The validity of bispectral index (BIS) monitoring in the elderly remains unclear, as the database that was used to develop the BIS algorithm did not contain any electroencephalogram (EEG) data from the elderly.1 The characteristics of the EEG are known to change with increasing age and we have often experienced some uncertainty regarding the accuracy of BIS monitoring, particularly in the elderly. In the present case report, an unsynchronised EEG during light anaesthesia was falsely evaluated as indicating a BIS value less than 20, because the small EEG amplitude typically seen in the elderly was mistakenly interpreted as burst suppression.
Written informed consent for publishing this case was obtained from the patient and her daughter.
An 82-year-old woman with hypothyroidism was scheduled for surgery for humeral bone fracture. No antipsychotics had been prescribed and she showed no evidence of dementia. Anaesthesia was induced using propofol and remifentanil and was maintained using sevoflurane and remifentanil. The EEG was monitored and recorded using an Aspect A-2000 BIS monitor (version 4.0; Aspect Medical Systems, Natick, Massachusetts, USA).
About 45 min after inducing general anaesthesia, BIS values suddenly decreased from 40 to 60 to 10 to 20, despite there being no changes or additions to the anaesthetic (Fig. 1, Point 1). Expiratory sevoflurane concentration was maintained at 0.9% and remifantanil was continuously administered at 0.2 μg min−1 kg−1. Circulatory dynamics were stable (SBP was kept between 90 and 130 mmHg, heart rate was 50 to 60 beats min−1). Although the expiratory sevoflurane concentration was reduced to 0.4%, BIS values remained below 20 (Fig. 1, Point 2). We checked the raw EEG data and found that the EEG amplitude was very small but included many waves, and that the appearance of the EEG was somewhat different from what is referred to as a suppressed EEG. We then set the expiratory sevoflurane concentration to 3%. About 1 min later, the BIS values suddenly returned to more than 50 (Fig. 1, Point 3). About 6 min after the expiratory sevoflurane concentration was set to 3%, BIS values again decreased abruptly to less than 20, accompanied by burst suppression (Fig. 1, Point 4). During that period, SBP was maintained above 90 mmHg using phenylephrine and heart rate was always above 50 beats min−1. After surgery, the patient woke up immediately and did not show any neurological complications. She had no experience of perioperative awakening.
Postoperative EEG analysis using BSA version 3.22B2 software (Bispectrum Analyser for A-2000BIS monitor)2 revealed that the EEG with an uncharacteristically low BIS index (Point 1) had a low amplitude of about 6 to 7 μV and the burst suppression ratio was 0.5 to 0.6. However, the EEG (Point 1) actually differed from the suppressed EEG at the time of real burst suppression (Point 4). The EEG (Point 1) consisted of numerous small waves and its power spectrum resembled the power spectrum associated with intermediate BIS values (Point 3), which had a peak alpha component. The burst phase was also not observed. As for the EEG when BIS increased markedly (Point 3, about 1 min after changing the expiratory sevoflurane concentration to 3%), the suppression ratio was reduced to less than 0.1 because the EEG with slightly increased amplitude was no longer considered to be a suppressed EEG.
The BIS algorithm employs a suppression ratio that quantifies the isoelectric activity of the burst suppression pattern to calculate the level of deep anaesthesia (particularly levels wherein BIS index is <30).3,4 Although suppression ratios less than 0.4 are not adequately reflected in the BIS index, deep anaesthesia causing suppression ratios more than 0.4 have been reported to be linearly correlated with BIS values in the range 0 to 30.4 The sudden recovery in BIS (i.e. from BIS <20 to BIS >50) described in the present report caused by a reduction in suppression ratio from 0.5 to 0.6 was consistent with a previous report.4
Concerning the detection of the suppression ratio, suppression is considered to occur when the EEG voltage does not exceed approximately +5.0 μV for periods longer than 0.50 s.3,4 Generally, 5 μV is used for the threshold value for suppression. However, in the elderly, low-amplitude, unsynchronised EEG near 5 μV during light anaesthesia might be misinterpreted as a suppression EEG, causing extremely low BIS values.
The ageing process is characterised by neuronal apoptosis and a loss of both synaptic density and neurotransmitters in various regions of the brain. It also causes a general reduction in EEG voltage, altering the functional neural networks of the brain.5 The physiology of unsynchronised, low-amplitude EEG in the elderly under light anaesthesia differs markedly from that associated with suppression processes, which are effected by strong, synchronised thalamic discharges to the unresponsive cortex wherein a depletion of extracellular cortical Ca2+ causes dysfunction of synaptic transmission.6,7
The elderly can easily be exposed to cerebral ischaemia and hypoperfusion because of a high incidence of cardiovascular complications, which can cause severe reductions in BIS values. However, this report indicates another possibility when extremely low BIS values are monitored in the elderly.
Acknowledgements relating to this article
Assistance with the letter: we thank Satoshi Hagihira for supplying BSA software.
Financial support and sponsorship: none.
Conflicts of interest: none.
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