http://health.yahoo.com/news/reuters/heart_cpr_dc.html
Cardiac arrest occurs when the heart stops circulating blood. Most often, people with cardiac arrest have a type of heart rhythm known as ventricular fibrillation, in which the heart quivers but does not pump blood.
If no shock is delivered in the first four minutes of this deadly rhythm, the heart stops altogether and it becomes much harder to get it restarted. During this phase, old-fashioned chest compressions can help push blood back into the heart, making it more likely to restart.
WAITING TO DEFIBRILLATE
As most emergency teams do not arrive on the scene in that critical first four minutes, the new resuscitation approach calls for a round of 200 chest compressions given in the first two minutes to improve the odds that the heart will restart.
"Traditionally, we've told them to defibrillate right away. When they do that, the patient dies frequently," Bobrow said in a telephone interview.
In 2004, only 3 percent of people in Arizona who had a cardiac arrest outside of a hospital survived.

(much more at link)