There is animal evidence which indicates that provision of chest compressions just prior to defibrillation may increase the ability of the heart to convert to a perfusing rhythm. Several minutes of CPR just prior to defibrillation may provide enough substrate to allow electrical activity to resume following defibrillation in some cases. We are analyzing the time interval from the cessation of chest compressions to delivery of the first shock using de-identified continuous ECG rhythm strips from out-of-hospital cardiac arrest cases.