Donald Witters

In vitro assessment of tissue heating near metallic medical implants by exposure to pulsed radio frequency diathermy

A patient with bilateral implanted neurostimulators suffered significant brain tissue damage, and subsequently died, following diathermy treatment to hasten recovery from teeth extraction. Subsequent MRI examinations showed acute deterioration of the tissue near the deep brain stimulator (DBS) leads electrodes which was attributed to excessive tissue heating induced by the diathermy treatment. Though not published in the open literature, a second incident was reported for a patient with implanted neurostimulators for the treatment of Parkinsons disease. During a diathermy treatment for severe kyphosis, the patient had a sudden change in mental status and neurological deficits. The diathermy was implicated in causing damage to the patients brain tissue. To investigate if diathermy induced excessive heating was possible with other types of implantable lead systems, or metallic implants in general, we conducted a series of in vitro laboratory tests. We obtained a diathermy unit and also assembled a controllable laboratory exposure system. Specific absorption rate (SAR) measurements were performed using fibre optic thermometry in proximity to the implants to determine the rate of temperature rise using typical diathermy treatment power levels. Comparisons were made of the SAR measurements for a spinal cord stimulator (SCS) lead, a pacemaker lead and three types of bone prosthesis (screws, rods and a plate). Findings indicate that temperature changes of 2.54 and 4.88 degrees C s(-1) with corresponding SAR values of 9129 and 17,563 W kg(-1) near the SCS and pacemaker electrodes are significantly higher than those found in the proximity of the other metallic implants which ranged from 0.04 to 0.69 degrees C s(-1) (129 to 2471 W kg(-1)). Since the DBS leads that were implanted in the reported human incidents have one-half the electrode surface area of the tested SCS lead, these results imply that tissue heating at rates at least equal to or up to twice as much as those reported here for the SCS lead could occur for the DBS leads.

On the mechanisms of interference between mobile phones and pacemakers: parasitic demodulation of GSM signal by the sensing amplifier

The aim of this study was to investigate the mechanisms by which the radiated radiofrequency (RF) GSM (global system for mobile communication) signal may affect pacemaker (PM) function. We measured the signal at the output of the sensing amplifier of PMs with various configurations of low-pass filters. We used three versions of the same PM model: one with a block capacitor which short circuits high-frequency signals; one with a ceramic feedthrough capacitor, a hermetically sealed mechanism connecting the internal electronics to the external connection block, and one with both. The PMs had been modified to have an electrical shielded connection to the output of the sensing amplifier. For each PM, the output of the sensing amplifier was monitored under exposure to modulated and non-modulated RF signals, and to GSM signals (900 and 1800 MHz). Non-modulated RF signals did not alter the response of the PM sensing amplifier. Modulated RF signals showed that the block capacitor did not succeed in short circuiting the RF signal, which is somehow demodulated by the PM internal non-linear circuit elements. Such a demodulation phenomenon poses a critical problem because digital cellular phones use extremely low-frequency modulation (as low as 2 Hz). which can be mistaken for normal heartbeat.

Testing the Immunity of Active Implantable Medical Devices to CW Magnetic Fields up to 1 MHz by an Immersion Method

This paper presents a magnetic-field system and the method developed for testing the immunity of the active implantable medical devices to continuous-wave magnetic fields in the frequency range up to 1 MHz. The system is able to produce magnetic fields of 150 A/m for frequencies up to 100 kHz and strengths decreasing as l/f between 100 kHz and 1 MHz, with uniformity of the field within plusmn2.5% in the volume for tests. To simulate human tissue, the medical device, together with its leads, is placed on a plastic grid in a saline tank that is introduced in the magnetic field of the induction coil. This paper offers an alternative for the injection voltage methods provided in the actual standards for assessing the protection of the implantable medical devices from the effects of the magnetic fields up to 1 MHz. This paper presents the equipment and signals used, the test procedure, and results from the preliminary tests performed at the Food and Drug Administration-Center for Devices and Radiological Health on implantable pacemakers and neurostimulators. The new system and test method are useful for the EMC research on the implantable medical devices.

EMC and wireless healthcare

Electromagnetic compatibility (EMC) is a critical part of addressing the risks related to the effects of electromagnetic interference (EMI) on active medical devices exposed to emissions from wireless technology. In addition, for wireless technology in healthcare to be safe, effective, reliable, and secure specific wireless issues must also be addressed including quality of service, coexistence with other wireless equipment, data integrity, and wireless security. Unfortunately, these issues pose risks that are poorly addressed in present medical devices standards or other consensus documents. This paper discusses risks for wireless technology in healthcare with examples from research examining the effects of emissions from wireless technology such as RFID on implantable cardiac pacemakers and defibrillators and EMC with other emitters. The paper goes into ways that the risks, including EMI, can be addressed and makes the case for substantive engagement by stakeholders, including the IT community, wireless developers, and clinical organizations. There is clear need to develop unbiased, consensus information and tools that will set the pathways and tools needed to meet the risks and challenges for widespread incorporation of wireless technology in healthcare.

