VF (Ventricular Fibrillation) – This is when the heart has gone nuts.  Nothing is firing properly and can be best describes as having an epileptic fit.  Whilst it is like this, it is filling with blood but not pumping anything.  The rhythm is shockable and often, shocking the patient will kick start the heart back into a proper rhythm (of sorts).  Unless there are underlying causes of course.  However, it is important that CPR is performed immediately because the more blood there is swelling the heart, the less likely a shock will work.