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Electromagnetic interference in healthcare environment
Chapter 55
Electromagnetic interference in healthcare environment Yadin Davida,b, W. David Papermanc a
Biomedical Engineering Consultants, LLC, Houston, TX, United States, bUniversity of Texas School of Public Health, Houston, TX, United States, cClinical Engineering Consultant, Cut and Shoot, TX, United States
Considerations for EM spectrum use in healthcare facilities The density of users on the electromagnetic (EM) spectrum has been increasing because of the expansion of wirelessbased services. The desire for mobility and anywhere connectivity creates conflicts between a variety of EM waves energy and intelligent devices or even human tissue (CBS, 1994). Despite the efforts by manufacturers to harden clinical devices to the effects of electromagnetic interference (EMI), reports of incidents of interference to previously unaffected medical devices appear in medical and scientific literature and anecdotally (CBS, 1994). Therefore, the role and knowledge base of clinical engineering practitioners must expand to include command understanding and proper management of these ever-increasing challenges (Paperman et al., 1994). According to research conducted at McGill University by Segal B. and colleagues “Wireless technology has been evolving from networks with a small number of relatively high-power sources to networks with a large number of relatively low-power sources. This (Muhlen et al., 2008) has made characterizing the hospital EM environment a complicated process and the associated potential for EMI difficult to determine. There is no globally accepted comprehensive protocol to evaluate the function/malfunction of medical devices exposed to radiofrequency (RF) fields. In addition, there is often uncertainty in determining the degree of clinical impact that a malfunction has.”
Electromagnetic radiation EM radiation occurs when an alternating current is generated. An EM field is created in the vicinity of this source (National Research Council (US), 1993). The range, or distance of the radiated EM field can be increased when coupled to a conductor. The magnetic field is further radiated 362
by the flow of the current along this conductor. Even at a reduced amplitude, a corresponding current will be generated when another conductor is subjected to the field of the radiating conductor. The radiated field has many characteristics. The most important of those characteristics are amplitude, periodicity, and waveform. The amplitude of the impressed alternating current defines the energy of a radiated field. This field is generally expressed in volts per meter (V/M). Amplitude at the source point (transmitting conductor) will define, in conjunction with frequency and distance, the amplitude of the EM field induced in the secondary (receiving) conductor subject to the inverse-square law. The periodicity or frequency of alternation is expressed in Hertz per second, which allows the calculation of the wavelength of the radiated EM field. Knowing the wavelength versus the physical length of both the radiator and the secondary conductor into which the radiated energy is induced provides an estimate of the potential amplitude developed by or within a device at risk (AAMI, 2010). Waveforms other than linear (sinusoidal) can produce multiple and variable frequencies. The foremost example of a nonlinear waveform is the square wave. The square wave produces many multiples, or harmonics, which have a range many times that of the fundamental frequency. Most digital devices are capable of generating square waves. Depending on the amplitude at the point of generation, the efficiency of the auxiliary radiator (antenna), the generated frequency, and the waveform, the potential for interference between digital devices and clinical devices can exist for many kilometers from the source of EM radiation. EMI threats come from a multitude of sources (Davis, 2019). These are broadly divided into two categories: devices that emit intentional EM radiation for communications and control, and devices that, as a by-product of their operation, emit unintentional (incidental) radiation (Fig. 1). Some of the major and more common sources of EMI Clinical Engineering Handbook. https://doi.org/10.1016/B978-0-12-813467-2.00056-0 Copyright © 2020 Elsevier Inc. All rights reserved.
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Radiator intentional
Target intentional
intents and purposes, into four parts: mitigation, detection, correction, and prevention. Although the mitigation and prevention phases appear to be similar, they differ in their approaches and applications.
Controlling the effect of EMI Mitigation
Radiator unintentional
Target unintentional
FIG. 1 The complexities of EMI-radiators and targets: culprits and victims.
e ncountered in the clinical environment are given in the following (Paperman et al., 1996): Intentional radiators: Television broadcast stations: Analog and digital Commercial radio stations: Analog and digital Land mobile radio: Fixed base (FB), mobile, and portable two-way radio sources (walkie talkies) Paging: One-way and two-way wireless messaging service transmitters Cellular telephones and sites Wireless personal digital assistants (PDAs) Wireless networking devices (an increasing threat in the 2.4-GHz industrial, scientific, and medical ISM band) Unlicensed and unauthorized users of two-way radio communications equipment (Pirates) Wearable personal instruments LASER instrumentation Unintentional radiators: Lighting systems (especially florescent), including energy saving electronic ballasts High-energy control systems (HVAC controllers), especially variable-speed controllers Malfunctioning electrical services Universal-type electric motors Pulse oximeters Displays (CRT and plasma), computer, television, and instrumentation Wired computing networks “Smart” fire detection and alarm devices Electrosurgical units (ESU) Defibrillators Electrical physical therapy equipment These sources of EM radiation, as seen from the previous lists, can be intentional or unintentional (ANSI, 1991). The challenges and the institutional responses imposed by the presence of these EMI sources can be divided, for all
In the mitigation phase, the principal concern is reducing risk to, or victimization of, clinical devices by devices emitting unintentional or intentional EM radiation. The sources of the devices may be internal or external to the institution (Knudson and Bulkeley, 1994). The devices may be intentional or unintentional radiators of EM fields, and the EM energy may be radiated or conducted. Fig. 1 illustrates the EM interaction among intentional and unintentional radiators and intentional and unintentional targets. Mitigation is accomplished in part by the careful analysis of a device (ECRI, 1992), including the accessories to be used with it, and the EM environment (US Government, 1991) in which it will be used. A successful approach has been implemented at several care environments. This approach is accomplished in two phases: thorough analysis of the environment in which the device is to be placed (Bennett, 1993), called “footprinting,” and an equally thorough analysis of the clinical device itself, called “fingerprinting.” The procedures of quantifying potential EMI and mitigating the effects through environment analysis are described in greater detail in the following. Another vital part of the mitigation process occurs during facility planning. Whether planning a new facility, an expansion of an existing facility, or remodeling of a facility, clear and concise communications between affected departments, architects, contractors, and the Clinical Engineering Department are vital at all stages of the project (ECRI, 1993). For example, one institution installed expensive, screened rooms at great cost. When the clinical engineer, acting on complaints of erratic EMG operation, determined that the room played a role in the culprit/victim relationship, the rooms were disassembled, also at great cost. In this case, the relocation of the affected department was not an option. The Clinical Engineering Department had not been made aware of these rooms, or of their intended application that involved the use of known culprit devices.
Footprinting At our healthcare provider institution, the EMI testing program has evolved from years of experience and analysis. This program is not static. Sources of EMI and susceptibility characteristics of devices are in constant change. As new threats arise, the plan is periodically reviewed and modified to contend with them. Further modifications to the plan and
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procedures are based on a continuous review of wireless industry trends. They represent a proactive response to perceived future threats. At present the program consists of the procedural plan outlined above and three series of tests: (1) area characterization and footprinting (Fig. 2), (2) device characterization, fingerprinting (Fig. 3), and ad hoc susceptibility testing according to IEEE guidelines (Knudson and Bulkeley, 1994). Footprinting an area means performing a series of spectrum/amplitude scans for EM radiation in a defined or designated area. Footprinting is an ongoing technique that defines EM radiation in a clinical facility or facilities in a multibuilding campus. Footprinting is also done on request from a department experiencing performance degradation of clinical, diagnostic, or therapeutic devices when EMI is the suspect. The procedure involves a series of 20-MHzwide spectrum sweeps, beginning at 2 MHz and ending at 1 GHz with antenna(s) in the horizontal plane. This procedure is repeated with the antenna(s) vertically polarized. Tunable standard antennas are adjusted for the correct resonant length to the center of each 20-MHz window. The footprinting procedures yield two results: an overview of the radiated EM fields presents at a specified location in the environment and the amplitudes and types of emission of those fields. Several incident investigations have been successful using this information. In cases where EMI has been site originated, the source has been removed and the problem has been corrected. In some cases, the affected device required maintenance to correct the problem. An added value of footprinting is that the data obtained during the process meets the basic requirements for a site
Calibrated antenna systems Biconical: Low frequency rod, dipoles,
Communications spectrum analyzer
Laptop computer
Printer
Data Archival storage FIG. 2 Typical equipment configuration for fingerprinting and footprinting.
Device Under Test
1-m distance
Antennas Selected and adjusted for frequency range of emissions under examination
Test equipment configured for Fingerprint
FIG. 3 Fingerprinting.
search similar to the OATS procedure for fingerprinting individual devices. The results of the footprinting scans are transferred to a storage medium and filed. They form a comparison database that is used to evaluate new devices before introducing them into a specified area.
