SHE'S GOT THE HEART TO DO IT SURGERY ALLOWS THURLBECK TO MAKE AN ATTEMPT TO COMPLETE L.A. MARATHON.

Byline: Lee Barnathan Staff Writer

PASADENA - Mercedes Thurlbeck notices the stares when she wears a bathing suit. People can't help but look at the seven-inch scar running down her chest.

And that's OK with her; she's not ashamed. She wears the scar like a personal trophy. It's her reminder not just of the heart surgery she had four years ago, it's a subject of her paintings and her symbol of how precious life is.

So partly on a whim and partly to show that nothing from her past is too big a hurdle, Thurlbeck, 18, will run the Los Angeles Marathon on Sunday. It will be her first 26.2-mile race.

``I am an artist and not an athlete,'' she said, ``so I am particularly pleased that I have been able to do this.''

Thurlbeck was born with ventricle septal defect, a hole in a ventricular wall that prevents blood from flowing between the ventricles.

She suffered through various side effects, including weight loss,fatigue and illness of one type or another 50 percent of the time, which often made it difficult for her parents to get her out of bed. She also ate poorly and grew slowly. She wore the same pair of shoes for as long as three years.

Her parents didn't tell her of her condition for years to create as normal a life for her as possible. Her mother, Nancy, encouraged the girl's artistic endeavors and said nothing when she played with friends and grew weary before everyone else.

``I never saw a difference,'' she said. ``I'd play when they did. As far as I knew, I was keeping up.... I thought everyone had to go to the doctor and had a microphone (to monitor her heart beat) with jelly on your chest. I told my friends, 'I've just been to the doctor and I hate that microphone.' They said, 'What?' ''

Thurlbeck didn't learn of her condition until she was 8. A friend overheard a conversation between her mother and Nancy, walked up to Thurlbeck and said, ``You have a hole in your heart.''

``I thought she was being cruel. I got defensive,'' Thurlbeck said. ``I told my mom and she reassured me.''

Thurlbeck's defect often corrects itself. If not, surgery is necessary.

Thurlbeck joked about the procedure beforehand, saying her friends now would have to be nice to her. Actually, her friends at Immaculate Heart High wouldn't talk to her as the date neared. They feared it was too sensitive a subject, for them and for Thurlbeck.

The surgery, at Children's Hospital, lasted five hours, but Thurlbeck was unconscious for 24.

At the time, she didn't realize how serious it was, how without surgery, her life expectancy was 30 years and she would have needed a pacemaker before she was 18.

Instead, after the initial pain subsided, her energy and appetite levels increased, she slept better, she studied better and she was happier.

``She was a changed person,'' Nancy Thurlbeck said. `'She had a more normal level of energy, which you take for granted until you don't have it.''

Thurlbeck had so much energy, she thought, why not try out for track? She had always been a fast runner, beating all girls and most boys in races around her neighborhood. She made the team and continued to feel better. Now, she hikes for hours and feels fine. She's even proud of her stomach and leg muscles.

Recently, she thought, 'Why not try the marathon?'

``It shocked the hell out of me when she told me she would do this marathon,'' said Erin Trefry, Thurlbeck's friend and her former art teacher at Los Angeles County High School for the Arts, where Thurlbeck studies. ``But it makes me love her more.''

To prepare, Thurlbeck runs almost five hours a week at school and competes in 10-kilometer and half-marathon runs. She hopes to finish in less than eight hours.

``I like to be able to finish the run put in front of me,'' she said.

And all she has to do is glance at the scar if she needs motivation.

``It's giving me more confidence in myself,'' she said, ``that I can do whatever I set out to do.''

AT A GLANCE

What: Los Angeles Marathon

When: Sunday

TV: Ch. 13 (coverage begins at 8 a.m.)

CAPTION(S):

photo, box

Photo: Mercedes Thurlbeck, 18, had heart surgery four years ago to correct an ailment. She's attempting to run the L.A. Marathon to show she's overcome it.

Box: AT A GLANCE (see text)