Key words: electromagnetic compatibility, electromagnetic interference
To promote awareness of the potential for problems stemming from electromagnetic interference (EMI) and cooperation toward solutions, CDRH and AAMI co-sponsored a 2-day forum entitled, "Electromagnetic Compatibility for Medical Devices." The forum was planned and initiated by the CDRH EMC Working Group, which was formed by the Deputy Director for Science for CDRH and charged with assessing medical devices EMI and coordinating CDRH solutions. The forum was conducted this past May in Anaheim, California, in conjunction with the annual AAMI conference.
The goal of the forum was to present the concerns of CDRH about medical device EMC and strategies being developed to address these concerns. The concerns for device EMI have been heightened in recent years because of the increased use of sensitive electronics in many devices, and several reports of device malfunctions relating to EMI. This comes at a time when there is a dramatic increase in the number of portable radio transmitters, from cell phones to the "wireless" computer communications. OST representatives moderated the presentations by representatives from the medical device industries, the electromagnetic source (e.g., radio, cellular telephone, AC power) industries, clinical device users (clinicians and biomedical engineers), the legal profession, and the Federal Communications Commission. Feedback from the presenters and audience indicated that there was a spirit of openness and cooperation which many had not experienced in dealings with FDA. The proceedings will be published in FY 96. [ProA]
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Key words: pacemaker, cellular telephone, electromagnetic interference
Testing has been performed on 24 different implantable cardiac pacemakers for EMI from digital cellular telephones. This work was done in response to public health concerns based upon information that digital cellular phones could interfere with the normal operation of pacemakers. Laboratory results indicate that about eight of the devices tested reacted to a digital cell phone (i.e., U.S. TDMA digital, GSM digital, or MIRS digital). Most reactions were only with the MIRS cellular phone technology. All but one device ceased reacting to the cell phone when the phone was vertically beyond about 9 cm from the pacemaker submerged in saline. This distance is much less than the 15.24 cm (6 inches) recommended in the draft HIMA labeling for implanted cardiac pacemakers. However, one device continued to react even when the cell phone was at a distance of more than 36 cm. The test results also indicate that there are several devices in which EMI could not be induced. Further testing is needed to assess the validity of our findings on a wider range and number of pacemaker devices.
Figure 06 illustrates the exposure set-up for testing the pacemakers. A torso simulator (saline filled tank) was used for all tests. This set-up allows for the precise mapping of the area of interaction (if any) in a plane above the pacer generator surface. The pacer generator was placed at two distances (1 cm and 0.5 cm) below the saline surface to examine the effects of implantation depth on any EMI. In most cases, an actual cellular telephone was used to expose the pacer device. However, some exposures were done with a dipole antenna (a more standardized radiator source) to simulate the cellular phone.
Illustration of the exposure setup for testing pacemakers. An actual cellular phone was used in most cases to expose the pacer device.
The major factors that seem to correlate with the EMI of pacemakers include cellular phone pulse rate and output power, pacer device and manufacturer, pacer cardiac signal sensitivity, depth of pacer below saline solution surface and, to a lesser extent, the polarity of the leads (unipolar/bipolar) and phone orientation. The results also impact implantable cardiac defibrillators (ICDs) because of the many similarities between these devices. Indeed, very preliminary EMI testing indicates that ICDs are also susceptible to cellular phone EMI.
The sample devices were obtained from voluntary loans by the pacemaker manufacturers. Each of the manufacturers who lent devices for testing also provided equipment to program and interrogate the pacers. They were also allowed to observe and comment on some of the preliminary laboratory testing. This work is being coordinated with a similar ongoing project at the University of Oklahoma. [ProA]
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Key words: electromagnetic compatibility, electrostatic discharge, power quality, intendeduse environment
FDA requires a reasonable assurance of safety and effectiveness for Class II and III medical devices in their intended-use environment. CDRH, historically, has addressed the issue of intended-use environment on a case-by-case basis. In recent years, CDRH has attempted to address the electromagnetic aspects of the environment in a more uniform and proactive manner. OST is now expanding this approach to encompass additional environmental factors.
OST began in FY 93 providing engineering support to postmarket evaluation of electrostatic discharge (ESD) incidents affecting medical device performance. Since then, OST has expanded its involvement in ESD issues with representation in the standards development process and with development of in-house testing capability.
In FY 95, OST initiated a program studying power line disturbances and their effects on medical device performance. As a first step, the current situation in a representative medical facility was assessed. OST conducted an electromagnetic interference and electromagnetic compatibility (EMI/EMC) risk assessment at Walter Reed Army Medical Center in cooperation with the engineering staff at the medical center. The purpose of this study was to determine the power quality environment within the medical facility. This documented the types of power quality problems that might be encountered by medical devices in a hospital environment. In parallel with the testing conducted at Walter Reed, OST staff conducted a survey of applicable standards and general literature on power line quality. The staff also investigated and evaluated test equipment that can be used in the laboratory to simulate such power line disturbances as voltage sags, swells, surges, and transients. OST established a liaison with the Electric Power Research Institute (EPRI). There are ongoing efforts to participate in the EPRI Health Care Initiative that addresses power quality in the health care industry.
