Medical Devices and Electromagnetic Interference

by Food and Drug Administration (FDA)

How could a nurse work without a beeper? An ambulance without a two-way radio? A doctor without a cellular phone?

Today's medical professionals rely heavily on wireless communication devices to help them do their jobs efficiently. And yet the proliferation of such gadgetry is not without its problems. Increasingly, medical and communications devices may be at odds with each other.

The problem is electromagnetic interference (EMI), and it's becoming a growing concern among hospital staffs, electronics manufacturers, and the Food and Drug Administration. Every electrical device emits electromagnetic energy. This energy can interfere with other devices the way a hair dryer creates "snow" on a nearby television.

Most of the time the problem is merely annoying. For example, EMI could cause static on the screen of a hospital computer. But whenever anything interferes with a lifesaving medical device like a pacemaker or an apnea monitor, the results can affect the patient.

Between 1979 and 1993, FDA received reports of more than 100 suspected incidents of EMI with medical devices. Because the interference was almost always fleeting and difficult to reproduce, most of those reports have not been verified or duplicated in laboratory settings. Nevertheless, FDA suspects EMI caused most of them, including the following:

• A pacemaker failed during an ambulance ride while the two-way radio was in use.
• A man in a powered wheelchair was seriously injured when his chair rode off a cliff at high speed. He was several miles from a radio tower and three blocks from a busy road, where mobile radios were likely in use.
• A pulse oximeter machine displayed a pulse rate and oxygen level on a dead body when a telemetry receiver that was part of the system was placed too close to the body.
• A fetal heartbeat detector picked up local radio and CB broadcasts instead of the baby's heartbeat.

As wireless technologies proliferate and the airways become more crowded, EMI is bound to increase. However, FDA, the medical device manufacturers, and members of the electronics industry are taking steps to minimize the danger it poses.

It will not be easy to make all medical devices immune to unwanted electromagnetic waves. The exposure, frequency, location, orientation, and design of a device all influence whether it will experience EMI.

"It's a complex phenomenon, and we don't yet know how it occurs in some cases," says Don Witters, an FDA physicist and chairman of an agency working group examining the problem.

"There are large uncertainties here. You can make a device immune to a certain level, but it really depends on several complex things interacting."

Clutter on the Airways

Sources of possible electromagnetic interference increase every year. Citizens Band radios, cellular telephones, wireless computer links, microwave signals, radio and television broadcast transmitters, pagers, and many other machines emit electromagnetic waves that could interfere with other devices.

For practical purposes, it's impossible to stop electromagnetic waves completely at their source. Modern society has become much too dependent on the convenience of instant communication. And since medical devices themselves often emit electromagnetic waves, using several machines at once in a hospital room can cause problems.

It is much more feasible to build electromagnetic compatibility (EMC) into new medical devices, so they can operate accurately in an environment flooded with electromagnetic waves, and so they don't give off any more waves than necessary.

"There's really no place to get away from electromagnetic waves. They're in the room, they're in the air," says Witters. "Even the body itself is electrical and acts as a transmitter for energy. Your body can generate electrostatic energy by walking across a wool rug on a winter day."

In an effort to investigate EMI problems with medical devices, FDA has already examined a few devices that are especially sensitive to electromagnetic waves.

Apnea monitors, for example, can be very sensitive. Used on premature babies and adults with sleeping disorders, the monitors are supposed to sound an alarm if the patient stops breathing while asleep.

In 1987, a physician in Nebraska reported to FDA that monitors in some neighborhoods of Omaha would not work properly. FDA tested monitors both at the site and in the laboratory and found that certain models are, indeed, very sensitive to electromagnetic waves.

The monitors can mistake low levels of modulated electromagnetic waves for breaths, therefore failing to sound the alarm if the patient stops breathing. Some monitors were so sensitive that a person walking across the room changed the waves in the room enough to fool the monitor.

FDA asked the manufacturer of the most sensitive monitor to recall the device. This machine malfunctioned when the monitor or the cables were touched or when an electrostatically charged fabric was waved over it.

While not as sensitive as the apnea monitors, powered wheelchairs may also encounter EMI-related problems. FDA has tested these and powered scooters in its laboratories after receiving a number of reports of the machines malfunctioning.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.