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 -- although no one knows if the lifesaving implants work as well under the stress of high-intensity activity.

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? 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.

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.

Defibrillators once were implanted mostly to prevent a second cardiac arrest in lucky first-time survivors. Now, at least than 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. Team physicians disqualify defibrillator recipients in college and professional sports more than in high school or community-level competition. 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.

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.

To volunteer, contact a cardiologist or email Yale at icdsports.registry@yale.edu.

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