Radio waves can induce currents within electrical circuits. Some electrical circuits are designed for this purpose (e.g., an antenna), which is the basis for the use of radio waves as communication signals. Unfortunately, radio waves can sometimes induce unwanted electrical currents within the circuitry of medical devices, thereby causing interference. This phenomenon is known as electromagnetic interference (EMI). Cell phones operate on radio frequencies that are reserved for their use, so even medical devices that are designed to receive data via radio waves are unlikely to have interference by mistaking a cell phone transmission for an appropriate incoming signal. Rather, cell phone radio waves can induce currents in circuits that were not designed to receive radio waves at all. Other wireless communications devices, such as some personal digital assistants (PDAs) and two-way pagers, transmit data in the same way that cell phones do and are theoretically capable of producing EMI.

The risk of cell phone use causing EMI with a medical device depends on the distance between the devices, telephone transmission power, and device construction. Risk decreases with distance, and increases with higher transmission power. A cell phone’s transmission power varies inversely with the strength of the signal it receives from its telephone tower. A cell phone receiving a strong signal will decrease its output to conserve battery life; a
phone receiving a weak signal will increase its output to ensure more reliable reception. However, power output is always at a maximum when the cell phone is ringing. Characteristics of the medical device, such as outer metal shielding, can decrease vulnerability to EMI.

The devices most commonly affected by EMI are electrocardiographic (ECG) monitors, which often display signal noise and baseline movement as a result of EMI. Interference with ECG monitors is worrisome because of possible effects on defibrillator function. One European study tested automated external defibrillator response to nearby cell phone use and demonstrated that monitor display and voice commands were often distorted; however, at no time did an automated external defibrillator fail to deliver or incorrectly deliver a defibrillator shock because of EMI. Loss of synchronization with QRS complexes has been demonstrated in a defibrillator as a result of nearby cell phone use.

Mechanical ventilators have also shown vulnerability to EMI. Effects include change of readouts, variation of operation (including changes in rate, tidal volume, and positive end-expiratory pressure), inappropriate sounding of alarms, and shutdown. Death has even occurred because of ventilator shutdown resulting from EMI caused by nearby cell phone use.