Background: The Osborn or J-wave, an upright deflection of the J-point on the electrocardiogram (ECG), is often observed during severe hypothermia. A possible relation between Osborn waves (OW) and increased risk of ventricular arrhythmia has been reported. We sought to determine whether the level of targeted temperature management (TTM) following out-of-hospital cardiac arrest (OHCA) affects the prevalence of OW and to assess the associations between OW and risk of ventricular arrhythmia and death. Methods and results: The present study is part of the TTM-trial ECG-substudy (including OHCA-patients randomized to TTM at 33 °C vs. 36 °C from 24 of 36 sites). Serial 12-lead ECGs from 680 (94%) patients were analysed and stratified by OW at predefined time-points (0, 4, 28, 36, 72-h after admission). On admission, the overall prevalence of OW was 16%, increasing to 32% at target temperature, with higher prevalence in the 33 °C-group (40% vs. 23%, p < 0.0001). No difference in prevalence was found between the 33 °C- and 36 °C-groups on admission (18% vs. 14%, p =.11) or after rewarming (13% vs. 10%, p =.44). OW were not associated with increased risk of ventricular arrhythmia (Odds ratio = 0.78 (0.51–1.20), p =.26), but associated with significantly lower 180-day mortality as compared to no OW (38% vs. 52%, plog-rank = 0.001) in univariable analyses only. Conclusion: OW are frequent during TTM, particularly in patients treated with 33 °C. OW are not associated with increased risk of ventricular arrhythmia, and may be considered a benign physiological phenomenon, associated with lower mortality in univariable analyses
