In a "Dear Doctor" advisory sent Friday, Little Canada-based St. Jude Medical said that low levels of cosmic radiation might damage older models of implantable cardioverter defibrillators (ICDs) still implanted in about 26,000 people.

That's right -- if you wear any one of several St. Jude ICD models, you're subject to getting zapped by cosmic rays.

No Deaths or Serious Injuries

"As you may be aware, background cosmic radiation bombards the Earth constantly," reads St. Jude's letter dated Oct. 6. "While the Earth's atmosphere acts as a shield and absorbs much of the cosmic radiation, some amount of high-energy particles do arrive at the Earth's surface."

Apparently, those rays can cause a memory chip to drain the battery, sapping the devices' ability to shock the heart, according to the letter. St. Jude, the smallest of the three players in the ICD market, was able to mimic the effect in a nuclear laboratory, prompting the advisory to doctors and US regulators.

No deaths or serious injuries have been linked to the flaw in the Photon DR, Photon Micro and Atlas VR/DR defibrillators, which are used to correct irregular heart rhythms, according to St. Jude. What's more, the incident rate is very low. Testing showed that perhaps 60 devices out of 36,000 might be affected, the company said.

Affected Memory Chip Is No Longer Used

Even so, St. Jude is advising physicians to monitor patients with these devices every three months -- about twice as often as these patients usually are monitored. St. Jude urged doctors and patients to weigh the low risk of a malfunction in considering whether to replace the devices.

"[St. Jude] told me if it hadn't been for the heightened awareness about problems with ICDs, they wouldn't have sent out a communication," said Jan Wald, a securities analyst with AG Edwards.

St. Jude spokeswoman Angela Craig said that the company decided to be "conservative and cautious" in issuing the advisory. "We erred on the side of communication," she said.

St. Jude offered to provide replacement devices at no cost, according to its letter to doctors. Glenn Reicin, a Morgan Stanley analyst, said he expects about one-fifth of the devices to be replaced, or about 5,200 of those now implanted, which would cost the company about $13 million in pretax income. None of the affected models currently is on the market and the affected memory chip is no longer used.

Problems Are Industry-Wide

St. Jude is the third manufacturer of the heart rhythm devices, which cost $20,000 to $30,000, to report potential failures this year. No. 2 maker Indianapolis-based Guidant Corp., which develops and makes the devices at its division headquarters in Arden Hills, has issued warnings that affect some 80 percent of its heart devices. No. 1 Fridley-based Medtronic Inc. issued warnings on 87,000 defibrillators earlier this year because of a potential battery defect.

Analysts expect that St. Jude's action will be classified as a recall by regulators. That may help take the pressure off Guidant, whose recall problems have put it in the hot seat for much of the summer, Wald said. That Guidant is not alone in having problems should bode well for its pending $26 billion merger with Johnson & Johnson, which has been held up as regulators investigate the circumstances around the recalls earlier this year.

"This shows that these problems are industry-wide," Wald said.

Smaller and More Complex

A recent study sponsored by the US Food and Drug Administration, which regulates medical devices, confirmed the "industry-wide" nature of the problem and determined that the number of implanted heart defibrillators that fail is growing as they get smaller and more complex.

FDA spokeswoman Julie Zawisza would only say that the agency is looking into the matter.

Analysts did not anticipate much impact on St. Jude's business, and investors seemed unaffected by the news as St. Jude shares fell 3 cents to $45.92. Bloomberg News and the Associated Press contributed to this report.