It's so important to recognize ventricular fibrillation in cardiac arrest and defibrillation early. At the same time Research findings have identified high-quality CPR with minimum interruption is critical to the success of defibrillation therapy. Both the guidelines by American Heart Association and the guidelines by European Resuscitation Council emphasize the importance of minimizing interruptions in chest compressions, specifically, duration of the pre-shock and post-shock pauses. So AHA indicate that there will be a new algorithm of VF during chest compression, which has high sensitivity and specificity. And the investigators can identify VF and defibrillate earlier.