● Frequency of the wireless telecommunication device; ● Transmitting time, in some cases ● Shielding of the medical device and cables
These test results implied that cell phones caused a larger effect when initially transmitting a call. Once the connection was established, the severity of the effect often diminished slightly. The reason could be that cell phones adjust their power outputs depending on the distance from the operating site to the base station. To analyze the EMI risk, all observed malfunctions were categorized into five classes (lowest risk to highest risk):
I Change in output (e.g., distortion of waveform) but operation of device and safety of patient is unaffected II Transient error message display, no reset of device required III False output readings, which may affect treatment of patient IV Error message display, manual reset of device required V Operation of device is affected, but there is no visual indication.
Figure 62-5 shows the distribution of malfunctions by risk category obtained with the five wireless telecommunication devices. The analog cell phones, which had lower power outputs than the two-way radios, produced fewer device malfunctions than the radios. The 165 MHz and 818 MHz two-way radios generated fewer device malfunctions than the 460 MHz and 810 MHz radios. Of all the malfunctions, 29% were Class IV and V (the two higher risk categories). Of these, 28% were caused by two-way radios, while 1% was caused by the analog cell phones.