Association of Age, Sex and ASA Physical Status With Bispectral Index Values During General Anaesthesia: A Large...

Depth of anaesthesia (DoA) is often guided by standard dosing protocols and evaluated through clinical signs, assuming these signs reliably indicate unconsciousness. However, this assumption is problematic: patients respond differently to anaesthetic drugs, and physiological markers can misrepresent brain state. Nociceptive responses may persist despite apparent unconsciousness. Excessive DoA is associated with cardiovascular instability and post-operative cognitive dysfunction, whereas insufficient DoA increases the risk of intraoperative awareness. Processed electroencephalography (pEEG), such as the Bispectral Index (BIS), was introduced to address these limitations by enabling monitoring of frontal cortical brain activity during general anaesthesia. We aimed to investigate how pEEG, measured as BIS, reflects anaesthetic depth across patient subgroups and anaesthetic techniques.

This large-scale prospective observational cohort study analysed electronic health records from patients who underwent general anaesthesia between 1 January 2017, and 31 December 2020. The study evaluated associations between BIS values and patient age, sex and health status measured by American Society of Anaesthesiologists (ASA) scores.

Clinical data were extracted from 97,733 patients, of whom 26,650 underwent continuous BIS monitoring. BIS monitoring was more frequent in patients classified as ASA scores of 1 or 2 and those receiving total intravenous anaesthesia. Patients with higher ASA scores, increasing age and female patients showed deeper anaesthesia with significantly lower BIS values.

In this large observational cohort, older patients, patients with higher ASA score and females exhibited lower BIS values during general anaesthesia compared with younger, healthier and male patients, even when anaesthetic dosing followed standard protocols. These observed associations may reflect practise patterns, unmeasured confounding or physiological differences, and should not be interpreted as causal. Further studies are warranted to explore more individualised anaesthetic strategies and tailored neuromonitoring approaches to optimise anaesthetic depth, particularly in vulnerable patient subgroups.

Using a large database of cases with processed EEG (BIS) and anaesthesiological data, this study found that older patients and those with higher ASA-class had the deepest anaesthetic depth despite standard anaesthetic dosing protocols. These findings support the need for more individualised EEG-monitoring, especially in the elderly and frail, but also how these BIS indicators should guide management. It remains unresolved if deeper anaesthesia with presumed benefits of reducing the surgical stress, can be balanced by improving brain perfusion rather than reducing anaesthetic depth.