People with pacemakers used to be told to steer clear of microwave ovens because of possible interference microwaves could cause to the electrical device implanted in their chests. Now they must worry about cell phones, security systems, magnetic resonance imaging (MRI), even therapeutic magnet mattress pads.

In the United States, nearly 2 million people have pacemakers and 250,000 have implantable cardioverter defibrillators - other devices to help the heart beat in a regular rhythm. Household electric and electronic devices aren't a problem for patients with cardiac pacing devices. And microwave ovens no longer present a danger because both the ovens and the medical devices of today are better shielded.

However, there are new threats to the pacemaker's circuitry in our gadget-happy culture. People now worry whether the electromagnetic fields in the environment created by cell phones, security gates, and magnets can turn off implantable cardioverter defirillators (ICDs) and alter pacemaker function, and affect other medical devices, such as spinal cord stimulators.

"Most environmental exposures (to electromagnetic fields) are not powerful enough or exposure is not long enough to be an issue," says Thomas A. Mattioni, MD, director of electrophysiology at Arizona Heart Institute, Phoenix.

Patients who have cardiac pacing devices implanted are told to avoid strong magnetic fields. They should never have an MRI scan, for example, because an MRI has a high magnetic field strength and the exposure is prolonged.

Mattioni and other researchers looked into the potential dangers of therapeutic magnets after some patients with cardiac pacing devices reported using them to relieve arthritis and muscle pain. The team's research, presented before the 21^st annual Scientific Session of the North American Society of Packing and Electrophysiology (NASPE) last month, found that while most therapeutic magnets are generally safe, mattress pads containing therapeutic magnets are not.

The 6-inch rule

Magnetic mattress pads can deactivate ICDs and pacemakers, according to the researchers. The difference between magnetic mattress pads and other therapeutic magnets, such as a headband, is in distance - the closer the magnet, the stronger the magnetic field.

To deactivate an ICD or alter pacemaker function, a magnet has to have a field strength of 10 gauss (a unit of magnetic strength) or greater next to the surface of the device. A magnet in a stereo speaker has a field strength of 100 gauss at the surface - but 0 gauss 6 inches away.

"If it's kept 6 inches or greater, it has no effect,'' Mattioni says of a magnet.

The problem with magnetic mattress pads isn't so much that they have a strong magnetic field strength - even though manufacturers claim the pads have a magnetic field of 15,000 gauss - but that a patient with an ICD or pacemaker would come closer than that 6-inch zone.

A short exposure is not a major risk, but the real problem is that with a mattress pad there is continuous exposure for the duration of sleep, and that, Mattioni says, "is potentially life threatening."

"When a magnet deactivates an ICD, it's like not having the device at all,'' says Paul Van Lake, field clinical engineer for St. Jed Medical, Sylmar, CA, and principal researcher of the magnet study. "It tells the device not to deliver therapy, which is a serious problem if the patient has an arrhythmia (irregular heartbeat)."

Being careful with cell phones

NASPE also recommends that patients with ICDs or pacemakers use the 6-inch rule when using cellular phones.

A study in the 1997 New England Journal of Medicine showed that there is indeed a risk of interference of the pacemaker and ICDs by cell phones, although it appears to be small. According to the NASPE, the interference is generally temporary. Moving the phone away from the pacemaker will return it to proper function.

Nevertheless, the NASPE and some healthcare professionals recommend patients take the following precautions:

• Maintain a distance of at least 6 inches between the cellular phone and your pacemaker.

• Do not carry a cell phone in a shirt pocket on the same side as the pacemaker while the telephone is on.

• Hold the device away from the body while dialing.

• Use the ear on the side away from the pacemaker when using the cell phone.

• Turn the cell phone off if possible when carried in clothing, etc.

Playing it safe with security systems

Anti-theft systems and metal detectors commonly found in airports and government buildings have the potential for interfering with the normal operation of pacemakers and ICDs, according to the Food and Drug Administration (FDA).

In 1998, Stuart Portnoy, a physician at the FDA who reviews manufacturer's marketing applications for cardiac devices including pacemakers, reported that in the previous 10 years there had been 46 adverse events reported to the FDA of interactions between security systems and medical devices.

Two thirds of the severe adverse interactions occurred between metal detectors and medical devices, most of them pacemakers, and ICDs. Severe interactions including those that were fatal or life threatening, resulted in permanent or significant impairment, or required surgical intervention or hospitalization.

Portnoy, speaking at an FDA Safety Standards Committee meeting, told of a woman in Phoenix whose heart sped up whenever she walked through an acoustomagnetic gate. "She gets palpitations, dizziness, nausea, and so on," Portnoy reported. "And when she walks out, she is okay."

Just who is responsible for minimizing interactions between medical devices and security systems and metal detectors (the electronic-field producer or the implanted-device producer) is much debated, and technical solutions aren't always simple. As a result, the FDA offers an easy, practical remedy for patients with pacemakers, ICDs or other medical devices - "Don't linger, don't lean."

"It's not a major health problem,'' Mattioni says. Someone with a pacemaker or ICD may set off the alarm walking through an airport Metal detector, but as long as people keep moving through - "don't linger, don't lean" - they really shouldn't worry about the detector interfering with a medical device.