Mechanical ventilation of a critical cardiac tamponade is a high-risk situation that can lead to asystolic cardiac arrest. We report a prehospital mechanical ventilation of a penetrating cardiac injury complicated with tamponade. Onset diagnosis of the circulatory arrest allowed by prehospital continuous ultrasonography led to earlier initiation of the resuscitation and might have favored successful outcome. Electrocardiographic signs are too late to diagnose circulatory arrest.