Monitoring Depth of Anaesthesia

There are, and have been, many monitors designed to monitor depth of anaesthesia and to give an indication of awareness during surgery, which use electrical signals obtained from the human body. Some have been designed as just research devices, some have been available commercially, but have been withdrawn, and some are still available. Most, but not all, are based on the spontaneous EEG and the AER. Some have been designed to use properties of the ECG. Although useful, all of the discussed monitors have some shortcomings, and not all are 100% sensitive and specific to discriminate between consciousness and unconsciousness, and none correlate exactly with clinical states and levels of anaesthesia. The design of the commercial monitor, the Cerebral Function Monitor (CFM) was based on simple time domain measures already discussed [Maynard et al. 1969]. The CFM took the EEG from a single pair of parietal electrodes. The signal was amplified and passed through a band-pass filter and differentiator, which had the effect of accentuating the gain of the higher end of the 2–15 Hz pass band. The output of this specialised filter was integrated to produce a voltage output, which varied with time. It was plotted on a logarithmic scale. The trace on the paper gave an indication of the power of the EEG and the width of the line gave an indication of the signal’s variability. A schematic of an example of a CFM trace is shown in Figure 19.1(a). The CFM although useful did have its problems [Sechzer 1977]. When used to monitor depth of anaesthesia, the machine was shown to be unreliable, especially when using inhalational agents. The response is biphasic, as has already been discussed in chapter 18. Also burst suppression, as already discussed, is smoothed out by the action of the filtering in the CFM, so effectively the burst suppression can artificially elevate the readings producing a paradoxical rise in cerebral function [Sinha 2007] The machine was further developed into the Cerebral Function Analysing Monitor (CFAM)[Maynard 1984]. This machine produced two chart recorder outputs, as shown in Figure 19.1. There was a chart similar to the CFM trace, and also a chart that produced frequency domain data consisting of the EEG displayed as traditional EEG frequency bands.