Aperiodic exponent ceases to track changes in propofol dose at deep surgery-level hypnotic depths

BackgroundAccurately tracking hypnotic depth during surgery is key to deliver personalized and precise anaesthesia. The aperiodic exponent of electroencephalographic (EEG) data has shown great promise as a marker of the balance between excitatory and inhibitory cortical activity and of hypnotic depth. In humans, this relation has been mostly evidenced by pairwise comparisons between awake and superficial anaesthesia states. However, it is unclear if this relation is also present across deeper hypnotic depths. Here we examined the relation between aperiodic exponent and hypnotic depth across different surgery-level propofol doses. MethodsThe present was an exploratory prospective observational analytical study where we recorded high-density EEG data from patients before surgery and during continuous target-controlled propofol infusions with different intraoperative target concentrations. We evaluated the relation between steady-state propofol dose and several EEG features including aperiodic exponent and spectral band powers. ResultsAll EEG measures were different when comparing awake versus anaesthesia conditions. At surgery-level hypnotic depths, some spectral band powers continued to track propofol dose displaying either with positive (delta R2 = 0.23, p = 0.001) or negative correlations (alpha R2 = 0.36, p < 0.001; beta R2 = 0.045, p < 0.001). Importantly, the aperiodic exponent was not associated with propofol doses absolute value (R2 = 0.011, p = 0.43) or with its relative change (R2 = 0.011, p = 0.51). ConclusionsOur results suggest that the relation between aperiodic exponent and excitatory/inhibitory balance is circumscribed to more superficial hypnotic depths than those typically employed for surgery.