An electroencephalogram (EEG) can be obtained using the standard 19-electrode method; however, this is 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.
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.
[i] Prediction of depth of sedation and anaesthesia by the NarcotrendTM EEG monitor. Br J Anaesth 2004; 92: 841-5.
Cerebral function monitor (CFM)This device is modified from the conventional EEG for use during anaesthesia.
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 seen in the accompanying examples. As a result of this processing, the output is no longer a regular EEG signal but is, rather, a representation 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.
The CFM has been used to monitor anaesthetic depth, but interest has fallen for several reasons. It can be unreliable, especially when using inhalational anaesthetic agents, and the response to increasing depth of anaesthesia is biphasic, complicating dose-response interpretation. Values similar to those seen in awake patients may be seen in anaesthetised individuals, while recovery from anaesthesia does not necessarily occur near baseline values. Additionally, burst suppression at deep levels of anaesthesia is characterised on the EEG by periods of normal or high-voltage activity alternating with periods of low or no activity. As the CFM provides a smoothed running average of the EEG voltage, early burst suppression artificially elevates the reading, producing an apparent, paradoxical rise in ‘cerebral function’.
[i] Non-expert use of the cerebral function monitor for neonatal seizure detection. Arch Dis Child Fetal Neonatal Ed 2004; 89: F37-40
[ii] Experience with the cerebral function monitor during deliberate hypotension. Br J Anaesth 1981; 53: 639-45
Cerebral function analysis monitor (CFAM)
This device produces a continuous display of an analysed EEG signal from two symmetrical pairs of scalp electrodes. The top trace displayed shows the mean amplitude of the signal plotted in time (90% confidence interval), while the bottom trace shows the power amplitude in the frequency band. Thus, at any instant the CFAM display shows the overall mean amplitude and relative power in each frequency band (a, ß, ? and d). It is said to be more useful than the CFM, but suffers from the same drawbacks.
In one study, five patients were anaesthetised with thiopentone, and nitrous oxide and halothane in oxygen. During maintenance of anaesthesia, there was a gradual decrease in EEG amplitude and shift towards lower frequency EEG activity. Discontinuation of nitrous oxide resulted in a marked increase in EEG amplitude and an increase in a- and ß-band activity. Discontinuation of halothane resulted in smaller alterations in the CFAM trace.
[i] Changes in cerebral electrical activity measured by the Cerebral Function Analysing Monitor following bolus injections of thiopentone. Br J Anaesth 1984; 56: 1075-81
[ii] The cerebral function analysing monitor (CFAM). A new microprocessor-based device for the on-line analysis of the EEG and evoked potentials. Br J Anaesth 1983; 55: 1265-70