['The prognostic implication of conversion from initially non-shockable to shockable rhythms in patients with out-of-hospital cardiac arrest (OHCA) remains unclear. Our objective is to determine whether the conversion to shockable rhythms is a reliable predictor of short- and long-term outcomes both in patients who initially presented with pulseless electrical activity (PEA) and in those with asystole.', 'A secondary analysis was performed on non-traumatic OHCA cases ≥18 years old with PEA or asystole as initial rhythms, who were treated in the field and enrolled in the Resuscitation Outcomes Consortium (ROC) PRIMED study (clinicaltrials.gov/ct2/show/NCT00394706). We reported the characteristics and outcomes for those patients with or without shocks delivered in the field. Logistic regression analysis assessed the association of shock delivery with pre-hospital return of spontaneous circulation (ROSC), survival to hospital discharge and favorable neurological outcome as well.', 'Of the 9902 included cases, 3415 (34.5%) were initially in PEA and 6487 (65.5%) were in asystole. 744 (21.8%) PEA and 1134 (17.5%) asystolic patients underwent rhythm conversions and received subsequent shocks. For asystolic patients, the adjusted odds ratios (ORs) of shock delivery for pre-hospital ROSC, survival to discharge and favorable neurological outcome were 1.862 (95%CI 1.590-2.180), 3.778 (95%CI 2.374-6.014) and 4.154 (95%CI 2.192-7.871) respectively, while for PEA patients they were 0.951 (95%CI 0.796-1.137), 1.115 (95%CI 0.720-1.726) and 1.373 (95%CI 0.790-2.385) respectively.'] Is conversion to shockable rhythms associated with better outcomes in out-of-hospital cardiac arrest patients with initial asystole but not in those with pulseless electrical activity?