The Ageing Brain: Age-dependent changes in the electroencephalogram during propofol and sevoflurane general anaesthesia

Abstract

Background: Anaesthetic drugs act at sites within the brain that undergo profound changes during typical ageing. We postulated that anaesthesia-induced brain dynamics observed in the EEG change with age.

Methods: We analysed the EEG in 155 patients aged 18-90 yr who received propofol (n=60) or sevoflurane (n=95) as the primary anaesthetic. The EEG spectrum and coherence were estimated throughout a 2 min period of stable anaesthetic maintenance. Age-related effects were characterized by analysing power and coherence as a function of age using linear regression and by comparing the power spectrum and coherence in young (18- to 38-yr-old) and elderly (70- to 90-yr-old) patients.

Results: Power across all frequency bands decreased significantly with age for both propofol and sevoflurane; elderly patients showed EEG oscillations ∼2- to 3-fold smaller in amplitude than younger adults. The qualitative form of the EEG appeared similar regardless of age, showing prominent alpha (8-12 Hz) and slow (0.1-1 Hz) oscillations. However, alpha band dynamics showed specific age-related changes. In elderly compared with young patients, alpha power decreased more than slow power, and alpha coherence and peak frequency were significantly lower. Older patients were more likely to experience burst suppression.

Conclusions: These profound age-related changes in the EEG are consistent with known neurobiological and neuroanatomical changes that occur during typical ageing. Commercial EEG-based depth-of-anaesthesia indices do not account for age and are therefore likely to be inaccurate in elderly patients. In contrast, monitoring the unprocessed EEG and its spectrogram can account for age and individual patient characteristics.

Keywords: EEG; ageing; brain monitoring; elderly; electroencephalography; propofol; sevoflurane.

Fig 1

Trends in the spectrum and coherence from 18 to 90 yr old during propofol anaesthesia. (a) The frontal EEG spectrum as a function of age. Slow (0.1–1 Hz) oscillations are present in all patients during general anaesthesia maintained solely with propofol. Alpha (8–12 Hz) oscillations appear to diminish with age. (b) The trend in alpha power appears to decrease with age. The green line represents a linear regression model describing the relationship between age and alpha power. (c) The frontal EEG coherence as a function of age. The alpha band coherence is robust in young patients and decreases with age, starting at about 65 yr. (d) The trend in alpha coherence decreases with age. The green line represents a linear regression model describing the relationship between age and alpha coherence.

Fig 2

Trends in the spectrum and coherence from 18 to 90 yr old during sevoflurane anaesthesia. (a) The frontal EEG spectrum as a function of age. Slow (0.1–1 Hz) oscillations are present in all patients during general anaesthesia maintained solely with sevoflurane. Alpha (8–12 Hz) oscillations are evident during young adulthood and appear to diminish with age. (b) The trend in alpha power appears to decrease with age. The green line represents a linear regression model describing the relationship between age and alpha power. (c) The frontal EEG coherence as a function of age. The alpha band coherence is robust in young patients and decreases with age, starting at about 65 yr. (d) The trend in alpha coherence decreases with age. The green line represents a linear regression model describing the relationship between age and alpha coherence.

Fig 3

Median spectra, coherence, spectrograms, and cohereogram of young and elderly age groups for propofol. (a) Power spectrum for both young patients (18–38 yr old), represented by blue line, and elderly patients (70–90 yr old), represented by green line. The power spectra show peaks in the slow (0.1–1 Hz) and alpha (8–12 Hz) frequency bands. The elderly patients show a marked decrease in power across all frequency bands. The power in the alpha band appears to decrease more than other bands, quantified in terms of alpha-to-slow ratio. (b) The group spectrogram for young patients shows prominent power in the slow and alpha frequency bands. (c) The group spectrogram for elderly patients shows a visible absence of power in the alpha frequency band. (d) Coherence for both young patients (18–38 yr old), represented by blue line, and elderly patients (70–90 yr old), represented by green line. The coherence is significantly lower in elderly patients, particularly in the alpha band. The peak coherent frequency is also lower in elderly patients. (e) The group cohereogram for young patients shows prominent coherence in the slow and alpha frequency bands. (f) The group cohereogram for elderly patients shows a visible absence of coherence in the alpha frequency band.

Fig 4

Median spectra, coherence, spectrograms, and cohereogram of young and elderly age groups for sevoflurane. (a) Power spectrum for both young patients (18–38 yr old), represented by blue line, and elderly patients (70–90 yr old), represented by green line. The power spectra show peaks in the slow (0.1–1 Hz) and alpha (8–12 Hz) frequency bands. The elderly patients show a marked decrease in power across all frequency bands. The power in the alpha band appears to decrease more than other bands, quantified in terms of alpha-to-slow ratio. (b) The group spectrogram for young patients shows prominent power in the slow and alpha frequency bands. (c) The group spectrogram for elderly patients shows a visible absence of power in the alpha frequency band. (d) Coherence for both young patients (18–38 yr old), represented by blue line, and elderly patients (70–90 yr old), represented by green line. The coherence is significantly lower in elderly patients, particularly in the alpha band. The peak coherent frequency is also lower in elderly patients. (e) The group cohereogram for young patients shows prominent coherence in the alpha frequency band. (f) The group cohereogram for elderly patients shows a visible absence of coherence in the alpha frequency band.

Fig 5

Analysis of episodes of burst suppression under propofol or sevoflurane. (a) Representation of the probability (Pr) that patients experience an episode of burst suppression during anaesthetic maintenance with propofol. (b) Representation of the probability that patients experience an episode of burst suppression during anaesthetic maintenance with sevoflurane. (c) The proportion of elderly patients who experience burst suppression is greater than that for young patients under propofol. (d) The proportion of elderly patients who experience burst suppression is greater than that for young patients under sevoflurane.