of anaesthetic drugs results in a delay in the transmission of the waveforms and a decrease in amplitude. The Alaris is the first commercially available AEP monitor designed to measure the depth of anaesthesia. It generates an index called Alaris AEP Index (AAI), which is scaled from 100 (awake) to 0. Initial data show some correlation between AAI and BIS during propofol-remifentanil anaesthesia, but there is wide variation in the awake values and also overlap of AAI readings between unconsciousness and consciousness.
Heart rate variability: other systems have been developed that correlate anaesthetic depth with various aspects of the ECG. At present, none is in routine use.
Liu WHD, Thorp TAS, Graham GSG et al. Incidence of awareness with recall during general anaesthesia. Anaesthesia 1999; 46: 435-7
Rampil IJ. A primer for EEG signal processing in anaesthesia. Anesthesiology 1998; 89: 980-1002
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