Fingerprinting In the process of fingerprinting, a device has several steps. Again, the spectrum analyzer and calibrated antenna system are the primary tools used to analyze the device under test (DUT) (see Fig. 3). To minimize the loss of information from the DUT due to masking by other sources of EM radiation, areas within the clinical physical plant should be tested using the footprinting procedure until a relatively quiet area is found. For the fingerprinting procedure, as in the footprinting procedures, the standards antenna should be located as far as possible from any conductive material. The standards antenna should be located at a height equal to the center of the DUT. Due to constantly changing EM fields and frequencies, they execute the footprinting procedure in the selected area immediately prior to fingerprinting the DUT. Once the data from the latest footprint has been stored, place the DUT on a nonmetallic stand located as far as possible from
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any conductive material. The standard antenna(s) is then mounted on a tripod and, in a horizontally polarized mode, placed 1 m from the front of the DUT. As in the footprint procedure, a series of 20-MHz-wide spectrum sweeps are performed, and the results are recorded. This procedure is repeated with the antenna(s) vertically polarized. When using tunable standard antennas, adjust them to the correct resonant length to the center of each 20-MHz window. Based on the review of the data collected, any emissions attributable to the DUT should be rescanned. To increase detail in the frequency range in which the emissions were observed, the spectrum analyzer window is narrowed to a 200-KHz/division or smaller (e.g., 5 kHz/division) sweep width. Fingerprinting tests are conducted for two reasons. First, compliance with hospital policy on acceptance testing requires that representative samples of all devices be tested prior to entering the clinical environment for the first time. Second, both theory and experience have demonstrated the need for device testing. A device that radiates unintentionally can victimize other devices. It may also be susceptible to victimization by ingress at their egress frequencies and modulation parameters. Those devices already in the environment are fingerprinted if there is reason to believe that they have the potential to be an EMI victim or a culprit. Examples of proactive mitigation include consulting services provided by the Biomedical Engineering Department and the Television Services Group during the latest expansion of Texas Children’s Hospital. These services, which involved the expertise of all groups within the Biomedical Engineering Department, included not only space design, but also the design, implementation, and expansion of wireless paging and communications systems. Using a distributed energy antenna systems (i.e., leaky coax-radiax), radio frequency power levels were maintained at levels deemed safe for clinical devices while providing the required coverage area for a nurse call-specific paging system. Safe wireless telephonic communications were implemented using low-power microcellular systems (Hoglund and Varga, 2018). Infant abduction warning systems were reviewed for specifications including emission characteristics, and a system was selected based in part on its low EM radiation levels (Southwick, 1992). The use of portable radio communication devices in a large campus environment is a basic requirement for security, engineering, and guest services. As part of the mitigation process, instruction in the safe use of two-way radio equipment in the clinical environment is mandatory for all personnel who use this equipment (ECRI, 1988).
that the cause of the malfunction, intermittent or permanent, may have been EMI. In the case of continuing or continuous device malfunction not otherwise attributable to defects within the device itself [e.g., no problem found (NPF) service reports], a careful investigation begins, using various test equipment, some of which may require specialized construction. This section presents a more detailed description of the basic equipment necessary to locate and identify sources of EMI and some of the basic techniques, referred to as “ghost hunting.” Once the type and source of the interference is detected and analyzed, the next step is to reduce or eliminate the interference to the victim device. However, the victim device is removed in some cases, because of an intensely hostile EM environment. Replacement of the victim device with an equivalent device that may have other, less sensitive responses is an option. The detection phase can eliminate the possibility of EMI as the culprit and cause of improper operation of a specific device. Indeed, subsequent investigation can reveal technical problems within a device that was previously attributed to EMI.
Detection
Sustainable EMI prevention must include institution-wide compliance with guidelines and with those policies created within an institution to limit possible sources of EMI as well as the selection and deployment of medical devices
Detection is implemented on the report of a device malfunction. A preliminary analysis of the incident may indicate
Correction Correction of victim/culprit relationship(s) can take many forms, some practical and some impractical. Correction may be part of a process to ensure that the victim device meets all specifications that can affect its susceptibility to EM radiation. For example, hospital staff should ensure that proper case-to-case contact is made in coated conductive coatings that have not been worn or abraded. In some instances, the victim device can and should be removed from the environment where it is at risk. If the culprit is local, removal may be one practical solution to the problem. Instances of EM radiation emanating from abandoned wiring (passive reradiation) have occurred that dictated removal of the wiring. Interference to clinical devices originating from active (non-fiberoptic) wiring, such as networking trunk conduits, mandates rerouting of the wiring. The same can apply to modern in-plant telephone systems, usually digital in nature. Experiences at Texas Children’s Hospital support the work of researchers (ECRI, 1988) in finding that EMI shielded rooms rarely correct problems when the source of EM radiation is contained within the local environment. In extreme cases, the existing shielded room must be disassembled or the victim equipment (and department), moved to other quarters within the institution.
Prevention
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that offer more effective immunity from EMI. Overall, observance of this two-pronged policy will have the effect of reducing the risk of EMI to the proper operation of clinical devices and therefore reducing the risk to the institution. At our healthcare provider institution, EMI prevention takes several forms (ECRI, 1988). A proactive policies and procedures manual that, guided by the Biomedical Engineering Department, defines allowable sources of radiation within the institution and mandates training procedures for employees of the institution that is required by job necessity to carry sources of EM radiation, i.e., intentional radiators. As an example, signage mandated by the policies and procedures manual directs that all cellular telephones should be turned off. Their use within the institutional campus is not allowed. The policies and procedures manual further mandates that all employees using radio equipment, especially handheld devices, are trained in their safe use within the institution. Additionally, a stipulation that clinical devices must be EMC compatible is incorporated into the condition of sale, issued, and agreed to by vendors, to define the technical conditions that must be met under the purchase contract.
Case histories Role reversal An unusual example occurred when the victim (i.e., a pager), normally considered a culprit (i.e., a physiological monitor), and the culprit, normally a victim, exchanged roles. Pagers had ceased receiving calls in a cardiovascular intensive care unit. Field intensity measurements, interrogation of employees, and other investigative procedures conducted by the clinical engineer yielded a scenario in which some pagers were not able to receive calls when near a wellknown intensive care monitoring system. Standards and practices relating to on-site testing for the presence of EM radiation were reviewed. A modified open antenna test site (OATS) procedure (Southwick, 1992) was used as the measurement guideline (Bennett, 1993). As for equipment, an Empire NF-105 field intensity metering system (loaned by a staff member previously engaged in field site RF measurements), was used as the measurement device. An area characterization (i.e., footprint) in the area of interest was taken as well as a characterization of a representative culprit device (i.e., fingerprint). The field intensity measurements of a representative unit of the monitor system showed the existence of an unintentional radiofrequency close to the operating frequency of the paging system. Once the source of the EMI was identified, the results were presented to the manufacturer. Initial responses from the manufacturer’s media representatives were not encouraging. During one conversation, the manufacturer’s EMI expert said that the solution was to increase the radiated
power of the paging transmitters. This solution was immediately dismissed from consideration, because increasing the level of intentionally radiated EM energy to compensate for the effect of existing unintentional EM radiation would increase the potential for risk to other clinical devices. Only when the Biomedical Engineering Department was ready to persuade the client department to cancel the purchase order for the remaining devices did the manufacturer discover a modification to reduce the severity of the interference. This modification, when installed, did not remove all of the unintentional radiation but did reduce the level of the interfering EM radiation sufficiently to relieve interference with pagers. The manufacturer essentially redistributed and dispersed the radiated energy to other points in the spectrum, which reduced the amplitude at the specific pager operating frequency. This incident was an interesting introduction to the practical effects of device compatibility (or lack of) and EMI in the clinical environment. After a technician from the Texas Children’s Hospital Biomedical Engineering Department installed the modification provided by the manufacturer, a fingerprint of the modified device was taken again. This incident was concluded so successfully that two mutually beneficial results were obtained: The problems of EMI in the clinical environment were clearly and graphically demonstrated within the institution, and the hospital obtained funding for the purchase of modern signal analysis equipment.
An unusual source Another “ghost hunt” occurred at one of the Biomedical Engineering Departments’ client hospitals. The wireless medical telemetry began intermittently displaying error codes (i.e., loss of data). Staff members were carefully questioned. The times when the interference occurred were established. Further probing revealed that a recently upgraded fire alarm system was undergoing acceptance tests during the periods of interference. As this would be an unusual source of interference, cooperation was sought from plant engineering personnel and the representatives of the fire detection company. Questions posed to the fire detection engineers yielded no prior indications of interference. A spectrum analyzer and broadband antenna system were set up at the site. The spectrum analyzer showed the presence of a recurrent but not time repetitive pulse, indicating that the interference source was radiating a quasi-random pulse. Working with the fire alarm and plant engineering personnel, circuits controlling the various alarm devices were isolated and shutdown individually. When no results were obtained on the affected floor, the same shut-down procedure was implemented on the floor below. When the enunciators—the audio-visual (A/V) units that proved audible and visual warnings of a fire—were shutdown one floor below
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the affected area, the pulse and the interference effects disappeared. The floor below the affected area was a mechanical plant floor, and so the fire alarm company had installed high-powered A/V units. These A/V units used strobe lights. On discharge, these devices emitted a fast-rising, short- duration pulse. The telemetry antennas were receiving these pulses. However, the preamplifiers in the antennas and in the receivers themselves were not saturating or being desensitized by these pulses because the pulse was of such short duration. The pulses passed right through the RF portions of the telemetry system and corrupted data bytes. Open junction boxes with excess wiring hanging in circular loops contributed to the radiation of the pulses. Properly closing the junction boxes and replacing the strobe lights with lower intensity devices within fire code guidelines resolved the situation.
