NORMAN, Okla. (AP) — When Lisa Marti woke on May 9, she had no idea her heart would stop that afternoon.

Marti, a Norman resident, said she felt fine that morning. It was a sunny day, and she was thinking about work and her uncle's birthday party that evening.

"I got dizzy a couple of times but that was about it," Marti said. At 39, she hadn't considered that the dizziness she felt that morning and had experienced a couple of days prior meant something was wrong with her heart.

At work, things were busy, but no more stressful than usual.

"I remember going to lunch, and then I don't remember an...