During cardiac arrest, the heart stops pumping blood, the blood pressure falls to zero and the pulse disappears. Within 10 seconds of cardiac arrest the person loses consciousness and becomes unresponsive. If you shake or shout at the victim, there will be no response.
Sometimes a person in cardiac arrest may make grunting, gasping or snoring type breathing sounds for a couple of minutes. Do not be confused by this abnormal type of breathing.
If a person is unresponsive (doesn't respond to shouts or shakes) and not breathing (or is breathing abnormally) then call 911 and begin CPR.