Risk prevention The following is an example of how the application of footprinting and fingerprinting may have prevented an EMI incident. The diagnostic imaging department bought new telemetry equipment. Footprinting revealed that the level of incidental emissions in that department was approximately 67 μV. Fingerprinting a representative telemetry transmitter showed that the transmitted energy level at the standard 1-m distance was 12 μV above the background level. The defined risk was that there was not sufficient signalto-noise ratio to preclude intolerably long periods of loss of useable signal throughput. Testing the telemetry transmitter (fingerprinting) is done under controlled conditions at a fixed distance that cannot be maintained under realworld conditions. A patient wearing a telemetry transmitter cannot be expected to maintain a 1-m distance from the receiving antennas. As the distance between a patient and the receiving antennas increases, the received signal degrades because of the various topographical conditions and the inverse-square law. As the signal degrades, the level of acceptable data degrades accordingly. If the degradation is significant, there is a chance that the telemetry system will not be able to recognize the emergency if a patient is in cardiac distress.
Interference of another type But not all ghosts are due to radiated or conducted EM fields. For example, a report was received of intermittent interference to an EMG device in the physical therapy department of the hospital. The department felt that the culprit was the MRI system located directly above the area containing the victim device. During footprinting, measurements entailing more than the normal broad-spectrum procedures were indicated. In this application, the fingerprinting equipment was set up to measuring any radiated EM fields at the
resonant frequency of the MRI that might leak from the shielded room environment that contains the MRI system. During a period of several hours, no leakage was detected. A broader spectrum scan showed that this department was in a remarkably quiet location with respect to radio frequency energy. Further interviews with the doctors and staff led the clinical engineer to perform a somewhat unorthodox series of tests. The filtration on the EMG was broadened and the leads laid out, unterminated, on the couch on which patients were placed. During this procedure the clinical engineer observed that, when pressure was applied to the couch cushion, the baseline of the EMG machine would vary synchronously. Several additional adjustments to the sensitivity (gain) of the device and the time base were made. It then appeared that any motion in the immediate vicinity of the device would cause this baseline shift. Based on the results of these tests and of an investigation that determined the material composition of the environment (vinyl cushions on the couch and highly waxed vinyl floor), the engineer concluded that the cause of the interference with the device was electrostatic, not EM. Furthermore, the intermittent appearance of the problem was attributed to the fact that hydrotherapeutic baths were located two doors away from the EMG room. Their intermittent operation would raise the humidity sufficiently to reduce the potential for the generation of intense electrostatic charges. This was a decidedly different ghost hunt.
Interference not caused by EMI One of the recent trends related to EMI is that of a manufacturer’s technical problems being attributed to EMI. Although EMI may play a significant role in the performance degradation in certain clinical devices, it is not always the cause. One example involved radiological equipment. X-ray films from one of the radiology labs displayed an artifact described by the radiologists as “chicken scratches.” This artifact was present primarily during a Temporo-Mandibular Joint (TMJ) procedure. This artifact was initially attributed to either conducted or radiated EMI, specifically conducted EMI from the power lines. An outside consultant measured radiation from the power lines, but there was insufficient indication that the artifact was due to the power system. After several months, the Biomedical Engineering Department was called in for consultation. Initial investigation, including timing the artifacts as they appeared on the film, indicated some degree of synchronicity (timing repeatability). This implied that the interference was time locked (synchronized) within the radiology system. Due to the characteristics of the interference, such as the timing (frequency) of the interference in relationship to the scan rate, a probe was designed, constructed, and tuned to resonate within the frequency range
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of the suspected interfering signal. A thorough investigation of the areas surrounding the radiology suite included probing of the three-phase electrical service panels that provided the distribution of power to the suite. No high levels of radiated EM energy were encountered within the frequency of interest. The investigation was then conducted within the radiology suite. Despite intensive probing of all devices within the suite, including video monitors, computers, and control systems, and examination of the system involved in the TMJ procedure during which these artifacts most often appeared, the source could not be attributed to EMI. A detailed report was filed with the radiology department. The manufacturer’s representative was called in. Based on the inconclusive findings, a greater in-depth analysis of the problem was conducted by the representative, the clinical engineer, and technicians from the Biomedical Engineering Department radiology group. During the analysis, the investigators discovered a disconnected bonding strap within the camera head. Reconnecting this strap improved the performance of the system and reduced the artifact. This incident further illustrates the benefits of cooperation to resolve interference issues, irrespective of the sources of interference. It also demonstrates the effectiveness of a proactive, competent, and supported EMI program in the clinical environment. Placing blame rarely mitigates risks.
at 5-kHz deviation). The results illustrated of the issue of variable susceptibility. One device failed repeatedly in a non-catastrophic condition, while the other device, located nearby, was unaffected. Demonstrations such as this, performed on demand, tend to support the many anecdotal EMI-related reports that our department receives daily from other institutions. Regrettably, many EMI incidents go unreported due to a lack of specific programs to address them. The varied nature of EMI-related equipment malfunctions and the associated risks mandate a proactive program of EMI identification and methodology for risk reduction. An effective program relies on the cooperation of all parties potentially affected by EMI; medical staff, plant engineering, information services, biomedical engineering, and device manufacturers. Due to the highly diversified knowledge and experience required to coordinate detection and mitigation of EMI, the Clinical Engineering Department and its personnel experienced in RF must take responsibility. An operational protocol must be developed to address EMI issues (see Fig. 4). This provides a defined structure to process requests for EMI investigations and a structure for processing and reporting the results of the investigations.
Variability: A demonstration of the problem
Testing for electromagnetic compatibility (EMC) in the clinical environment introduces a host of complex conditions not normally encountered in laboratory situations. In the clinical environment, various RF sources of EMI may be present anywhere. Isolating and analyzing the impact from the sources of interference involves a multidisciplined approach based on training in and knowledge of the following:
Variability in the repeat cause and effect (culprit/victim) device relationship poses a problem when instituting proactive programs designed to reduce the risks attributable to EMI with the support of management. In 1996, at the request of NHK (Japan Educational Television), the television services group of the Biomedical Engineering Department at the Texas Children’s Hospital in Houston was asked to contribute a segment to an educational program about EMI issues in the hospital environment. The hospital administrators decided that a demonstration of EMI using actual medical equipment would meet the program’s objectives of demonstrating the variability of effects of a common source radiator on identical clinical devices. Two of the same model of hemodialysis machines from the same manufacturer were used in the demonstration. Under the supervision of the clinical engineer, the machines were prepared in such a way that accessories, calibrations, wiring positions within the devices, and location within the demonstration area were as identical as possible. An intentional radiating source was placed in transmit mode at a distance of 1 m from the clinical devices. The source was a walkie-talkie commonly encountered in the environment (151.625 MHz frequency band, measured power output of 4 W at the transmitter, antenna efficiency of approximately 40%, frequency modulation
Programs and procedures
Operation of medical devices and their susceptibility to EMI RF propagation modalities and interaction theory Spectrum-analysis systems and technique (preferably with signature analysis capabilities) and calibrated antennas. Established methodology of investigating suspected EMI problems, which includes testing protocols and standards. Both standard test procedures adapted for the clinical environment and personnel trained in RF behavior increase the odds of proactively controlling EMI in the clinical environment, thus providing a safer and more effective patient care environment. The methods employed in the following procedures are variations of the OATS technique (Bennett, 1993; Southwick, 1992; AAMI, 2010), a standard for open site testing and ANSI C63.4-1991 (ANSI, 1991) and ANSI C63.18-1997. The selection of the spectrum analyzer and the options installed in it were influenced by several factors. A spectrum analyzer of the communications system test
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Education
Mandated training (hospital policy)
Information sharing
Training
"Using your radio safely in the clinical environment"
Publications
Hospital personnel
Contractors
Biomedical engineering staff
Medical / patient support staff
"Ghosthunting" tests and measurements
Observing reporting
Seminars workshops
Security Plant engineering Guest services Others FIG. 4 Flow diagram of hospital educational program in electromagnetic compatibility and interference relating to medical device technologies.
type was deemed desirable because there is no better way to characterize devices that emit RF—intentional or incidental—and the environment in which those devices operate. Broadband devices indicate relative RF activity but do not indicate the operating frequencies or modulation types. Both characteristics, independently and together, affect the susceptibility of clinical devices. The test equipment and standards used by Texas Children's Hospital Biomedical Engineering Department complied with the description above. A digitally based communications analyzer was needed to archive the results of the EMI tests; both fingerprints and footprints. The flexibility of performance requirements of the device was important, and as a cost-saving benefit, our healthcare provider institution also uses it to maintain of the hospital radio communications systems.
