In ventricular Fibrillation the heart beats (quivers rather) too fast and chaotically to produce any pulse. Without blood supply to the brain the person falls down unconscious within seconds and then stops breathing. So the symptoms are pulselessness (cannot feel any pulse or hear heart sounds) and unconsciousness rapidly leading to stoppage of breathing. The treatment is a thump on the chest followed by cardio-pulmonary resuscitation and defibrillation with a defibrillator.