Cardiac arrest can strike without warning. Do you suspect that someone is experiencing cardiac arrest? Here are the signs: Sudden loss of responsiveness The person doesn’t respond, even if you tap him or her hard on the shoulders, or ask loudly if he or she is OK. The person doesn’t move, speak, blink or otherwise react.
https://www.heart.org/en/health-topics/cardiac-arrest/emergency-treatment-of-cardiac-arrest