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Cellphones make wheelchair dangerously uncontrollable

Fibreglass ambulance roof causes death of patient

Interference from medical diathermy and electro-surgery

Walkie-talkies and cellphones interfere with medical equipment

Numerous other interference problems with medical devices

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Cellphones make wheelchair dangerously uncontrollable Top of page button

Description

In the Netherlands recently a wheelchair unintentionally drove off a subway-platform, badly injuring the person in it. Her insurance company started an investigation with help of an EMC laboratory, who found out that the chair would move uncontrollably in a field of only a few Volts/meter at 1.89 GHz.

Cartoon of interference causing wheelchair malfunction

The chair’s manufacturer argued that it complied with the relevant EMC product standard for wheelchairs. But the radiated susceptibility test in this standard did not go beyond 1GHz and the judge decided that the manufacturer could have known that 1.89 GHz was a commonly used cellphone handset transmitter frequency.

The manufacturer was sentenced because he had put an unsafe product on the market.


Commentary

Most (if not all) of the EMC and Safety standards used for regulatory compliance either do not cover ‘EMC-related functional safety’ or cover it badly. Clearly, a safety-critical machine like a powered wheelchair must remain under control in any reasonably foreseeable electromagnetic environment, which obviously must include nearby cellphone handsets. A cellphone handset at a distance of a foot or so creates a field of over 50V/m at its transmitter frequency.

Most wheelchair manufacturers are not electrical/electronic engineers and buy their drive and control systems from companies who are. It is important that these systems are correctly designed so that they do not risk the health and safety of the wheelchair users and third parties, and that the designers communicate all necessary information on correct assembly/installation methods to maintain EMC performance to the wheelchair manufacturer.

It is important that the wheelchair manufacturer follows all the EMC assembly/installation advice given by the drive and control system suppliers, and also that the finished wheelchair designs are fully tested to ensure they are immune enough for safety reasons.


References and links

“EMC and Functional Safety, Impact of IEC 61000-1-2”, Dick Groot Boerle (Teamleader EMC Laboratory for Thales Nederland B.V), IEEE 2002 International EMC Symposium, Minneapolis, August 2002, http://www.ewh.ieee.org/soc/emcs.

Examples of other wheelchair interference problems can be found in the “Banana Skins compendium”, via a link from www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers:11, 129, 219 and 220.

“Radiowaves may interfere with control of powered wheelchairs and motorized scooters”, Department of Health and Human Services, FDA, September 20, 1994. Can be downloaded from http://www.fda.gov/cdrh/emc/emcref.html, or via: http://www.fda.gov.

The IEE’s guide on “EMC and Functional Safety”, 2000, can be downloaded from: http://www.iee.org/Policy/Areas/EMC/index.htm.


Links to Mitigation Techniques

  Installation Design & Development Resources
Shielding of enclosures Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding of cables Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Circuit design to reduce demodulation   Click to go to Design technique Click to go to Resources technique
Filtering with CM cable-mounted chokes Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering Click to go to installation technique Click to go to Design technique Click to go to Resources technique

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Fibreglass ambulance roof causes death of patient Top of page button

Description

Medical technicians taking a heart-attack victim to the hospital in 1992 attached her to a monitor/defibrillator. Unfortunately, the heart machine shut down every time the technicians turned on their radio transmitter to ask for advice, and as a result the woman died.

Analysis showed that the monitor unit had been exposed to exceptionally high fields because the ambulance roof had been changed from metal to fibreglass (to let in more daylight) and also fitted with a long-range radio antenna. The reduced shielding from the vehicle roof combined with the strong radiated signal proved to be too much for the monitor/defibrillator.


Commentary

Ambulances in the U.K. are typically fitted with radio transmitters with 100W of transmitted power (ERP), and the resulting field strengths near the roof, where the medical equipment is often installed, can exceed 70V/m in a vehicle with a metal roof.

Compare this with the 3V/m and 10V/m RF field susceptibility tests required by the medical devices safety standard EN 60601-1-2. Equipment for fitting in ambulances and similar environments must be able to function correctly despite such strong RF fields, so they need to have greater immunity.


References and links

“Medical Horror Stories”, Compliance Engineering Magazine, September/October 1994, pages 100-101, http://www.ce-mag.

The same example is item 2.3.3.2 in NASA Reference Publication 1374 (RP-1374), “Electronic Systems Failures and Anomalies Attributed to Electromagnetic Interference”, which can be downloaded from: http://trs.nis.nasa.gov/archive/00000296

Examples of other ambulance interference can be found in the “Banana Skins compendium”, via a link from http://www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 3, 39, 179 and 251.


