Background. We sought to identify the factors that were associated with higher incidence of ventricular fibrillation and survival with good neurologic function in the ResQTrial, which compared standard cardiopulmonary resuscitation (S-CPR) versus active compression–decompression CPR with an inspiratory impedance threshold device (ACD+ITD) in patients with out-of-hospital cardiac arrest (OHCA). Methods. A retrospective analysis of a randomized multicenter clinical study of 1,655 patients with OHCA. 88.3% (106/120) of the patients discharged with good neurologic function (modified Rankin score [MRS] ¡Â3) had a first recorded rhythm of ventricular fibrillation/pulseless ventricular tachycardia (VF). The first rhythm was recorded in 99.4% (1,645/1,655) of the cases about 9.5 minutes after the 9-1-1 call, on average 3 minutes after the arrival of EMS on the scene and after CPR was performed for at least 2 minutes. Results. A total of 32.8% of the patients had VF as the presenting rhythm and 42.8% received bystander CPR. The presence of bystander CPR was associated with a higher VF incidence only in the S-CPR group (40.8% versus 23.1% with no bystander, p = 0.001), but survival was 7.6% versus 4.6%, p = 0.09. Presence or absence of bystander CPR led to similar VF incidence and survival in the ACD+ITD group: 36.1% versus 33.9% and 9.0% versus 8.9%, respectively, p > 0.2. After propensity adjustment for witnessed arrest, age <67 years, gender, and public location, bystander CPR lost significance. In the absence of bystander CPR, ACD+ITD significantly increased the incidence of first-recorded VF compared with S-CPR from 106 of 459 patients (23.1%) to 164 of 484 patients (33.9%) (odds ratio [OR] 1.71, 95% confidence interval [CI] 1.27, 2.30, p < 0.001), and in patients with VF, return of spontaneous circulation increased from 65 of 459 patients (14.2%) to 104 of 484 patients (21.5%) (OR 1.66, 95% CI 1.16, 2.37, p = 0.004), leading to an overall doubling of survival with MRS ¡Â3 from 21 of 455 patients (4.6%) to 43 of 482 patients (8.9%) (OR 2.02, 95% CI 1.15, 3.65, p = 0.009). After propensity adjustment, ACD+ITD remained a significant predictor of an MRS ¡Â3 (p = 0.02). Conclusions. VF was the most important predictor of survival with MRS ¡Â3. In the absence of bystander CPR, ACD+ITD increased VF incidence as the first-recorded rhythm and doubled survival to hospital discharge with MRS ¡Â3 compared with S-CPR.