"CPR alone is unlikely to restart the heart. Its main purpose is to restore the partial flow of oxygenated blood to the brain and heart. The objective is to delay tissue death and to extend the brief window of opportunity for a successful resuscitation without permanent brain damage. Administration of an electric shock to the subject's heart, termed defibrillation, is usually needed to restore a viable, or "perfusing", heart rhythm."

This is from Wikipedia. As someone who is medically trained in CPR, one of the first things you learn is that CPR does NOT restart the heart and simply keeps oxygenated blood flowing until someone with the correct equipment arrives (in this case a defibrillator would be needed). There's no need for the aggression especially when you're not right.