PHILADELPHIA (CBS) –  Sudden cardiac arrest is normally associated with middle-aged or older individuals but younger people can experience it as well without warning and sometimes with devastating results.

The Philadelphia Chapter of The American Heart Association is hosting a Heart Walk in Philadelphia on November 13th to raise money and awareness.

Thomas Jefferson University Hospital Cardiologist Dr. Arnold Greenspon says cardiac arrest can occur without warning due to a life threatening arrhythmia, ventricular fibrillation, which can be corrected with a shock from an AED or defibrillator.

“People who are fit have a lower incidence of sudden death. Yet, even fit people are at highest risk for heart problems at peak exercise.”

Main Line Personal Trainer Mark Peters was just 22-years-old and training a client in the gym when he suffered sudden cardiac arrest:

“My heart completely stopped. I was revived about two minutes later with a defibrillator.”

As for Conshohocken swimmer Fran Crippen who died during a Middle East race on Saturday:

Dr. Greenspon:

“We need more information. But think about it. In any type of extreme sport, the body is really under tremendous stress. Imagine if somebody was swimming and the water temperature was high.”

It could possibly alter the body’s core temperature and metabolism leading to cardiac arrest but Greenspon says we may never know.

Reported by: Michelle Durham, KYW Newsradio.