Since no two modified OATS environments are identical, no two results obtained under the same testing parameters will be identical. Many factors affect the detailed test results, including complex absorption and reflection variables that are totally site dependent. The procedure used to initiate and track an EMI investigation is shown in Fig. 5. Members of the clinical staff are instructed to call the Biomedical Engineering Department and television services group in any case of a device malfunction not attributable to a routine failure. In cases of a new device entering the clinical environment for the first time or a new application for an existing device, a request to test the device for compatibility is generated. This call is referred to the clinical engineer responsible for EMI investigations. When an incident might be attributable to EMI, the engineer visits the site as a part of the initial investigation.
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Service call request EMI or no other problem found
View site and affected equipment interview personnel
Clinical engineer
Yes
Yes
Further investigation required?
No
Problems solved non EMI-related Footprint of affected area
Fingerprint
Suspected radiator
Victim device Full spectrum scan Full spectrum scan
Narrow spectum scan
Narrow spectum scan
Full spectrum scan
Narrow spectum scan
Analysis and interpretation of spectrum test
Reports and recommendations Director, Biomedical Engineering
Clinical department review
Device modification
EMI trend report
Safety committee FIG. 5 EMI problem resolution flowchart.
Environment modification
Compliance agency report
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The possible victim device is viewed and tested to determine whether it might have been, or is being, affected by EMI. Interviews with the personnel responsible for the area and the operation of the device(s) must be conducted. Based on the results of this preliminary investigation, a decision is reached as to the desirability or feasibility of further investigation. The decision to continue the investigation is based on several factors: Is there a high probability of operator error? Is this a very rare occurrence? Is this either a very old device that might be reaching the end of its reliable life cycle or a new device experiencing infant mortality. As part of the preliminary investigation process, maintenance histories of the device are reviewed. Another component involves the review of equipment added to the environment that might have increased the overall RF hostility in the area enough to cause interference. Footprinting records of the area are reviewed as fingerprints of the victim devices(s). The mode of device failure is an important part of the evaluation. Is the victim device alarming? Is it operating erratically? Is it changing its operational parameters either temporarily or permanently? Is it shutting down? Is there a latching change that created the alarm? The answers to these questions can all point to the criminal device or devices. If the failure mode can be duplicated and if the failure appears to have been caused by an intentional or incidental radiator, the culprit device can usually be identified. If it is within the clinical environment, it is silenced or removed. Many times, no further investigation is required. An incident report is generated and filed. A copy is provided to the department initiating the service request. A copy, if deemed necessary, is submitted to the appropriate reporting agency, such as the Food and Drug Administration, Center for Devices and Radiological Health (FDA, CDRH, FCC). If a complete testing procedure is required and a recent footprint of the area exists, a new set is acquired and compared to the older set. Changes in the area environment are noted and analyzed. This is also compared with any existing fingerprints of the victim device. If no fingerprints exist for a representative victim device, or when a device shows signs that its ability to resist ingress may be compromised, it will be fingerprinted. After a cursory footprint of the quiet area, to ensure that no significant changes have occurred in that environment, the victim device, if practical (size and weight can affect the test location), can be moved to this area for fingerprinting. Generally, the fingerprinting procedure depends on the operating characteristics of the device. For example, intentional radiators are tested for emissions at their operating frequencies, modulations, and at second and third harmonic frequencies. Unintentional radiators (i.e., microprocessor controlled devices) are tested from 1/4 clock frequency to 300 MHz. There are exceptions to this guideline, such as when relatively strong emissions continue to the 300 MHz point. In these cases, readings are continued to 1 GHz. The
new digital cellular telephones (GSM) are now in service. Third-generation (3G) wireless communications devices should be in service in the near future. The Biomedical Engineering Department has already received reports of interference to clinical devices by digital telephones. As both the fingerprint and footprint tests proceed, the records presented by the spectrum analyzer are reviewed. Any RF emissions exceeding a predetermined value are noted, especially frequencies not easily correlated to known sources of intentional radiation. The window of observation is then narrowed to obtain a greater resolution, or magnification, of the emission of interest. This typically identifies the type of modulation of the intentional radiator. After the series of footprinting and fingerprinting tests have been completed, both the clinical engineer and the technician performing the tests analyze the results and generate reports and recommendations. These are discussed with the Director of Biomedical Engineering and possible resolutions of the problem are analyzed and reviewed. There is not always an ideal solution to an interference problem. Many factors may contribute to device victimization, some of which are not easily or practically resolved. For example, if a recently added high-power paging transmitter were installed on an adjacent building, the additional energy radiated could affect devices that exhibited no previous EMI-related reactions. Historically, if the licensed transmitter is installed on a building over which the institution has no control, it will be difficult to remove. Such transmitters are licensed by the Federal Communications Commission and the medical device has no statutory protection; basically a one-way street. Modifications to the environment are recommended if they are practical. These might include screening an area or relocating a device within an area. If modification of a medical device appears to be the only solution to the problem, then the manufacturer of the device is advised of the problem. The Biomedical Engineering Department can and does advise the manufacturer. Ad hoc modifications of medical devices to mitigate susceptibility or egress are improper and should not be attempted. Performing such modifications would, in most cases, be a violation of FDA regulations and would increase the institution’s risk for liability.
Summary Despite gains in spectrum protection for some categories of patient monitoring devices through the development of standards, the overall electromagnetic environment is still hostile to the safe operation of clinical devices. The establishment and maintenance of a safe environment for the operation of clinical devices requires a multidisciplined approach. Manufacturers, users, facility managers, and patients need to collaborate in cohesive program. Such a program must include education involving technical,
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c linical, maintenance, and management personnel and staff. A clinical engineer experienced in RF and EMI, as well as in appropriate test and measurement equipment, must be involved with the program. While this chapter focuses on the healthcare provider environment, the promotion of patient safety extends the applicability of this material to the home care environment as well. A proactive testing and evaluation program that includes ongoing measurements of plant and incoming devices must be established. Maintained devices—especially those that have demonstrated a potential for electromagnetic wave be spot tested periodically, using the fingerprint method. This chapter has presented the need for a proactive EMI management program designed to limit the destructive effects of EMI on clinical devices. Some of the previously mentioned causes of EMI and methods used to test both the environment and the devices within it are based on experiences at care provider institutions in Texas.
References AAMI, 2010. Guidance on Electromagnetic Compatibility of Medical Devices in Healthcare Facilities, Technical Information Report, TIR18. Association for the Advancement of Medical Instrumentation. http:// my.aami.org/aamiresources/previewfiles/TIR181003_preview.pdf. ANSI, 1991. American National Standard for Methods of Measurement of Radio-Noise Emissions From Low-Voltage Electrical and Electronic Equipment in the Range of 9 KHz to 40 GHz. C63.4. American National Standards Institute, New York. Bennett, W.S., 1993. Making OATS measurements reproducible from site to site. EMC Test & Design 4, 34. CBS, 1994. Haywire. Eye-to-Eye with Connie Chung. CBS. December 1. Davis, D.L., 2019. Peer Reviewed Research Studies on Wi-Fi Radiation. The Environmental Health Trust, Teton Village, WY. https://ehtrust. org/science/peer-reviewed-research-studies-on-wi-fi/.
ECRI, 1988. Patient-owned equipment. Health Devices 17, 98. ECRI, 1992. Ventilators, High Frequency. Health Device Alert. 2. December 18. ECRI, 1993. Guidance article: cellular telephones and radio transmittersinterface with clinical equipment. Health Devices 22 (8,9), 416. Hoglund, D., Varga, V., 2018. Building a reliable wireless medical device network. Global Clin. Eng. J. Special Issue No. 1 https://www.globalce.org/index.php/GlobalCE/issue/view/3. Knudson, T., Bulkeley, W.M., 1994. Stray signal, clutter on airwaves can block workings of medical electronics. Wall Street J. 1. June 15. Muhlen, S., Davis, B., Segal, B., Vazquez, G., January 2008. New challenges in controlling EMI in hospitals. In: Software de Chequeo para el Módulo de Control de Equipos Neuronica 5. pp. 834–837. https:// www.researchgate.net/publication/288787002_New_Challenges_in_ Controlling_EMI_in_Hospitals. National Research Council (US), 1993. Committee on Assessment of the Possible Health Effects of Ground Wave Emergency Network (GWEN)—Effects of Electromagnetic Fields on Organs and Tissues. National Academies Press (US), Washington, DC. ISBN-10: 0-30904777-3 https://www.ncbi.nlm.nih.gov/books/NBK208990/. Paperman, W.D., David, Y., McKee, K.A., 1994. Electromagnetic Compatibility: Causes and Concerns in the Hospital Environment. ASHE Health Care Facilities Management Series. ASHE, Chicago, IL. Paperman, W.D., David, Y., Martinez, M., 1996. Testing for EMC in the clinical environment. J. Clin. Eng. 21 (3). May/June. Southwick, R., 1992. EMI signal measurements at open antenna test sites. EMC Test & Design 3, 44. US Government, 1991. CFR 47; Part 15.103 (c, e). Code of Federal Regulations of Telecommunications. Code of Federal Regulations.
Further reading David, Y., 1993. Safety and risk control issues: biomedical systems. In: Dorf, R.C. (Ed.), The Electrical Engineering Handbook. CRC Press, Boca Raton, FL.