Over the long term, medical device performance under actual conditions of use is expected to improve. Experience with EMC has shown that, as the industry is made aware of problems with interference and is given guidelines regarding needed design specifications, medical device performance has improved.
In FY 96, this project will develop a test protocol for evaluating the performance of selected medical devices subjected to typical power line disturbances. [PreME, PostME, Stds]
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Key words: electromagnetic interference, ventilator
During the last year, OST scientists have received reports of ventilator malfunctions due to radiofrequency interference (RFI) generated by various radio sources. The environment is increasingly filled with radiofrequency (RF) sources from personal transceivers, cellular telephones, mobile radios, and fixed-base station transmitters. In response to these reports, OST scientists embarked on a program to test ventilators for susceptibility to radiated RF fields. Tests were performed using methods described in the IEC 1000-4-3 standard for electromagnetic compatibility (EMC) testing and measurement techniques.
A servo-ventilator was tested in a gigahertz transverse electromagnetic (GTEM) cell using standardized methods for interference of function from RF electric field strengths of 3 and 10 volts per meter. The test RF fields from 30 to 1000 megahertz (MHz) were modulated using three different waveforms: 0.5 Hz square wave, 1 KHz kilohertz 80% amplitude modulated (AM), and unmodulated constant wave (CW). The performance of the ventilator was monitored by a ventilator tester that measured breaths per minute, tidal volume, minute volume, and relative pressure. A personal computer controlled the strength of the RF electromagnetic fields (EMI) and collected data from the ventilator tester.
Interference patterns were compared to determine which modulation caused the most ventilator performance degradation. The ventilator was most susceptible to RF electromagnetic fields when it was tested in the infant mode. For 10 V/m exposures using 1 kilohertz (kHz) 80% AM modulation, complete cessation of operation was observed for a narrow frequency band between 157 to 168 MHz. The data shown includes 20% error brackets. RF electric fields at frequencies that cause the ventilator performance to degrade to values outside of the 20% performance limits are considered failures due to RF EMI. Significant performance degradation also occurred from 130-345 MHz, 290-295 MHz, and 820-865 MHz.
In the infant mode, when exposed to 10 V/m RF electric fields, the ventilator tidal volume performance varied beyond the 20% limits for the 0.5 Hz square wave modulation in the 680-825 MHz frequency range. The 1 KHz, 80% AM modulation created the most interference with normal operation from 130-345 MHz. Complete ventilation cessation occurred during exposures to frequencies of 157 to 168 MHz. The unmodulated RF did not interfere with the ventilator as much as the other modulations that were used. Different modulations would sometimes cause the ventilator to perform differently at given frequencies. At 160 MHz, the 0.5 Hz modulation caused the relative pressure to increase, whereas the 1 kHz modulation suppressed the relative pressure. The unmodulated RF caused broader interference patterns below 350 MHz. Of the three sides tested, the front exposure yielded the greatest sensitivity.
The duration of exposure at each frequency did affect the test results. The ventilator tester required two breaths to acquire data at each frequency tested. The pilot testing was done at 5 MHz intervals. For the case of the ventilator in the infant mode, this corresponded to a dwell time of 11 seconds at each frequency tested. When the capability to measure relative pressure was added to the testing protocol, the time to acquire all of the data at each frequency increased to 16 seconds. When detailed testing was conducted at 1 MHz intervals, frequent alarm sounding occurred, and failure magnitudes were significantly greater than those measured during the pilot testing.
The GTEM cell and the computer-controlled EMC testing facilities in CDRH provide the capability to effectively evaluate the performance of medical devices, such as ventilators, in the presence of RF fields. Devices can be exposed to RF fields with different amplitude modulations and field strengths to determine how they react to the different parameters. RFI testing in the GTEM cell provided a simple, repeatable means of testing the ventilator over a broad frequency range. A reviewer guidance for environmental and electromagnetic compatibility testing for respiratory devices resulted from this work. [PostMS]
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Key words: electromagnetic inteference, hearing aids, cellular phones
As a result of recent reports of hearing aid interference from digital cellular
telephones, the Office of Device Evaluation requested that OST conduct a
laboratory study to quantify the extent of the interference. Seven "in-the-ear"
(ITE) hearing aids were tested for audible interference using a stethoscope
earpiece and tubing at various distances from two types of digital cellular
telephones. Interference, which took the form of a buzzing tone, was perceived
up to 47 cm away from the transmitting phone. Within a few centimeters of
the phone, the induced buzzing tone precluded normal use of the phone. The
level of induced interference in the ITE hearing aids as well as six behind-the-ear
(BTE) hearing aids was quantified using a sound pressure level (SPL) meter
with a calibrated microphone. The highest interference measured was slightly
above the normal threshold for pain within a centimeter of the phone.
These preliminary results demonstrate that there is not an immediate health
risk to hearing aid wearers from cellular telephone use in their general
vicinity. However, results also show that the hearing aids tested could
not be used in conjunction with GSM and US TDMA digital cellular telephones.
In some cases, potentially dangerous tones were produced within a couple
centimeters of the phone. Further testing will be conducted on a larger
representative sampling of hearing aids in order to verify these results.
Information learned from these experiments will be incorporated into revised
testing protocols in future hearing aid standards and guidances. [ProA]
Continue to the Electrical Stimulation Medical Devices section.
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