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Sedation-related Electroencephalographic Patterns in Acute Hypoxemic Respiratory Failure. BACKGROUND: There is no universal objective measure of the effect of sedation on brain activity and how to differentiate it from sleep. In patients with early acute hypoxemic respiratory failure (AHRF), the authors used the odds ratio product, an electroencephalography (EEG)-based metric used to quantify the sleep-wake continuum. Despite patients behaviorally appearing asleep, the authors observed and quantified novel EEG patterns previously unobserved during natural sleep, and hypothesized that these unnatural EEG patterns (EEG Ups ) reflect the effect of sedation. The objective of the study was to explore the relevance of EEG Ups (never or extremely rarely seen in sleep studies) and their association with sedation at the early phase of AHRF. METHODS: This was a prospective cohort study including patients mechanically ventilated for AHRF and Pa o2 /fraction of inspired oxygen less than 200 mmHg receiving various sedation-opioid regimens and doses as per clinical indication. Continuous EEG monitoring was performed from study inclusion until extubation, death, or up to 7 days. EEG quantified the relative power of each frequency band (slow delta, fast delta plus theta, alpha-sigma, beta) and determined the frequency of EEG Ups . RESULTS: A total 1,832 h of EEG recordings were analyzed (mean ± SD, 43 ± 25 h/patient) from 23 patients (median [interquartile range, 25 to 75%], 58 [48 to 70] yr; 87% male; Pa o2 /fraction of inspired oxygen, 150 [116 to 198] mmHg; intensive care unit mortality, 22%). EEG Ups accounted for 42% of the total recording time overall, differed among drug combinations, and exceeded 50% with some sedation-opioid combinations. Brief wake intrusions, a marker of physiologic sleep, were extremely low. EEG Ups prevalence was higher with sedation-opioid combinations ( P ≤ 0.029), high sedation dose ( P ≤ 0.035), and deeper clinical sedation score ( P ≤ 0.024), and was associated with intensive care unit mortality ( P < 0.001). CONCLUSIONS: Continuous intravenous sedation results in EEG Ups that are not present in natural sleep, correlate with dose of sedation, clinical sedation score, and clinical outcomes.