EEG pre-burst suppression: characterization and inverse association with preoperative cognitive function in older adults

Abstract

The most common complication in older surgical patients is postoperative delirium (POD). POD is associated with preoperative cognitive impairment and longer durations of intraoperative burst suppression (BSup) - electroencephalography (EEG) with repeated periods of suppression (very low-voltage brain activity). However, BSup has modest sensitivity for predicting POD. We hypothesized that a brain state of lowered EEG power immediately precedes BSup, which we have termed "pre-burst suppression" (preBSup). Further, we hypothesized that even patients without BSup experience these preBSup transient reductions in EEG power, and that preBSup (like BSup) would be associated with preoperative cognitive function and delirium risk. Data included 83 32-channel intraoperative EEG recordings of the first hour of surgery from 2 prospective cohort studies of patients ≥age 60 scheduled for ≥2-h non-cardiac, non-neurologic surgery under general anesthesia (maintained with a potent inhaled anesthetic or a propofol infusion). Among patients with BSup, we defined preBSup as the difference in 3-35 Hz power (dB) during the 1-s preceding BSup relative to the average 3-35 Hz power of their intraoperative EEG recording. We then recorded the percentage of time that each patient spent in preBSup, including those without BSup. Next, we characterized the association between percentage of time in preBSup and (1) percentage of time in BSup, (2) preoperative cognitive function, and (3) POD incidence. The percentage of time in preBSup and BSup were correlated (Spearman's ρ [95% CI]: 0.52 [0.34, 0.66], p < 0.001). The percentage of time in BSup, preBSup, or their combination were each inversely associated with preoperative cognitive function (β [95% CI]: -0.10 [-0.19, -0.01], p = 0.024; -0.04 [-0.06, -0.01], p = 0.009; -0.04 [-0.06, -0.01], p = 0.003, respectively). Consistent with prior literature, BSup was significantly associated with POD (odds ratio [95% CI]: 1.34 [1.01, 1.78], p = 0.043), though this association did not hold for preBSup (odds ratio [95% CI]: 1.04 [0.95, 1.14], p = 0.421). While all patients had ≥1 preBSup instance, only 20.5% of patients had ≥1 BSup instance. These exploratory findings suggest that future studies are warranted to further study the extent to which preBSup, even in the absence of BSup, can identify patients with impaired preoperative cognition and/or POD risk.

Keywords: EEG; anesthesia; burst suppression; non-cardiac surgery; perioperative; postoperative delirium; pre-burst suppression; preoperative cognition.

FIGURE 1

Consort diagram of participant data from the MADCO-PC and INTUIT studies.

FIGURE 2

PreBSup concept and algorithm. (A) Conceptual model of pre-burst suppression (preBSup) as an intermediate neural state between the normal anesthetized state and burst suppression (BSup). (B) BSup is characterized by periods of repeated bursts of EEG activity separated by low-amplitude isoelectric activity called suppression. (C) In this study, instances of suppression were marked (magenta), and in subjects with >0 burst suppression instances, the 1 s of EEG data preceding each suppression instance (cyan) was extracted. These data were used to create a preBSup threshold, which was then used to mark preBSup in all subjects.

FIGURE 3

Creation of a preBSup threshold. In subjects with >0 BSup instances (N = 17), we created a preBSup threshold by calculating the average 3–35 Hz EEG power (dB) decrease for epochs occurring 1 s prior to suppression events relative to “normal spectra” during the rest of the EEG recording (excluding periods of BSup or artifact). (A) In an example subject, 138 instances of BSup from their surgical recording were overlaid and aligned to suppression onset (the magenta dotted line). The cyan dotted line represents the point in time 1 s before the onset of BSup in all 138 aligned EEG traces. (B) The average of all spectrograms from the 138 EEG traces plotted in panel (A) using non-overlapping 1-s windows. (C) The same data redrawn for smoother visualization using a 1-s moving window and a 0.025-s step size. (D) The averaged power across 1-s periods before suppression (i.e., average preBSup power, shown in red) among the 17 subjects with >0 BSup instances. The average normal spectra among the 17 subjects with >0 BSup instances are shown in blue. (E) The bold purple line indicates the average power (dB) decrease by frequency of the preBSup epoch [the red line in part (D)] from normal spectral power [the blue line in part (D)] with a 95% confidence interval depicted in lighter purple. We used the average power decrease from 3–35 Hz (a 2.32 dB drop) from the 17 subjects with >0 BSup instances as our threshold to detect and mark preBSup in all subjects, independent of BSup.

FIGURE 4

Forest plots of the relationship between intraoperative EEG patterns (percentage of first hour of surgical case spent in BSup, preBSup, or their combination) and preoperative cognitive measures and postoperative delirium incidence. Each point with error bars represents the linear regression beta coefficient and 95% confidence interval from a separate statistical model, for the effects of BSup, preBSup, or their combination on continuous cognitive index and the 5 preoperative cognitive factor domains (Randt verbal memory, Hopkins verbal memory, executive function, visual memory, and attention/concentration). The odds ratios from the simple Firth-corrected logistic regression models of the effect of BSup, preBSup, and their combination on postoperative delirium incidence are shown in the bottom panel.