July 30, 1999 DALLAS -- People with implanted defibrillators that shock the heart to regulate its rhythm may safely walk through electronic anti-theft systems, but should not linger there, according to a study in today's Circulation: Journal of the American Heart Association.
The anti-theft machines use an electronic beam to detect security-tagged merchandise being carried out of a store. The defibrillator can interpret the electronic beam as a rapid heartbeat for which the defibrillator is programmed to deliver a shock.
Douglas P. Zipes, M.D., the study's lead author, says, "There is absolutely no danger from a slow stroll through the gates, even if it takes 10 or 15 seconds."
In the study one person went through in a walker without any problems. "Now, one should not lean on or linger in theft detection devices because adverse events can happen, no question. But in normal use, there should be nothing to worry about," says Zipes, Distinguished Professor of Medicine and Pharmacology at the Indiana University School of Medicine in Indianapolis.
An estimated 400,000 people worldwide have implanted defibrillators and as many as 800,000 security systems are in use around the globe, Zipes says.
There were three recent reports of anti-theft systems causing defibrillators to fire unnecessarily, but in each case the individual lingered in the machine and in at least one instance the patient was holding on to the transmitter pole.
Previous studies were either too problematic or too small to clearly say whether the theft detectors were harmful or safe, according to Zipes.
Researchers at the Indianapolis Medical School, Methodist Hospital in Indianapolis and the Southwest Florida Heart Group in Fort Myers decided to investigate the device's response to normal use, he says.
"We attempted to replicate a normal situation of someone shopping and just passing through the machines," he says. In tests on 169 patients, they found no problems from a normal, 10-to-15 second walk through the security gates.
The researchers used a magnetic programmer tool placed over the defibrillators to reprogram the patients' implants during the tests. Although the devices continued to record data on heart rate and whether a shock should be delivered, the shocking mechanism was turned off so that the patients were in no danger of having their devices fire during the test.
"When an implanted defibrillator fires, it is like a giant mule kick in the chest. It's not pleasant," Zipes explains.
When the patients received extreme exposure by standing within six inches of the transmitter pole for two minutes, 19 of the 169 individuals had evidence of defibrillator-surveillance system interference. In 12 of those, the interference was only minor and was considered clinically irrelevant by researchers. However, seven experienced significant incidents in which the defibrillators would have or might have fired, Zipes says.
The researchers' findings concur with a Food and Drug Administration letter to physicians in September 1998, which said that although the anti-theft machines appeared to be safe, patients should not lean on or linger in the machines. That same letter advised patients undergoing scans with hand-held metal detectors, such as those used in airports, to warn security personnel that they have an electronic medical device and ask them not to hold the metal detector near the device any longer than absolutely necessary.
The current study was partly supported by a grant from an electronics company which makes some of the most common security devices. "However, physicians wrote the protocol, then collected, analyzed and interpreted the data and wrote the abstract and the manuscript without company interference," Zipes says.
Co-authors were William J. Groh, M.D.; Scott A. Boschee, B.S.; Erica D. Engelstein, M.D.; William M. Miles, M.D.; M. Erick Burton, M.D.; Peter R. Foster, M.D.; and Barry J. Crevey, M.D.
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