Links to Mitigation Techniques

  Installation Design & Development

Resources

Circuit design to reduce demodulation   Click to go to Design technique Click to go to Resources technique
Filtering with CM cable-mounted chokes Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding enclosures Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding cables Click to go to installation technique Click to go to Design technique Click to go to Resources technique

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Interference from medical diathermy and electro-surgery Top of page button

Description


Commentary

Medical diathermy and electrosurgery is well known as a source of significant interference problems that most surgeons simply learn to cope with.

Medical diathermy used for physiotherapy typically operates at 27MHz with RF powers up to 400W, although modern pulsed diathermy uses average RF powers around 40W. There is also a technique known as Interferential Therapy which operates at 4kHz.

Electrosurgery equipment used for cutting and coagulation typically uses 500kHz at up to 1A, 2kV and 400W (the high frequency prevents electric shock to the patient). Typical electrosurgery equipment radiates at harmonics of its basic operating frequency, especially between 1 and 10MHz, and was adapted during the Second World War for jamming radar. It can generate fields of 40-50V/m at 1m, making it difficult to use monitors in the same room. Shielding is not practical as the patient forms part of the ‘accidental’ radiating antenna.

There are significant levels of emissions from the diathermy/electrosurgery leads, although they can be reduced by careful lead routing. ‘Bipolar’ diathermy technology reduces the interference caused, and the use of sinusoidal output waveforms reduces the emissions of harmonics.

The long leads which connect pacemakers to their wearers’ hearts make good ‘accidental’ receiving antennas – picking up the RF fields from diathermy/electrosurgery and injecting strong RF currents into the heart. These currents damage tissues and reduce their sensitivity to pacemaker signals, possibly preventing the pacemaker from functioning. External pacemakers have much longer leads than implanted pacemakers and so pick up the fields from diathermy and electrosurgery with greater facility, causing greater problems.


References and links

“Surveying a hospital for electromagnetic interference”, Lindsay Grant, Consultant Clinical Engineer, Royal United Hospital, Bath, U.K.; IPEM conference “Practical Methods for Mitigation of EMI and EMF Hazards within Hospitals”, York, 28th January 2003, http://www.ipem.org.uk, http://www.ipem.org.uk/meetings/28jan03abs.pdf.

“Electromagnetic Interference in Medical Devices: Health Canada’s Past and Current Perspectives and Activities” by Kok-Swang Tan et al, Medical Devices Bureau, Therapeutic Products Directorate, Health Canada, IEEE 2001 International EMC Symposium, Montreal, August 13-17 2001, page 1283, http://www.ewh.ieee.org/soc/emcs/

“Electromagnetic Interference (EMI) in an Operating Theatre Environment”, Nigel Beaumont-Rydings of the Royal Oldham Hospital, in a paper given at a meeting of the “CE North West” club on 30th March 1998.

The “1998 EMC Encyclopaedia” from Emf-Emi Control, Inc.

“EMC of Medical Equipment”, Dr Martin P Robinson, University of York, N. J. Wainwright York EMC Services Ltd., presented at EMV'99 Dusseldorf, Germany and available from: http://www.yorkemc.co.uk/Technical/Papers/Docs/Emv99/emv99.htm

“Practical problems of electrosurgery”, J Gardner, IEE Colloquium on “Electromagnetic interference in hospitals”, Digest no. 1994/190, sec. 6, 1994, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.

“How Physiotherapy generates problems”, D Wilton, IEE Colloquium on “Electromagnetic interference in hospitals”, Digest no. 1994/190, sec. 7, 1994, from sales@iee.org.uk, http://www.iee.org/shop or http://www.iee.org.uk/Library.

“Electric field strengths created by electrosurgical units”, Robert Nelson and Hualiang Ji, IEEE 1994 International EMC Symposium, p. 366, http://www.ewh.ieee.org/soc/emcs.


Links to Mitigation Techniques

  Installation Design & Development Resources
Balanced interconnections and twisted pair Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding enclosures Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding cables Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering with CM cable-mounted chokes Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering Click to go to installation technique Click to go to Design technique Click to go to Resources technique

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Walkie-talkies and cellphones interfere with medical equipment Top of page button

Description

Many types of hospital equipment are susceptible to RF radiation from hand-portable mobile radio transmitters – diagnostic equipment such as ECGs, EEGs, pulse oximeters and other physiological monitoring equipment; and therapeutic equipment such as infusion pumps, ventilators and defibrillators. Physiological monitoring equipment is very sensitive – hence very susceptible.

The type of modulation employed by the mobile transmitter can be significant. For example, an external pacemaker withstood a GSM signal (modulated at 217Hz) at 30V/m field strength, but TETRA modulation (17Hz) caused interference at 3V/m.