Electromagnetic interference in healthcare environment Yadin Davida,b, W. David Papermanc a
Biomedical Engineering Consultants, LLC, Houston, TX, United States, bUniversity of Texas School of Public Health, Houston, TX, United States, cClinical Engineering Consultant, Cut and Shoot, TX, United States
Considerations for EM spectrum use in healthcare facilities The density of users on the electromagnetic (EM) spectrum has been increasing because of the expansion of wirelessbased services. The desire for mobility and anywhere connectivity creates conflicts between a variety of EM waves energy and intelligent devices or even human tissue (CBS, 1994). Despite the efforts by manufacturers to harden clinical devices to the effects of electromagnetic interference (EMI), reports of incidents of interference to previously unaffected medical devices appear in medical and scientific literature and anecdotally (CBS, 1994). Therefore, the role and knowledge base of clinical engineering practitioners must expand to include command understanding and proper management of these ever-increasing challenges (Paperman et al., 1994). According to research conducted at McGill University by Segal B. and colleagues “Wireless technology has been evolving from networks with a small number of relatively high-power sources to networks with a large number of relatively low-power sources. This (Muhlen et al., 2008) has made characterizing the hospital EM environment a complicated process and the associated potential for EMI difficult to determine. There is no globally accepted comprehensive protocol to evaluate the function/malfunction of medical devices exposed to radiofrequency (RF) fields. In addition, there is often uncertainty in determining the degree of clinical impact that a malfunction has.”
Electromagnetic radiation EM radiation occurs when an alternating current is generated. An EM field is created in the vicinity of this source (National Research Council (US), 1993). The range, or distance of the radiated EM field can be increased when coupled to a conductor. The magnetic field is further radiated 362
by the flow of the current along this conductor. Even at a reduced amplitude, a corresponding current will be generated when another conductor is subjected to the field of the radiating conductor. The radiated field has many characteristics. The most important of those characteristics are amplitude, periodicity, and waveform. The amplitude of the impressed alternating current defines the energy of a radiated field. This field is generally expressed in volts per meter (V/M). Amplitude at the source point (transmitting conductor) will define, in conjunction with frequency and distance, the amplitude of the EM field induced in the secondary (receiving) conductor subject to the inverse-square law. The periodicity or frequency of alternation is expressed in Hertz per second, which allows the calculation of the wavelength of the radiated EM field. Knowing the wavelength versus the physical length of both the radiator and the secondary conductor into which the radiated energy is induced provides an estimate of the potential amplitude developed by or within a device at risk (AAMI, 2010). Waveforms other than linear (sinusoidal) can produce multiple and variable frequencies. The foremost example of a nonlinear waveform is the square wave. The square wave produces many multiples, or harmonics, which have a range many times that of the fundamental frequency. Most digital devices are capable of generating square waves. Depending on the amplitude at the point of generation, the efficiency of the auxiliary radiator (antenna), the generated frequency, and the waveform, the potential for interference between digital devices and clinical devices can exist for many kilometers from the source of EM radiation. EMI threats come from a multitude of sources (Davis, 2019). These are broadly divided into two categories: devices that emit intentional EM radiation for communications and control, and devices that, as a by-product of their operation, emit unintentional (incidental) radiation (Fig. 1). Some of the major and more common sources of EMI Clinical Engineering Handbook. https://doi.org/10.1016/B978-0-12-813467-2.00056-0 Copyright © 2020 Elsevier Inc. All rights reserved.
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Radiator intentional
Target intentional
intents and purposes, into four parts: mitigation, detection, correction, and prevention. Although the mitigation and prevention phases appear to be similar, they differ in their approaches and applications.
Controlling the effect of EMI Mitigation
Radiator unintentional
Target unintentional
FIG. 1 The complexities of EMI-radiators and targets: culprits and victims.
e ncountered in the clinical environment are given in the following (Paperman et al., 1996): Intentional radiators: Television broadcast stations: Analog and digital Commercial radio stations: Analog and digital Land mobile radio: Fixed base (FB), mobile, and portable two-way radio sources (walkie talkies) Paging: One-way and two-way wireless messaging service transmitters Cellular telephones and sites Wireless personal digital assistants (PDAs) Wireless networking devices (an increasing threat in the 2.4-GHz industrial, scientific, and medical ISM band) Unlicensed and unauthorized users of two-way radio communications equipment (Pirates) Wearable personal instruments LASER instrumentation Unintentional radiators: Lighting systems (especially florescent), including energy saving electronic ballasts High-energy control systems (HVAC controllers), especially variable-speed controllers Malfunctioning electrical services Universal-type electric motors Pulse oximeters Displays (CRT and plasma), computer, television, and instrumentation Wired computing networks “Smart” fire detection and alarm devices Electrosurgical units (ESU) Defibrillators Electrical physical therapy equipment These sources of EM radiation, as seen from the previous lists, can be intentional or unintentional (ANSI, 1991). The challenges and the institutional responses imposed by the presence of these EMI sources can be divided, for all
In the mitigation phase, the principal concern is reducing risk to, or victimization of, clinical devices by devices emitting unintentional or intentional EM radiation. The sources of the devices may be internal or external to the institution (Knudson and Bulkeley, 1994). The devices may be intentional or unintentional radiators of EM fields, and the EM energy may be radiated or conducted. Fig. 1 illustrates the EM interaction among intentional and unintentional radiators and intentional and unintentional targets. Mitigation is accomplished in part by the careful analysis of a device (ECRI, 1992), including the accessories to be used with it, and the EM environment (US Government, 1991) in which it will be used. A successful approach has been implemented at several care environments. This approach is accomplished in two phases: thorough analysis of the environment in which the device is to be placed (Bennett, 1993), called “footprinting,” and an equally thorough analysis of the clinical device itself, called “fingerprinting.” The procedures of quantifying potential EMI and mitigating the effects through environment analysis are described in greater detail in the following. Another vital part of the mitigation process occurs during facility planning. Whether planning a new facility, an expansion of an existing facility, or remodeling of a facility, clear and concise communications between affected departments, architects, contractors, and the Clinical Engineering Department are vital at all stages of the project (ECRI, 1993). For example, one institution installed expensive, screened rooms at great cost. When the clinical engineer, acting on complaints of erratic EMG operation, determined that the room played a role in the culprit/victim relationship, the rooms were disassembled, also at great cost. In this case, the relocation of the affected department was not an option. The Clinical Engineering Department had not been made aware of these rooms, or of their intended application that involved the use of known culprit devices.
Footprinting At our healthcare provider institution, the EMI testing program has evolved from years of experience and analysis. This program is not static. Sources of EMI and susceptibility characteristics of devices are in constant change. As new threats arise, the plan is periodically reviewed and modified to contend with them. Further modifications to the plan and
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procedures are based on a continuous review of wireless industry trends. They represent a proactive response to perceived future threats. At present the program consists of the procedural plan outlined above and three series of tests: (1) area characterization and footprinting (Fig. 2), (2) device characterization, fingerprinting (Fig. 3), and ad hoc susceptibility testing according to IEEE guidelines (Knudson and Bulkeley, 1994). Footprinting an area means performing a series of spectrum/amplitude scans for EM radiation in a defined or designated area. Footprinting is an ongoing technique that defines EM radiation in a clinical facility or facilities in a multibuilding campus. Footprinting is also done on request from a department experiencing performance degradation of clinical, diagnostic, or therapeutic devices when EMI is the suspect. The procedure involves a series of 20-MHzwide spectrum sweeps, beginning at 2 MHz and ending at 1 GHz with antenna(s) in the horizontal plane. This procedure is repeated with the antenna(s) vertically polarized. Tunable standard antennas are adjusted for the correct resonant length to the center of each 20-MHz window. The footprinting procedures yield two results: an overview of the radiated EM fields presents at a specified location in the environment and the amplitudes and types of emission of those fields. Several incident investigations have been successful using this information. In cases where EMI has been site originated, the source has been removed and the problem has been corrected. In some cases, the affected device required maintenance to correct the problem. An added value of footprinting is that the data obtained during the process meets the basic requirements for a site
Calibrated antenna systems Biconical: Low frequency rod, dipoles,
Communications spectrum analyzer
Laptop computer
Printer
Data Archival storage FIG. 2 Typical equipment configuration for fingerprinting and footprinting.
Device Under Test
1-m distance
Antennas Selected and adjusted for frequency range of emissions under examination
Test equipment configured for Fingerprint
FIG. 3 Fingerprinting.
search similar to the OATS procedure for fingerprinting individual devices. The results of the footprinting scans are transferred to a storage medium and filed. They form a comparison database that is used to evaluate new devices before introducing them into a specified area.
