Pushing the limit?

If you have a heart-zapping defibrillator implanted in your chest, you're not supposed to compete in sports any more intense than bowling or golf.

Lots of patients ignore those guidelines, trying everything from school basketball teams and community tennis leagues to running marathons and rock climbing -- although no one knows if the lifesaving implants work as well under that kind of stress.

A new nationwide registry is recruiting these determined athletes, aiming to track them and finally settle whether they're taking a big risk in pursuing a beloved sport.

Do their hearts need shocking more often than if they sat on the sidelines or tried gentler exercise? Do the implants fire hard enough to overcome the physical duress and surge of adrenalin? Could a sport's repetitive motions or a hit to the chest break the implants?

With little scientific evidence to tell, leading cardiac guidelines recommend against most competitive sports for recipients of "implantable cardioverter defibrillators," or ICDs.

But with more teenagers and young adults receiving the implants as a precaution against cardiac arrest -- often before they ever have symptoms -- how much to curtail activity is becoming a tougher question.

"We all take care of patients with ICDs, and we know that lots of them just say, 'I don't care what you say, I'm going to do this anyway,'æ" says Dr. Rachel Lampert, a heart specialist at Yale University School of Medicine who is leading the new study.

"We should try to gain some data on whether it is or isn't safe."

More than 100,000 defibrillators are implanted a year in people at risk of a life-threatening irregular heartbeat, because of damage from a survived heart attack, genetic disorders or other conditions. An implanted defibrillator constantly checks for abnormal beats and automatically zaps the heart to short-circuit any dangerous arrhythmia it senses forming.

The past decade brought a major shift: Defibrillators once were implanted mostly to prevent a second cardiac arrest in lucky first-time survivors. Now, at least 60 percent are implanted as a precaution to prevent that first strike, says Dr. Bruce Lindsay of Washington University School of Medicine, president of the Heart Rhythm Society.

That's where the most angst occurs. These so-called "primary prevention" patients tend to be younger, healthier and may never have noticed symptoms.

But even people who have survived a cardiac arrest can be reluctant to quit their sport. Among well-known cases: University of Washington basketball player Kayla Burt was allowed to return to the team after her implant; she quit in January 2006 after the defibrillator fired during a game. In the 1990s, basketball player Nick Knapp sued Northwestern University when team doctors benched him because of his defibrillator; he eventually gave up the court battle and played briefly elsewhere.

Amanda Kurovski, 16, of Ankeny, Iowa, got her parents to sign a release saying they knew the consequences so that she could resume high school track and volleyball after getting a defibrillator in July 2006. She was diagnosed with a genetic abnormality in the heart's electrical system called long QT syndrome after her father found her not breathing.

"I could be shocked during this because my heart would be accelerating," she said. "If we want to take that risk, then we can. I wanted to, because I don't like not doing stuff."

Team physicians disqualify defibrillator recipients in college and professional sports more than in high school or community-level competition, but it's judged case-by-case, says Ron Courson, the University of Georgia's sports medicine director and spokesman for the National Athletic Trainers' Association.

How common are athletes with defibrillators? There are no good counts, but a survey of more than 600 heart specialists last year found almost three-quarters had patients who kept competing, particularly in basketball, running and skiing.

Forty percent reported their athlete-patients had experienced ICD shocks during the sport. There were few reports of serious consequences, including two "shock failures" suggesting further treatment was needed, and two people who fell and suffered head injuries when the ICD zapped.

That survey couldn't assess patient safety. Enter the new registry, which is funded by major defibrillator manufacturers but has researchers at influential heart hospitals guiding the research and recruiting 800 patient-athletes.

The registry does not condone competitive sports for defibrillator patients, Lampert stresses.

But when someone like Amanda Kurovski, the Iowa teen, who's competing anyway signs up, it will track his or her medical history and how the implant fires over two years. (To volunteer, contact a cardiologist or e-mail Yale at icdsports.registry@yale.edu).

"It's quite likely that arrhythmias will be more likely during sports," says Lampert, whose previous research shows emotion and stress can trigger them. "But if the defibrillator works and the patients are willing to accept that they might get shocked, it's an issue of quality of life."

Whatever the registry finds, athletic trainer Courson offers an important reminder: Most cardiac arrest victims didn't know they were at risk, so ask if your coaches know what to do when someone collapses. An external defibrillator saves lives, too, but only if used within minutes.