Purpose: The 2021 guidelines endorsed by the European Resuscitation Council (ERC) and the European Society of Intensive Care Medicine (ESICM) recommend using highly malignant electroencephalogram (EEG) patterns (HMEP; suppression or burst-suppression) at>24 h after cardiac arrest (CA) in combination with at least one other concord‑ ant predictor to prognosticate poor neurological outcome. We evaluated the prognostic accuracy of HMEP in a large multicentre cohort and investigated the added value of absent EEG reactivity.

Methods: This is a pre-planned prognostic substudy of the Targeted Temperature Management trial 2. The presence of HMEP and background reactivity to external stimuli on EEG recorded>24 h after CA was prospectively reported. Poor outcome was measured at 6 months and defned as a modifed Rankin Scale score of 4–6. Prognostication was multimodal, and withdrawal of life-sustaining therapy (WLST) was not allowed before 96 h after CA.

Results: 845 patients at 59 sites were included. Of these, 579 (69%) had poor outcome, including 304 (36%) with WLST due to poor neurological prognosis. EEG was recorded at a median of 71 h (interquartile range [IQR] 52–93) after CA. HMEP at>24 h from CA had 50% [95% confdence interval [CI] 46–54] sensitivity and 93% [90–96] specifcity to predict poor outcome. Specifcity was similar (93%) in 541 patients without WLST. When HMEP were unreactive, specifcity improved to 97% [94–99] (p=0.008).

Conclusion: The specifcity of the ERC-ESICM-recommended EEG patterns for predicting poor outcome after CA exceeds 90% but is lower than in previous studies, suggesting that large-scale implementation may reduce their accuracy. Combining HMEP with an unreactive EEG background signifcantly improved specifcity. As in other prog‑ nostication studies, a self-fulflling prophecy bias may have contributed to observed results.