Depending on the circumstances of the cardiac arrest a person may receive only one of these treatments or both of these treatments. The purpose of the research study is to determine if more people live when either the real ITD is used or if additional CPR is given before looking at the heart rhythm the first time. This study is being conducted in 9 different areas throughout the United States and Canada by the Resuscitation Outcomes Consortium (ROC). About 15,000 patients will be enrolled in this research study.