Fingerprinting In the process of fingerprinting, a device has several steps. Again, the spectrum analyzer and calibrated antenna system are the primary tools used to analyze the device under test (DUT) (see Fig. 3). To minimize the loss of information from the DUT due to masking by other sources of EM radiation, areas within the clinical physical plant should be tested using the footprinting procedure until a relatively quiet area is found. For the fingerprinting procedure, as in the footprinting procedures, the standards antenna should be located as far as possible from any conductive material. The standards antenna should be located at a height equal to the center of the DUT. Due to constantly changing EM fields and frequencies, they execute the footprinting procedure in the selected area immediately prior to fingerprinting the DUT. Once the data from the latest footprint has been stored, place the DUT on a nonmetallic stand located as far as possible from
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any conductive material. The standard antenna(s) is then mounted on a tripod and, in a horizontally polarized mode, placed 1 m from the front of the DUT. As in the footprint procedure, a series of 20-MHz-wide spectrum sweeps are performed, and the results are recorded. This procedure is repeated with the antenna(s) vertically polarized. When using tunable standard antennas, adjust them to the correct resonant length to the center of each 20-MHz window. Based on the review of the data collected, any emissions attributable to the DUT should be rescanned. To increase detail in the frequency range in which the emissions were observed, the spectrum analyzer window is narrowed to a 200-KHz/division or smaller (e.g., 5 kHz/division) sweep width. Fingerprinting tests are conducted for two reasons. First, compliance with hospital policy on acceptance testing requires that representative samples of all devices be tested prior to entering the clinical environment for the first time. Second, both theory and experience have demonstrated the need for device testing. A device that radiates unintentionally can victimize other devices. It may also be susceptible to victimization by ingress at their egress frequencies and modulation parameters. Those devices already in the environment are fingerprinted if there is reason to believe that they have the potential to be an EMI victim or a culprit. Examples of proactive mitigation include consulting services provided by the Biomedical Engineering Department and the Television Services Group during the latest expansion of Texas Children’s Hospital. These services, which involved the expertise of all groups within the Biomedical Engineering Department, included not only space design, but also the design, implementation, and expansion of wireless paging and communications systems. Using a distributed energy antenna systems (i.e., leaky coax-radiax), radio frequency power levels were maintained at levels deemed safe for clinical devices while providing the required coverage area for a nurse call-specific paging system. Safe wireless telephonic communications were implemented using low-power microcellular systems (Hoglund and Varga, 2018). Infant abduction warning systems were reviewed for specifications including emission characteristics, and a system was selected based in part on its low EM radiation levels (Southwick, 1992). The use of portable radio communication devices in a large campus environment is a basic requirement for security, engineering, and guest services. As part of the mitigation process, instruction in the safe use of two-way radio equipment in the clinical environment is mandatory for all personnel who use this equipment (ECRI, 1988).
that the cause of the malfunction, intermittent or permanent, may have been EMI. In the case of continuing or continuous device malfunction not otherwise attributable to defects within the device itself [e.g., no problem found (NPF) service reports], a careful investigation begins, using various test equipment, some of which may require specialized construction. This section presents a more detailed description of the basic equipment necessary to locate and identify sources of EMI and some of the basic techniques, referred to as “ghost hunting.” Once the type and source of the interference is detected and analyzed, the next step is to reduce or eliminate the interference to the victim device. However, the victim device is removed in some cases, because of an intensely hostile EM environment. Replacement of the victim device with an equivalent device that may have other, less sensitive responses is an option. The detection phase can eliminate the possibility of EMI as the culprit and cause of improper operation of a specific device. Indeed, subsequent investigation can reveal technical problems within a device that was previously attributed to EMI.
Detection
Sustainable EMI prevention must include institution-wide compliance with guidelines and with those policies created within an institution to limit possible sources of EMI as well as the selection and deployment of medical devices
Detection is implemented on the report of a device malfunction. A preliminary analysis of the incident may indicate
Correction Correction of victim/culprit relationship(s) can take many forms, some practical and some impractical. Correction may be part of a process to ensure that the victim device meets all specifications that can affect its susceptibility to EM radiation. For example, hospital staff should ensure that proper case-to-case contact is made in coated conductive coatings that have not been worn or abraded. In some instances, the victim device can and should be removed from the environment where it is at risk. If the culprit is local, removal may be one practical solution to the problem. Instances of EM radiation emanating from abandoned wiring (passive reradiation) have occurred that dictated removal of the wiring. Interference to clinical devices originating from active (non-fiberoptic) wiring, such as networking trunk conduits, mandates rerouting of the wiring. The same can apply to modern in-plant telephone systems, usually digital in nature. Experiences at Texas Children’s Hospital support the work of researchers (ECRI, 1988) in finding that EMI shielded rooms rarely correct problems when the source of EM radiation is contained within the local environment. In extreme cases, the existing shielded room must be disassembled or the victim equipment (and department), moved to other quarters within the institution.
Prevention
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that offer more effective immunity from EMI. Overall, observance of this two-pronged policy will have the effect of reducing the risk of EMI to the proper operation of clinical devices and therefore reducing the risk to the institution. At our healthcare provider institution, EMI prevention takes several forms (ECRI, 1988). A proactive policies and procedures manual that, guided by the Biomedical Engineering Department, defines allowable sources of radiation within the institution and mandates training procedures for employees of the institution that is required by job necessity to carry sources of EM radiation, i.e., intentional radiators. As an example, signage mandated by the policies and procedures manual directs that all cellular telephones should be turned off. Their use within the institutional campus is not allowed. The policies and procedures manual further mandates that all employees using radio equipment, especially handheld devices, are trained in their safe use within the institution. Additionally, a stipulation that clinical devices must be EMC compatible is incorporated into the condition of sale, issued, and agreed to by vendors, to define the technical conditions that must be met under the purchase contract.
Case histories Role reversal An unusual example occurred when the victim (i.e., a pager), normally considered a culprit (i.e., a physiological monitor), and the culprit, normally a victim, exchanged roles. Pagers had ceased receiving calls in a cardiovascular intensive care unit. Field intensity measurements, interrogation of employees, and other investigative procedures conducted by the clinical engineer yielded a scenario in which some pagers were not able to receive calls when near a wellknown intensive care monitoring system. Standards and practices relating to on-site testing for the presence of EM radiation were reviewed. A modified open antenna test site (OATS) procedure (Southwick, 1992) was used as the measurement guideline (Bennett, 1993). As for equipment, an Empire NF-105 field intensity metering system (loaned by a staff member previously engaged in field site RF measurements), was used as the measurement device. An area characterization (i.e., footprint) in the area of interest was taken as well as a characterization of a representative culprit device (i.e., fingerprint). The field intensity measurements of a representative unit of the monitor system showed the existence of an unintentional radiofrequency close to the operating frequency of the paging system. Once the source of the EMI was identified, the results were presented to the manufacturer. Initial responses from the manufacturer’s media representatives were not encouraging. During one conversation, the manufacturer’s EMI expert said that the solution was to increase the radiated
power of the paging transmitters. This solution was immediately dismissed from consideration, because increasing the level of intentionally radiated EM energy to compensate for the effect of existing unintentional EM radiation would increase the potential for risk to other clinical devices. Only when the Biomedical Engineering Department was ready to persuade the client department to cancel the purchase order for the remaining devices did the manufacturer discover a modification to reduce the severity of the interference. This modification, when installed, did not remove all of the unintentional radiation but did reduce the level of the interfering EM radiation sufficiently to relieve interference with pagers. The manufacturer essentially redistributed and dispersed the radiated energy to other points in the spectrum, which reduced the amplitude at the specific pager operating frequency. This incident was an interesting introduction to the practical effects of device compatibility (or lack of) and EMI in the clinical environment. After a technician from the Texas Children’s Hospital Biomedical Engineering Department installed the modification provided by the manufacturer, a fingerprint of the modified device was taken again. This incident was concluded so successfully that two mutually beneficial results were obtained: The problems of EMI in the clinical environment were clearly and graphically demonstrated within the institution, and the hospital obtained funding for the purchase of modern signal analysis equipment.
An unusual source Another “ghost hunt” occurred at one of the Biomedical Engineering Departments’ client hospitals. The wireless medical telemetry began intermittently displaying error codes (i.e., loss of data). Staff members were carefully questioned. The times when the interference occurred were established. Further probing revealed that a recently upgraded fire alarm system was undergoing acceptance tests during the periods of interference. As this would be an unusual source of interference, cooperation was sought from plant engineering personnel and the representatives of the fire detection company. Questions posed to the fire detection engineers yielded no prior indications of interference. A spectrum analyzer and broadband antenna system were set up at the site. The spectrum analyzer showed the presence of a recurrent but not time repetitive pulse, indicating that the interference source was radiating a quasi-random pulse. Working with the fire alarm and plant engineering personnel, circuits controlling the various alarm devices were isolated and shutdown individually. When no results were obtained on the affected floor, the same shut-down procedure was implemented on the floor below. When the enunciators—the audio-visual (A/V) units that proved audible and visual warnings of a fire—were shutdown one floor below
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the affected area, the pulse and the interference effects disappeared. The floor below the affected area was a mechanical plant floor, and so the fire alarm company had installed high-powered A/V units. These A/V units used strobe lights. On discharge, these devices emitted a fast-rising, short- duration pulse. The telemetry antennas were receiving these pulses. However, the preamplifiers in the antennas and in the receivers themselves were not saturating or being desensitized by these pulses because the pulse was of such short duration. The pulses passed right through the RF portions of the telemetry system and corrupted data bytes. Open junction boxes with excess wiring hanging in circular loops contributed to the radiation of the pulses. Properly closing the junction boxes and replacing the strobe lights with lower intensity devices within fire code guidelines resolved the situation.
