Postscript: Cell phone interference

March 1, 2007

A look at how cell phones, PDAs, and wireless networks interact with medical devices.

Many hospitals prominently display signs at their entrances that warn visitors to turn off cell phones. The signs state that cell phones interfere with the functioning of critical medical equipment. This turns out to be an urban legend, based on studies with analog cell phones in the early '90s.

To date, there have been no deaths reported due to cell phone interference with medical devices. However-given the ubiquitous nature of cell phones, PDAs, and wireless networks in our lives-it is worth considering how these devices interact with one another.

A brief physics lesson

EMI's likelihood is determined by three factors: the frequency and power of the transmitting device, the distance between devices, and the degree of electromagnetic shielding of the potentially affected device.1

Low-frequency devices (such as pagers, which operate in the 150 to 470 MHz frequency band) transfer minimal energy. High-frequency devices such as Bluetooth (2.4 GHz band) and wireless networks (2.4 and 5 GHz bands) are easily shielded, and usually do not cause EMI either. The mid-frequency band, where most cell phones operate, have the most potential for EMI.

Cell phones can have a power output as little as 60 mW in standby mode. But that can climb to 2 W during ringing. Since the energy level falls dramatically with distance, though, cell phones must be less than one meter from a device to cause EMI. This is a rare situation in most medical environments.1

In the 1990s, older high–power-output cell phones (up to 3 W) reportedly interfered with infusion pumps, ventilators, pulse oximeters, and infant incubator heaters. Modern cell phones have less power output, and medical equipment has improved shielding.1

However, the potential for EMI with hospital equipment still exists. In a study performed at the Mayo Clinic in 2004, six different cell phones were used with 16 medical devices. In the 510 tests performed, EMI was detected in 21.2% of the tests, although only in 1.2% was the EMI felt to be clinically relevant. Most interference occurred with older devices that displayed ECG or EEG waveforms, and rarely in ventilators, and only when the cell phone was within 32 inches of the device.2

Today's medical equipment must comply with 2002 International Electrotechnical Commission Standards, with higher standards for critical care and life support systems. The FDA even requires EMI data for all new medical devices. Most hospitals have lifted bans on cell phone use, and many have even deployed in-house cellular networks. And hospital staff routinely considers EMI a potential factor in device failure or malfunction.

There will likely be an explosive growth in the number of wireless medical devices over the next few years, as well as the introduction of new wireless technologies. We must keep in mind that we need to make sure that all our high-tech devices play well together.

DR. SCHUMAN is adjunct assistant professor of pediatrics at Dartmouth Medical School, Lebanon, N.H., and a contributing editor for Contemporary Pediatrics. He has nothing to disclose in regard to affiliations with, or any financial interests in, any organization that may have an interest in any part of this article.

References

1. Lapinsky SE, Easty AC: Electromagnetic interference in critical care. J Critical Care 2006;21:267

2. Tri JL, Severson RP, Firi AR, et al: Cellular telephone Interference with medical equipment. Mayo Clin Proc 2005;80:1286