When Julie Llamas Rickman went to the emergency room five years ago, she thought her asthma was acting up. She felt short of breath and tired.

But it turned out she’d had a heart attack. Rickman was only 41 and didn’t think she was at particular risk until a doctor told her.

“He said, ‘Mrs. Rickman, you have two blockages in your heart… and had a heart attack’. I thought immediately, what did he just say? I started crying,” Rickman, who lives in Overland Park, Kansas, told NBC News.

“The only thing that ran through my mind was will I be there for my son?”

Rickman is far from alone. A new study released Monday found that 45 percent of heart attacks in the United States are “silent” — the people having them don’t realize.