Significance Anesthesiologists reversibly manipulate the brain function of nearly 60,000 patients each day, but brain-state monitoring is not an accepted practice in anesthesia care because markers that reliably track changes in level of consciousness under general anesthesia have yet to be identified. We found specific behavioral and electrophysiological changes that mark the transition between consciousness and unconsciousness induced by propofol, one of the most commonly used anesthetic drugs. Our results provide insights into the mechanisms of propofol-induced unconsciousness and establish EEG signatures of this brain state that could be used to monitor the brain activity of patients receiving general anesthesia.

Time and frequency domain analyses of scalp EEG recordings are widely used to track changes in brain states under general anesthesia. Although these analyses have suggested that different spatial patterns are associated with changes in the state of general anesthesia, the extent to which these patterns are spatially coordinated has not been systematically characterized. Global coherence, the ratio of the largest eigenvalue to the sum of the eigenvalues of the cross-spectral matrix at a given frequency and time, has been used to analyze the spatiotemporal dynamics of multivariate time-series. Using 64-lead EEG recorded from human subjects receiving computer-controlled infusions of the anesthetic propofol, we used surface Laplacian referencing combined with spectral and global coherence analyses to track the spatiotemporal dynamics of the brain's anesthetic state. During unconsciousness the spectrograms in the frontal leads showed increasing α (8-12 Hz) and δ power (0-4 Hz) and in the occipital leads δ power greater than α power. The global coherence detected strong coordinated α activity in the occipital leads in the awake state that shifted to the frontal leads during unconsciousness. It revealed a lack of coordinated δ activity during both the awake and unconscious states. Although strong frontal power during general anesthesia-induced unconsciousness-termed anteriorization-is well known, its possible association with strong α range global coherence suggests highly coordinated spatial activity. Our findings suggest that combined spectral and global coherence analyses may offer a new approach to tracking brain states under general anesthesia.alpha rhythm | delta rhythm | loss of consciousness T ime-domain and frequency displays of continuous surface EEG recordings have been used for many years to track changes in the state of the brain under general anesthesia (1-7). Although these analyses have shown that different spatial patterns appear over the scalp as the state of general anesthesia changes, the extent to which these patterns are spatially coordinated and may indicate interesting dynamics in the underlying brain networks has not been systematically characterized. Several experimental and analysis issues must be addressed to carry out this characterization. First, analysis of this coordinated activity with an appropriate level of spatial resolution requires use of high-density (≥64 leads) EEG recordings (8). With few exceptions, EEG studies of general anesthesia use no more than 20 electrodes (9). Second, although in principle high-density EEG recordings should provide higher resolution information about the spatial structure in brain activity under general anesthesia, the increased amount of data creates the analysis challenge of determining how to make informative temporal assessments of the spatial dynamics in these multivariate time-series. Analyzing the cross-spectral matrix for a range of relevant frequencies as a function of time would offer a way to conduct a temporal analysis of coordinated spatial...

Background Electroencephalogram patterns observed during sedation with dexmedetomidine appear similar to those observed during general anesthesia with propofol. This is evident with the occurrence of slow (0.1–1 Hz), delta (1–4 Hz), propofol-induced alpha (8–12 Hz), and dexmedetomidine-induced spindle (12–16 Hz) oscillations. However, these drugs have different molecular mechanisms and behavioral properties, and are likely accompanied by distinguishing neural circuit dynamics. Methods We measured 64-channel electroencephalogram under dexmedetomidine (n = 9) and propofol (n = 8) in healthy volunteers, 18–36 years of age. We administered dexmedetomidine with a 1mcg/kg loading bolus over 10 minutes, followed by a 0.7mcg/kg/hr infusion. For propofol, we used a computer controlled infusion to target the effect-site concentration gradually from and 0 µg/mL to 5 µg/mL. Volunteers listened to auditory stimuli and responded by button-press to determine unconsciousness. We analyzed the electroencephalogram using multitaper spectral and coherence analysis. Results Dexmedetomidine was characterized by spindles with maximum power and coherence at ~13 Hz, (mean±std; power, −10.8dB±3.6; coherence, 0.8±0.08), while propofol was characterized with frontal alpha oscillations with peak frequency at ~11 Hz (power, 1.1dB±4.5; coherence, 0.9±0.05). Notably, slow oscillation power during a general anesthetic state under propofol (power, 13.2dB±2.4) was much larger than during sedative states under both propofol (power, −2.5dB±3.5) and dexmedetomidine (power, −0.4dB±3.1). Conclusion Our results indicate that dexmedetomidine and propofol place patients into different brain states, and suggests that propofol enables a deeper state of unconsciousness by inducing large amplitude slow oscillations that produce prolonged states of neuronal silence.