Cartoon of mobile phone user causing interference with a patient's ECG machineYork EMC Services recommend a distance of 1.2 metres be maintained between typical hand-portable transmitters and medical equipment. Rice and Smith found that 10 out of 14 devices failed with a 0.6W mobile phone at distances of under 500mm. Irnich and Tobisch tested 224 devices and recommend a safe distance of at least 1 metre (but 1.5 metres for emergency services’ mobile handsets, because they are more powerful). The U.K.’s Medical Devices Agency tested 178 devices and found that 4% exhibited effects with mobile transmitters at 1 metre distance, although only 0.1% of them had serious problems at that distance (Bulletin BD 9702).

It is estimated that 1 out of every 200 patients admitted to US hospitals die due to medical errors, an annual rate exceeding that due to road accidents or heart disease. In the vast majority of cases, excellent physicians and medical staff make such errors because they do not have access to appropriate information (e.g., medical textbooks, correct dosages for medication; patient health history records, etc.).

At least some of these deaths would be preventable if wireless information technology were widely used to make the necessary information quickly and easily available wherever it is needed throughout a hospital. However, the associated increased RF emissions may cause increased medical-device malfunctions.


Commentary

Demodulation of RF signals occur in all semiconductors and ICs, whether analogue or digital, and is sometimes called audio rectification. Digital circuits have more noise immunity than analogue, and so high-sensitivity analogue circuits – which are widely applied in medical electronics, since the patient-coupled signals are so small - are the worst affected.

Hand-held mobile transmitters (such as walkie-talkies, ‘handyphones’, cellphones, radio-telephones, mobile phones, etc.) use up to 4W of RF power and can be close enough to expose equipment to fields which are greatly in excess of the 3V/m or 10V/m immunity tests typically required by medical equipment EMC standards. Mobile transmitters used by the emergency services can be up to 10W RF power, whilst (legal) vehicle transmitters can be up to 100W.

It is possible to design analogue circuits to demodulate less, but the problem cannot be eliminated and so shielding and filtering techniques are required to reduce the levels of the RF signals which actually get through to the circuits.

Due to the ubiquity of hand-portable and vehicle mobile radio transmitters, this is a common problem which very few EMC or safety standards address.


References and links

“Mobile communication systems and hospital equipment”, M P Robinson, I D Flintoff and A C Marvin, York Electromagnetics, University of York.

“Interference to medical equipment from mobile phones”, M P Robinson, I D Flintoft and A C Marvin, J. Med. Eng. Technol. vol. 21, p. 141, 1997.

“Study of electromagnetic interference between portable cellular phones and medical equipment’, M L Rice and J M Smith, Proc. Canadian Med. Biol. Eng. Conf. p330, 1993.

“Effect of mobile phone on life-saving and life-sustaining systems.” Irnich W, Tobisch R, Biomed Tech (Berl) 43(6):164-173, 1998, in German. An English précis is available at http://www.electric-words.com/cell/abstracts/index-misc.html.

“Electromagnetic compatibility of medical devices with mobile communications”, Bulletin MDA DB 9702 March 1997 from the U.K. Medical Devices Agency, http://www.medical-devices.gov.uk.

“Safety Notice SN 2001 (06)”, the U.K. Medical Devices Agency, http://www.medical-devices.gov.uk.

“Assessing the risk to medical equipment of interference from mobile phones”, Steve Smye, EMC York ‘98 Conference Proceedings, July 1998, http://www.yorkemc.co.uk.

“The mobile phone problem in hospitals” Martin Robinson and Ian Flintoff, EMC York ‘98 Conference Proceedings, July 1998, http://www.yorkemc.co.uk.

“Interference with cardiac pacemakers by cellular telephones”, D L Hayes, P J Wang, D W Reynolds, M Estes 3rd, J L Griffith, R A Steffens, G L Carlo, G K Findlay, C M Johnson, New England Journal of Medicine 336(21):1473-1479, 1997, précis at: http://www.electric-words.com/cell/abstracts/index-misc.html.

The IEE’s guide on “EMC and Functional Safety”, 2000, can be downloaded from: http://www.iee.org/Policy/Areas/EMC/index.htm.

“Risk of patient injury due to electromagnetic interference malfunctions: Estimation and minimization”, B.Segal et al, IEEE International EMC Symposium, Montreal, August 13-17 2001, page 1308 in the Symposium Record.

“Risk analysis and EMI Risk Abatement Strategies for Hospitals: Scientific and Legal Approaches”, David A Townsend, Faculty of Law, University of New Brunswick, Canada, IEEE International EMC Symposium, Montreal, August 132-17 2001, page 1304 in the Symposium Record, http://www.ewh.ieee.org/soc/emcs


Links to Mitigation Techniques

  Installation Design & Development Resources
PCB layout   Click to go to Design technique Click to go to Resources technique
Reducing demodulation   Click to go to Design technique Click to go to Resources technique
Shielding enclosures Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Shielding cables Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering with CM cable-mounted chokes Click to go to installation technique Click to go to Design technique Click to go to Resources technique
Filtering Click to go to installation technique Click to go to Design technique Click to go to Resources technique

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Numerous other interference problems with medical devices Top of page button

Cartoon of doctor hearing The Archers through his stethescope when examining a patient with a pacemakerDescription

In the USA, the Food and Drug Administration (FDA) collects reports of medical equipment failure. Jeffrey Silberberg of the FDA’s Centre for Devices and Radiological Health (CDRH) is responsible for raising the awareness of the medical community to the problems of interference.

