Background: For patients suffering from an out-of-hospital cardiac arrest (OHCA), having an initial shockable rhythm is a marker of good prognosis. However, the prognostic implication of converting from a non-shockable to a shockable rhythm, as compared to having an initial shockable rhythm, remains uncertain. The objective of this study was to evaluate the association between the initial rhythm and its subsequent conversion and survival following an OHCA. It was hypothesized that such a conversion would be associated with a higher survival.
Methods: The present cohort study used a registry of adult OHCA between 2010 and 2015 in Montreal, Canada. Adult patients suffering from a non-traumatic OHCA for whom the initial rhythm was known were included. The primary outcome measure was survival to hospital discharge. The association of interest were assessed using a multivariable logistic regression controlling for demographic and clinical variables (e.g. age, gender, witnessed arrest, bystander cardiopulmonary resuscitation). Assuming a survival rate of 3% and 25% of the variability explained by the control variables, including more than 4580 patients would allow to detect an absolute difference of 4% in survival between groups with a power of more than 90%.
Results: A total of 6681 patients (male=64%, mean age = 70 years [SD 17], survival=11%) were included, of whom 1788 (27%) had an initial shockable rhythm, 1749 (26%) had pulseless electrical activity (PEA) and no subsequent shockable rhythm, 295 (4%) had PEA and a subsequent shockable rhythm, 2694 (40%) had asystole and no subsequent shockable rhythm, and 155 (2%) asystole and a subsequent shockable rhythm. As compared to patients having an initial shockable rhythm, patients in all other groups had significantly lower odds of survival (adjusted odds ratio [AOR] between 0.15 [95%CI 0.12-0.18] and 0.017 [95%CI 0.010-0.030]; p<0.001 for all comparisons). Among patients with a PEA and asystole, there was no association between evolving to a shockable rhythm and survival to hospital discharge (AOR=0.74 [95%CI 0.40-1.35], p=0.32, and AOR=1.37 [95%CI 0.17-10.83], p=0.77, respectively).
Conclusion: There is no clinically significant association between the conversion to a shockable rhythm and survival in patients suffering from OHCA. The initial rhythm remains a much better outcome predictor than subsequent rhythms.