Risk prevention The following is an example of how the application of footprinting and fingerprinting may have prevented an EMI incident. The diagnostic imaging department bought new telemetry equipment. Footprinting revealed that the level of incidental emissions in that department was approximately 67 μV. Fingerprinting a representative telemetry transmitter showed that the transmitted energy level at the standard 1-m distance was 12 μV above the background level. The defined risk was that there was not sufficient signalto-noise ratio to preclude intolerably long periods of loss of useable signal throughput. Testing the telemetry transmitter (fingerprinting) is done under controlled conditions at a fixed distance that cannot be maintained under realworld conditions. A patient wearing a telemetry transmitter cannot be expected to maintain a 1-m distance from the receiving antennas. As the distance between a patient and the receiving antennas increases, the received signal degrades because of the various topographical conditions and the inverse-square law. As the signal degrades, the level of acceptable data degrades accordingly. If the degradation is significant, there is a chance that the telemetry system will not be able to recognize the emergency if a patient is in cardiac distress.
Interference of another type But not all ghosts are due to radiated or conducted EM fields. For example, a report was received of intermittent interference to an EMG device in the physical therapy department of the hospital. The department felt that the culprit was the MRI system located directly above the area containing the victim device. During footprinting, measurements entailing more than the normal broad-spectrum procedures were indicated. In this application, the fingerprinting equipment was set up to measuring any radiated EM fields at the
resonant frequency of the MRI that might leak from the shielded room environment that contains the MRI system. During a period of several hours, no leakage was detected. A broader spectrum scan showed that this department was in a remarkably quiet location with respect to radio frequency energy. Further interviews with the doctors and staff led the clinical engineer to perform a somewhat unorthodox series of tests. The filtration on the EMG was broadened and the leads laid out, unterminated, on the couch on which patients were placed. During this procedure the clinical engineer observed that, when pressure was applied to the couch cushion, the baseline of the EMG machine would vary synchronously. Several additional adjustments to the sensitivity (gain) of the device and the time base were made. It then appeared that any motion in the immediate vicinity of the device would cause this baseline shift. Based on the results of these tests and of an investigation that determined the material composition of the environment (vinyl cushions on the couch and highly waxed vinyl floor), the engineer concluded that the cause of the interference with the device was electrostatic, not EM. Furthermore, the intermittent appearance of the problem was attributed to the fact that hydrotherapeutic baths were located two doors away from the EMG room. Their intermittent operation would raise the humidity sufficiently to reduce the potential for the generation of intense electrostatic charges. This was a decidedly different ghost hunt.
Interference not caused by EMI One of the recent trends related to EMI is that of a manufacturer’s technical problems being attributed to EMI. Although EMI may play a significant role in the performance degradation in certain clinical devices, it is not always the cause. One example involved radiological equipment. X-ray films from one of the radiology labs displayed an artifact described by the radiologists as “chicken scratches.” This artifact was present primarily during a Temporo-Mandibular Joint (TMJ) procedure. This artifact was initially attributed to either conducted or radiated EMI, specifically conducted EMI from the power lines. An outside consultant measured radiation from the power lines, but there was insufficient indication that the artifact was due to the power system. After several months, the Biomedical Engineering Department was called in for consultation. Initial investigation, including timing the artifacts as they appeared on the film, indicated some degree of synchronicity (timing repeatability). This implied that the interference was time locked (synchronized) within the radiology system. Due to the characteristics of the interference, such as the timing (frequency) of the interference in relationship to the scan rate, a probe was designed, constructed, and tuned to resonate within the frequency range
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of the suspected interfering signal. A thorough investigation of the areas surrounding the radiology suite included probing of the three-phase electrical service panels that provided the distribution of power to the suite. No high levels of radiated EM energy were encountered within the frequency of interest. The investigation was then conducted within the radiology suite. Despite intensive probing of all devices within the suite, including video monitors, computers, and control systems, and examination of the system involved in the TMJ procedure during which these artifacts most often appeared, the source could not be attributed to EMI. A detailed report was filed with the radiology department. The manufacturer’s representative was called in. Based on the inconclusive findings, a greater in-depth analysis of the problem was conducted by the representative, the clinical engineer, and technicians from the Biomedical Engineering Department radiology group. During the analysis, the investigators discovered a disconnected bonding strap within the camera head. Reconnecting this strap improved the performance of the system and reduced the artifact. This incident further illustrates the benefits of cooperation to resolve interference issues, irrespective of the sources of interference. It also demonstrates the effectiveness of a proactive, competent, and supported EMI program in the clinical environment. Placing blame rarely mitigates risks.
at 5-kHz deviation). The results illustrated of the issue of variable susceptibility. One device failed repeatedly in a non-catastrophic condition, while the other device, located nearby, was unaffected. Demonstrations such as this, performed on demand, tend to support the many anecdotal EMI-related reports that our department receives daily from other institutions. Regrettably, many EMI incidents go unreported due to a lack of specific programs to address them. The varied nature of EMI-related equipment malfunctions and the associated risks mandate a proactive program of EMI identification and methodology for risk reduction. An effective program relies on the cooperation of all parties potentially affected by EMI; medical staff, plant engineering, information services, biomedical engineering, and device manufacturers. Due to the highly diversified knowledge and experience required to coordinate detection and mitigation of EMI, the Clinical Engineering Department and its personnel experienced in RF must take responsibility. An operational protocol must be developed to address EMI issues (see Fig. 4). This provides a defined structure to process requests for EMI investigations and a structure for processing and reporting the results of the investigations.
Variability: A demonstration of the problem
Testing for electromagnetic compatibility (EMC) in the clinical environment introduces a host of complex conditions not normally encountered in laboratory situations. In the clinical environment, various RF sources of EMI may be present anywhere. Isolating and analyzing the impact from the sources of interference involves a multidisciplined approach based on training in and knowledge of the following:
Variability in the repeat cause and effect (culprit/victim) device relationship poses a problem when instituting proactive programs designed to reduce the risks attributable to EMI with the support of management. In 1996, at the request of NHK (Japan Educational Television), the television services group of the Biomedical Engineering Department at the Texas Children’s Hospital in Houston was asked to contribute a segment to an educational program about EMI issues in the hospital environment. The hospital administrators decided that a demonstration of EMI using actual medical equipment would meet the program’s objectives of demonstrating the variability of effects of a common source radiator on identical clinical devices. Two of the same model of hemodialysis machines from the same manufacturer were used in the demonstration. Under the supervision of the clinical engineer, the machines were prepared in such a way that accessories, calibrations, wiring positions within the devices, and location within the demonstration area were as identical as possible. An intentional radiating source was placed in transmit mode at a distance of 1 m from the clinical devices. The source was a walkie-talkie commonly encountered in the environment (151.625 MHz frequency band, measured power output of 4 W at the transmitter, antenna efficiency of approximately 40%, frequency modulation
Programs and procedures
Operation of medical devices and their susceptibility to EMI RF propagation modalities and interaction theory Spectrum-analysis systems and technique (preferably with signature analysis capabilities) and calibrated antennas. Established methodology of investigating suspected EMI problems, which includes testing protocols and standards. Both standard test procedures adapted for the clinical environment and personnel trained in RF behavior increase the odds of proactively controlling EMI in the clinical environment, thus providing a safer and more effective patient care environment. The methods employed in the following procedures are variations of the OATS technique (Bennett, 1993; Southwick, 1992; AAMI, 2010), a standard for open site testing and ANSI C63.4-1991 (ANSI, 1991) and ANSI C63.18-1997. The selection of the spectrum analyzer and the options installed in it were influenced by several factors. A spectrum analyzer of the communications system test
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Education
Mandated training (hospital policy)
Information sharing
Training
"Using your radio safely in the clinical environment"
Publications
Hospital personnel
Contractors
Biomedical engineering staff
Medical / patient support staff
"Ghosthunting" tests and measurements
Observing reporting
Seminars workshops
Security Plant engineering Guest services Others FIG. 4 Flow diagram of hospital educational program in electromagnetic compatibility and interference relating to medical device technologies.
type was deemed desirable because there is no better way to characterize devices that emit RF—intentional or incidental—and the environment in which those devices operate. Broadband devices indicate relative RF activity but do not indicate the operating frequencies or modulation types. Both characteristics, independently and together, affect the susceptibility of clinical devices. The test equipment and standards used by Texas Children's Hospital Biomedical Engineering Department complied with the description above. A digitally based communications analyzer was needed to archive the results of the EMI tests; both fingerprints and footprints. The flexibility of performance requirements of the device was important, and as a cost-saving benefit, our healthcare provider institution also uses it to maintain of the hospital radio communications systems.
