Cell Phones and Other RF Transmitters May Interfere with Medical Devices
Case example: Staff reported problems with patient monitors, but the equipment worked properly when tested in the biomedical engineering department. Problem tracking found that patient monitors experienced intermittent malfunctions mostly on one particular side of one particular floor, located above a loading dock. Citizens’ band (CB) or other mobile radio transmissions from vehicles arriving at the loading dock were strongly suspected to have been the cause of the patient monitor interference. Truck drivers were asked to use house phones on arriving at the hospital, and the problems were resolved. (Reference 1)
CHPSO received several reports of equipment failure probably related to cell phone use. In each case, the cell phone had been placed upon the equipment. In none was the clinical engineering department able to replicate the issue.
Radiofrequency (RF) electromagnetic interference (EMI) from cell phones is a recognized risk. Yet, cell phones may also present a significant benefit to health care delivery, and are increasingly being incorporated into nursing practice at many facilities. Key to balancing the benefits and risks is understanding how to reduce risks and test for possible adverse effects. Also, recognize that there are many other potential EMI sources in a hospital, including electrocautery devices, MRIs, two-way radios, wireless-enabled laptop computers and similar devices, RFID readers, two-way pagers and wireless medical devices. Two-way radios for emergency communications, in particular, may produce significant EMI.
Following are some key points to consider:
EMI risk decreases rapidly as distance of device to potential EMI source increases. For two-way radios, at least three meters separation is recommended. One-meter separation from cell phones is generally safe, and most medical devices continue to function normally at much closer ranges. Never place a cell phone on top of or in direct contact with a device. Walls, excluding those specifically built with shielding (e.g., MRI) cannot be treated as EMI shields, so this separation distance needs to be honored through walls, as well.
Leads and wires act as antennas. Testing for EMI should mimic the layout encountered during the incident to replicate the antenna structure that was present.
Cell phone transmission power varies with signal quality. Cell tower locations and hospital design may result in markedly varied cell phone EMI potential within the building. Improving cell phone reception by installing microcells or other communications infrastructure in the institution can reduce EMI risk.
Ad hoc immunity testing of devices (Reference 2) should be considered when EMI susceptibility is unknown, separation of device from transmitter is less than the manufacturer’s recommendation, the device will be in an uncontrolled environment (e.g., cafeteria), or when new RF sources or devices are likely to operate beside each other. However, if the new RF source transmits less than 10 mW, it is unlikely to pose a problem unless placed closer than 20 cm to a medical device.
CHPSO will be producing a full report on cell phone use, policy considerations, and ad hoc testing methods; circulating to its members for comment, then releasing to the general public. Look for it in your email soon.