Depth of anaesthesia monitoring using the spontaneous EEG in the elderly compared with younger adults

EEG-based depth of anaesthesia monitors are an important tool in the operating theatre. They provide a safeguard against intraoperative awareness (under-dosing) and over-dosing of anaesthetic (hypnotic) drugs by providing quantitative analysis of the EEG, reflecting cortical activation. These monitors have generally been assumed to be equally accurate across adult ages. However, conflicting results are emerging that indicate that monitor performance alters with advancing age. Elderly patients undergo surgery up to four times more often than the rest of the population, and anaesthesia becomes a greater risk in the elderly due to increased effect of anaesthetic agent and occurrence of coexisting disease. In a clinical sense, there is a special interest in avoiding deep anaesthesia because of a possible relationship with reduced survival in the months and years following surgery, which has particular relevance to elderly patients. It is of importance to ensure that the EEG-based depth of anaesthesia monitors perform accurately in this age group. The work presented has been conducted to gain further insight into the differences in selected EEG parameters during anaesthesia in the elderly compared to younger adults, and the impact these differences have on EEG-based depth of anaesthesia monitoring. Subjects (n = 44) were recruited from theatre lists at the Alfred Hospital, and the spontaneous EEG was recorded throughout the surgical procedure. The subject data were divided into the age groups of young adults (18 – 34 yrs, n=10), middle-aged (35 – 64 yrs, n=18) and the elderly (65 yrs +, n=16) for analysis. Parameters as recorded by the BIS monitor (BIS index and SEF95) and spectral parameters of the raw EEG (absolute and relative powers, beta ratio and WSMF8-30Hz) were compared between young adults and the elderly to establish if any differences exist. Then the ability to distinguish between consciousness and unconsciousness, and the ability to track graduated levels of anaesthesia, were compared across young adults, middle age and the elderly, using the analytical methods of linear analysis using spectral power of the gamma band, non-linear analysis using bicoherence and complexity analysis using permutation entropy. Results of this analysis confirm that there is a decrease in delta activity and an increase in alpha, beta and gamma activity in the EEG of elderly subjects during deep anaesthesia in comparison to younger adults. Analysis has highlighted the occurrence of some elderly subjects (2/16 or 13%) with particularly high gamma activity present during surgical anaesthesia. The reason behind this occurrence remains unclear, however it is apparent that this gamma activity is not directly related to muscle activity. Generally, the differences in the EEG of the elderly make it more difficult to accurately distinguish the moderate to deep levels of anaesthesia using the EEG parameters investigated. Therefore, there is an increased risk of unintentionally over-dosing elderly subjects when using EEG-based depth of anaesthesia monitors.