Objective Treatment: protocols for cardiac arrest rely upon rhythm analyses performed at two-minute intervals, neglecting possible rhythm changes during the intervening period of CPR. Our objective was to describe rhythm profiles (patterns of rhythm transitions during two-minute CPR cycles) following attempted defibrillation and to assess their relationship to survival. Methods The study included out-of-hospital cardiac arrest cases presenting with ventricular fibrillation from 2011 to 2015. The rhythm sequence was annotated during two-minute CPR cycles after the first and second shocks of each case, and the rhythm profile of each sequence was classified. We calculated absolute survival differences among rhythm profiles with the same rhythm at the two-minute check. Results Of 569 rhythm sequences after the first shock, 46% included a rhythm transition. Overall survival was 47%, and survival proportion varied by rhythm at the two-minute check: ventricular fibrillation (46%), organized (58%) and asystole (20%). Survival was similar between profiles which ended with an organized rhythm at the two-minute check. Likewise, survival was similar between profiles with asystole at the two-minute check. However, in patients with ventricular fibrillation at the two-minute check, survival was twice as high in those with a transient organized rhythm (69%) compared to constant ventricular fibrillation (32%) or transient asystole (28%). Conclusion Rhythm transitions are common after attempted defibrillation. Among patients with ventricular fibrillation at the subsequent two-minute check, transient organized rhythm during the preceding two-minute CPR cycle was associated with favorable survival, suggesting distinct physiologies that could serve as the basis for different treatment strategies.
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