Possible overexposure of pregnant women to emissions from a walk through metal detector

This paper presents a systematic procedure to evaluate the induced current densities and electric fields due to walk-through metal detector (WTMD) exposure. This procedure is then used to assess the exposure of nine pregnant women models exposed to one WTMD model. First, we measured the magnetic field generated by the WTMD, then we extracted the equivalent current source to represent the WTMD emissions and finally we calculated the induced current densities and electric fields using the impedance method. The WTMD emissions and the induced fields in the pregnant women and fetus models are then compared to the ICNIRP Guidelines and the IEEE C95.6 exposure safety standard. The results prove the consistency between maximum permissible exposure (MPE) levels and basic restrictions for the ICNIRP Guidelines and IEEE C95.6. We also found that this particular WTMD complies with the ICNIRP basic restrictions for month 1-5 models, but leads to both fetus and pregnant women overexposure for month 6-9 models. The IEEE C95.6 restrictions (MPEs and basic restrictions) are not exceeded. The fetus overexposure of this particular WTMD calls for carefully conducted safety evaluations of security systems before they are deployed.

Medical device EMI: FDA analysis of incident reports, and recent concerns for security systems and wireless medical telemetry

FDA has evaluated reports of medical device malfunctions caused by electromagnetic interference (EMI), performed device testing, and developed standardized test procedures. Over 500 incident reports are suspected to be attributable to EMI affecting cardiac devices. More than 80 of these reports involve cardiac and other medical device interactions with electronic security systems. EMI presents a risk to patient safety and medical device effectiveness that is likely to continue as the use of electromagnetic energy in the medical device environment increases (e.g., cell phones, security systems). Developments can reduce these risks, such as the allocation of dedicated frequency bands for the new wireless medical telemetry service (WMTS) designed to protect transmissions of patient vital signs from interference by other intentional transmitters.

Wireless Medical Systems: Risks, Challenges, And Opportunities

Envision a wireless patient monitoring system at a large busy hospital suddenly loses connection with several patients. The staff scrambles to reconnect the patients to wired monitors while the clinical engineering department tries to figure out the problem. The cause is traced to the new digital television broadcast which has completely overwhelmed the medical system.1 Consider drug infusion pumps, active implantable medical devices, wireless nurse call units, or blood collection systems where the wireless link is slowed, intermittent, disrupted, or cannot be reliably established. Visualize a patient just home from a procedure where a new pacemaker generator was implanted because the old device battery was at the end of its life. He is awakened by an alarm that indicates battery end of life only to find that the alarm was from the old pacemaker that the patient had placed near his bed.2 These are real events. The benefits and opportunities for innovation via wireless technology are numerous and can outweigh the risks. This article focuses on key technical aspects that, if managed well, can help make the path to market and market adoption much more likely.

Electromagnetic Compatibility Testing of Implantable Neurostimulators Exposed to Metal Detectors

This paper presents results of electromagnetic compatibility (EMC) testing of three implantable neurostimulators exposed to the magnetic fields emitted from several walk-through and hand-held metal detectors. The motivation behind this testing comes from numerous adverse event reports involving active implantable medical devices (AIMDs) and security systems that have been received by the Food and Drug Administration (FDA). EMC testing was performed using three neurostimulators exposed to the emissions from 12 walk-through metal detectors (WTMDs) and 32 hand-held metal detectors (HHMDs). Emission measurements were performed on all HHMDs and WTMDs and summary data is presented. Results from the EMC testing indicate possible electromagnetic interference (EMI) between one of the neurostimulators and one WTMD and indicate that EMI between the three neurostimulators and HHMDs is unlikely. The results suggest that worst case situations for EMC testing are hard to predict and testing all major medical device modes and setting parameters are necessary to understand and characterize the EMC of AIMDs.

Tests for Radio Frequency Interference to Medical Telemetry Operating in the Private Land Mobile Radio Service (PLMRS) from Newly Allocated PLMRS Channels

Abstract Radio frequency interference (RFI) is a well-known risk for wireless medical telemetry, particularly for older telemetry systems still operating in the Private Land Mobile Radio Service (PLMRS) frequencies between 450 and 470 MHz. Testing was performed with medical telemetry systems at two local hospitals using the older 25 kHz wide, 460 to 470 MHz channel PLMRS telemetry to assess the RFI potential with transmissions using newly allocated 12.5 kHz and 6.25 kHz wide channels. Significant interference and loss of telemetry was observed on the telemetry monitors when the new channels both overlapped with and were adjacent to the medical telemetry channels. This work demonstrates the vulnerability of older medical telemetry using the radio frequencies between 460 and 470 MHz to new PLMRS transmitters that will operate in these frequencies after December 31, 2005. Telemetry users are urged to assess their equipment RFI vulnerabilities, particularly in the PLMRS frequency band, and migrate to frequenc...

Characterizing the 2.4 GHz Spectrum in a Hospital Environment: Modeling and Applicability to Coexistence Testing of Medical Devices

The increasing use of shared, unlicensed spectrum bands by medical devices and nonmedical products highlights the need to address wireless coexistence to ensure medical device safety and effectiveness. This paper provides the first step to approximate the probability of a device coexisting in its intended environment by providing a generalized framework for modeling the environment. The application of this framework is shown through an 84day spectrum survey of the 2.4-2.48 GHz industrial, scientific, and medical band in a hospital environment in the United States. A custom platform was used to monitor power flux spectral density and record received power. Channel utilization of three nonoverlapping channels of 20 MHz bandwidth-relative to IEEE 802.11 channels 1, 6, and 11-were calculated and fitted to a generalized extreme value distribution. Low channel utilization was observed (<;10%) in the surveyed environment with sporadic occurrences of higher channel utilization (>50%). Reported findings can be complementary to wireless coexistence testing. This paper can provide input to the development of a consensus standard for wireless device coexistence test methods and a consensus document focused on wireless medical device coexistence risk management.