Purpose of the study: In case of an out-of-hospital cardiac arrest (OHCA) due to ventricular fibrillation (VF) guidelines recommend early defibrillation followed by chest-compressions for two minutes before analyzing shock success. If rhythm analysis reveals VF again, it is obscure whether VF persisted or reoccurred within the two-minutes-cycle of chest-compressions after successful defibrillation. 
We investigated the point in time of VF-recurrence.
Materials and Methods: Between February 2014 and March 2018 we examined retrospectively 185 consecutive shocks of all patients presenting with initial VF at arrival of ALS-ambulance in OHCA (Marburg-Biedenkopf-County, 252.000 inhabitants). Three independent investigators analyzed ECG-recordings of the defibrillator Corpuls3. We enclosed ECG-data from CPR-beginning until 2min after the third shock. Using filters from 2-10Hz we reduced chest-compression-artifacts. Successful shock was defined as VF-termination within 5s after a shock. A relapse was defined as VF-recurrence in the interval 5s after a shock and the following shock.
Results: We enclosed 185 shocks of 82 patients. 74.1% (n=137) of the shocks were successful, recurrence-rate of VF was 88.3% (n=121). The median of point in time of VF-recurrence was 33s (16s/106s) after shock. 47.1% (n=57) of VF-recurrence occurred ≤30s after shock, 12.4% (n=15) of VF-recurrence occurred 31-60s after shock, 13.7% (n=16) of VF-recurrence occurred 61-90s after shock, 27.3% (n=33) of VF-recurrence occurred >90s after shock.
Conclusions: Although VF was terminated by rectilinear-waveform-defibrillation in 74%, VF recurred within two minutes of chest-compressions in 88%. Thus, VF reappears frequently and early. It is unclear to which extend chest-compressions influence VF-relapse. Further studies need to reevaluate the best shock-compression-analysis-algorithm and the regime of antiarrhythmic therapy in the early phase of CPR in OHCA with initial VF.

1 - Defibrillation success in out-of-hospital cardiac arrest: Point in time of ventricular fibrillation recurrence after successful shock during early phase of cardiopulmonary resuscitation