In numerous conference papers and articles he has described over 100 FDA reports attributed to EMI between 1979 and 1993. These include interference to a wide range of devices, including ECG, ventilators, infusion pumps and apnoea monitors, from a variety of sources including electrosurgery, fluorescent lights and radio transmitters.


Commentary

Silberberg’s EMI-related FDA reports form only a small portion of the (typically) 95,000 incidents reported to the FDA each year, but he and others believe there is widespread under-reporting of EMI incidents due to the lack of education and training that medical personnel have in EMC.

A number of useful sources of information on medical device EMC problems are given below.


References and links

“EMC of Medical Equipment”, Dr Martin P Robinson, University of York, N. J. Wainwright York EMC Services Ltd., EMV'99 Dusseldorf, Germany

“Performance degradation of electronic medical devices due to electromagnetic interference”, Jeffrey L Silberberg, Compliance Engineering vol. 10 p. 25 1993. An updated version was published in Compliance Engineering’s European Edition’s 1995 Annual Reference Guide as: “Electronic medical devices and EMI” (pages F-10 - F-15), http://www.ce-mag.com

“What can/should we learn from reports of medical device electromagnetic interference?”, J L Silberberg, Proceedings of a Workshop on Electromagnetics, Health Care and Health, held in association with the 17th Annual International Conference of the IEEE Engineering in Medicine and Biology Society and the 21st Canadian Medical and Biological Engineering Conference, Montreal, Canada, pp. 10-19, September 19-20, 1995. This was also published in Compliance Engineering, Vol. XIII, No. 4, May - June 1996, pp.41-57, http://www.ce-mag.com

“What can/should we learn from reports of medical device electromagnetic interference?”, J L Silberberg, Proceedings of the 1995 IEEE Engineering in Medicine and Biology 17th Annual Conference and the 21st Canadian Medical and Biological Engineering Conference, Montreal, Canada, CD-ROM, Web Edition, September 20-23, 1995.

“Medical device electromagnetic interference issues, problem reports, standards, and recommendations”, J L Silberberg, Proceedings of the Health Canada Medical Devices Bureau Round-Table Discussion on Electromagnetic Compatibility in Health Care, Ottawa, Canada, pp. 11-20, September 22-23, 1994.

“An FDA perspective on wireless radio interference and medical devices”, J L Silberberg, Workshop Notes, IEEE 1995 International Symposium on Electromagnetic Compatibility, Atlanta, GA, August 14-18, 1995.

“Electromagnetic compatibility for medical devices: Issues and solutions”, FDA/AAMI Conference 1995. Conference report edited by Stephen Sykes of the U.S. Food and Drug Administration, 1996. ISBN 1-57020-054-8, published by the Association for the Advancement of Medical Instrumentation, http://www.aami.org.

“Electromagnetic compatibility / electromagnetic interference: Solutions for medical devices”, FDA/AAMI Conference 1997. Conference report published by the Association for the Advancement of Medical Instrumentation, http://www.aami.org.

“Technical Information Report TIR-18 – 1997: Guidance on electromagnetic compatibility of medical devices for clinical/biomedical engineers – Part 1: Radiated radio-frequency electromagnetic energy”, AAMI, http://www.aami.org.

“Practical methods for mitigation of EMI and EMF hazards within hospitals”, IPEM Conference, York, 28th January 2003, http://www.ipem.org.uk/meetings/28jan03abs.pdf, http://www.ipem.org.uk.

“Current EMC issues in healthcare”, IEEE 2002 International EMC Symposium, Minneapolis, August 19-23 2002, Workshops and Tutorials sessions, http://www.ewh.ieee.org/soc/emcs.

“Consultant on call: The case of the talking EEG machine”, Mark J Nave, Compliance Engineering – European Edition, July/August 1994,m pages 43-47, http://www.ce-mag.com

The FDA’s EMC web site has more information on EMI and medical devices, and many related links, at: http://www.fda.gov/cdrh/emc/index.html and also at http://www.fda.gov/cdrh/emc/emc-resources.html

Other examples of interference with medical devices and equipment can be found in the “Banana Skins compendium”, via a link from http://www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 3, 16, 20, 38 55, 67, 72, 121, 122, 244 - 253 + etc…???? more will appear during August.

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