Since no two modified OATS environments are identical, no two results obtained under the same testing parameters will be identical. Many factors affect the detailed test results, including complex absorption and reflection variables that are totally site dependent. The procedure used to initiate and track an EMI investigation is shown in Fig. 5. Members of the clinical staff are instructed to call the Biomedical Engineering Department and television services group in any case of a device malfunction not attributable to a routine failure. In cases of a new device entering the clinical environment for the first time or a new application for an existing device, a request to test the device for compatibility is generated. This call is referred to the clinical engineer responsible for EMI investigations. When an incident might be attributable to EMI, the engineer visits the site as a part of the initial investigation.
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Service call request EMI or no other problem found
View site and affected equipment interview personnel
Clinical engineer
Yes
Yes
Further investigation required?
No
Problems solved non EMI-related Footprint of affected area
Fingerprint
Suspected radiator
Victim device Full spectrum scan Full spectrum scan
Narrow spectum scan
Narrow spectum scan
Full spectrum scan
Narrow spectum scan
Analysis and interpretation of spectrum test
Reports and recommendations Director, Biomedical Engineering
Clinical department review
Device modification
EMI trend report
Safety committee FIG. 5 EMI problem resolution flowchart.
Environment modification
Compliance agency report
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The possible victim device is viewed and tested to determine whether it might have been, or is being, affected by EMI. Interviews with the personnel responsible for the area and the operation of the device(s) must be conducted. Based on the results of this preliminary investigation, a decision is reached as to the desirability or feasibility of further investigation. The decision to continue the investigation is based on several factors: Is there a high probability of operator error? Is this a very rare occurrence? Is this either a very old device that might be reaching the end of its reliable life cycle or a new device experiencing infant mortality. As part of the preliminary investigation process, maintenance histories of the device are reviewed. Another component involves the review of equipment added to the environment that might have increased the overall RF hostility in the area enough to cause interference. Footprinting records of the area are reviewed as fingerprints of the victim devices(s). The mode of device failure is an important part of the evaluation. Is the victim device alarming? Is it operating erratically? Is it changing its operational parameters either temporarily or permanently? Is it shutting down? Is there a latching change that created the alarm? The answers to these questions can all point to the criminal device or devices. If the failure mode can be duplicated and if the failure appears to have been caused by an intentional or incidental radiator, the culprit device can usually be identified. If it is within the clinical environment, it is silenced or removed. Many times, no further investigation is required. An incident report is generated and filed. A copy is provided to the department initiating the service request. A copy, if deemed necessary, is submitted to the appropriate reporting agency, such as the Food and Drug Administration, Center for Devices and Radiological Health (FDA, CDRH, FCC). If a complete testing procedure is required and a recent footprint of the area exists, a new set is acquired and compared to the older set. Changes in the area environment are noted and analyzed. This is also compared with any existing fingerprints of the victim device. If no fingerprints exist for a representative victim device, or when a device shows signs that its ability to resist ingress may be compromised, it will be fingerprinted. After a cursory footprint of the quiet area, to ensure that no significant changes have occurred in that environment, the victim device, if practical (size and weight can affect the test location), can be moved to this area for fingerprinting. Generally, the fingerprinting procedure depends on the operating characteristics of the device. For example, intentional radiators are tested for emissions at their operating frequencies, modulations, and at second and third harmonic frequencies. Unintentional radiators (i.e., microprocessor controlled devices) are tested from 1/4 clock frequency to 300 MHz. There are exceptions to this guideline, such as when relatively strong emissions continue to the 300 MHz point. In these cases, readings are continued to 1 GHz. The
new digital cellular telephones (GSM) are now in service. Third-generation (3G) wireless communications devices should be in service in the near future. The Biomedical Engineering Department has already received reports of interference to clinical devices by digital telephones. As both the fingerprint and footprint tests proceed, the records presented by the spectrum analyzer are reviewed. Any RF emissions exceeding a predetermined value are noted, especially frequencies not easily correlated to known sources of intentional radiation. The window of observation is then narrowed to obtain a greater resolution, or magnification, of the emission of interest. This typically identifies the type of modulation of the intentional radiator. After the series of footprinting and fingerprinting tests have been completed, both the clinical engineer and the technician performing the tests analyze the results and generate reports and recommendations. These are discussed with the Director of Biomedical Engineering and possible resolutions of the problem are analyzed and reviewed. There is not always an ideal solution to an interference problem. Many factors may contribute to device victimization, some of which are not easily or practically resolved. For example, if a recently added high-power paging transmitter were installed on an adjacent building, the additional energy radiated could affect devices that exhibited no previous EMI-related reactions. Historically, if the licensed transmitter is installed on a building over which the institution has no control, it will be difficult to remove. Such transmitters are licensed by the Federal Communications Commission and the medical device has no statutory protection; basically a one-way street. Modifications to the environment are recommended if they are practical. These might include screening an area or relocating a device within an area. If modification of a medical device appears to be the only solution to the problem, then the manufacturer of the device is advised of the problem. The Biomedical Engineering Department can and does advise the manufacturer. Ad hoc modifications of medical devices to mitigate susceptibility or egress are improper and should not be attempted. Performing such modifications would, in most cases, be a violation of FDA regulations and would increase the institution’s risk for liability.
Summary Despite gains in spectrum protection for some categories of patient monitoring devices through the development of standards, the overall electromagnetic environment is still hostile to the safe operation of clinical devices. The establishment and maintenance of a safe environment for the operation of clinical devices requires a multidisciplined approach. Manufacturers, users, facility managers, and patients need to collaborate in cohesive program. Such a program must include education involving technical,
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c linical, maintenance, and management personnel and staff. A clinical engineer experienced in RF and EMI, as well as in appropriate test and measurement equipment, must be involved with the program. While this chapter focuses on the healthcare provider environment, the promotion of patient safety extends the applicability of this material to the home care environment as well. A proactive testing and evaluation program that includes ongoing measurements of plant and incoming devices must be established. Maintained devices—especially those that have demonstrated a potential for electromagnetic wave be spot tested periodically, using the fingerprint method. This chapter has presented the need for a proactive EMI management program designed to limit the destructive effects of EMI on clinical devices. Some of the previously mentioned causes of EMI and methods used to test both the environment and the devices within it are based on experiences at care provider institutions in Texas.
References AAMI, 2010. Guidance on Electromagnetic Compatibility of Medical Devices in Healthcare Facilities, Technical Information Report, TIR18. Association for the Advancement of Medical Instrumentation. http:// my.aami.org/aamiresources/previewfiles/TIR181003_preview.pdf. ANSI, 1991. American National Standard for Methods of Measurement of Radio-Noise Emissions From Low-Voltage Electrical and Electronic Equipment in the Range of 9 KHz to 40 GHz. C63.4. American National Standards Institute, New York. Bennett, W.S., 1993. Making OATS measurements reproducible from site to site. EMC Test & Design 4, 34. CBS, 1994. Haywire. Eye-to-Eye with Connie Chung. CBS. December 1. Davis, D.L., 2019. Peer Reviewed Research Studies on Wi-Fi Radiation. The Environmental Health Trust, Teton Village, WY. https://ehtrust. org/science/peer-reviewed-research-studies-on-wi-fi/.
ECRI, 1988. Patient-owned equipment. Health Devices 17, 98. ECRI, 1992. Ventilators, High Frequency. Health Device Alert. 2. December 18. ECRI, 1993. Guidance article: cellular telephones and radio transmittersinterface with clinical equipment. Health Devices 22 (8,9), 416. Hoglund, D., Varga, V., 2018. Building a reliable wireless medical device network. Global Clin. Eng. J. Special Issue No. 1 https://www.globalce.org/index.php/GlobalCE/issue/view/3. Knudson, T., Bulkeley, W.M., 1994. Stray signal, clutter on airwaves can block workings of medical electronics. Wall Street J. 1. June 15. Muhlen, S., Davis, B., Segal, B., Vazquez, G., January 2008. New challenges in controlling EMI in hospitals. In: Software de Chequeo para el Módulo de Control de Equipos Neuronica 5. pp. 834–837. https:// www.researchgate.net/publication/288787002_New_Challenges_in_ Controlling_EMI_in_Hospitals. National Research Council (US), 1993. Committee on Assessment of the Possible Health Effects of Ground Wave Emergency Network (GWEN)—Effects of Electromagnetic Fields on Organs and Tissues. National Academies Press (US), Washington, DC. ISBN-10: 0-30904777-3 https://www.ncbi.nlm.nih.gov/books/NBK208990/. Paperman, W.D., David, Y., McKee, K.A., 1994. Electromagnetic Compatibility: Causes and Concerns in the Hospital Environment. ASHE Health Care Facilities Management Series. ASHE, Chicago, IL. Paperman, W.D., David, Y., Martinez, M., 1996. Testing for EMC in the clinical environment. J. Clin. Eng. 21 (3). May/June. Southwick, R., 1992. EMI signal measurements at open antenna test sites. EMC Test & Design 3, 44. US Government, 1991. CFR 47; Part 15.103 (c, e). Code of Federal Regulations of Telecommunications. Code of Federal Regulations.
Further reading David, Y., 1993. Safety and risk control issues: biomedical systems. In: Dorf, R.C. (Ed.), The Electrical Engineering Handbook. CRC Press, Boca Raton, FL.