The regular “Banana Skins” column was published in the EMC Journal, starting in January 1998. Alan E. Hutley, a prominent member of the electronics community, distinguished publisher of the EMC Journal, founder of the EMCIA EMC Industry Association and the EMCUK Exhibition & Conference, has graciously given his permission for In Compliance to republish this reader-favorite column. The Banana Skin columns were compiled by Keith Armstrong, of Cherry Clough Consultants Ltd, from items he found in various publications, and anecdotes and links sent in by the many fans of the column. All of the EMC Journal columns are available at: https://www.emcstandards.co.uk/emi-stories, indexed both by application and type of EM disturbance, and new ones have recently begun being added. Keith has also given his permission for these stories to be shared through In Compliance as a service to the worldwide EMC community. We are proud to carry on the tradition of sharing Banana Skins for the purpose of promoting education for EMI/EMC engineers.


247.  Experiences of interference with medical devices in Canada

Electromagnetic interference (EMI) has been responsible for many medical device malfunctions, raising concerns about the safety of patients who depend on these devices. However, the incidence of unreported EMI malfunctions is unknown. Between 1984 and 2000, Health Canada’s Medical Devices Bureau received thirty-six reports of medical device malfunction attributed to EMI. These included 4 reports of medical device malfunctions caused by wireless cellular phones, two cases of EMI interference from electronic article surveillance (EAS) systems on implantable cardiac pacemakers and possibly one case of premature failure of a pacemaker.

The Bureau also investigated reports of interference from other radiofrequency sources. These included (1) Interference of an electrosurgical device with the electrocardiogram signals displayed on the monitor of an automated defibrillator; (2) Complete inhibition of the pacing signal of a pacemaker by a pulsating magnetic field from a video display terminal; (3) Failure of the R-wave detection circuitry of a cardiac defibrillator in the presence of a simulated muscle artifact signal from an electrocardiogram simulator; and (4) Interference of the line isolation system in an intensive care unit with the performance of a defibrillator. These reports highlighted the need for guidelines on the management of EMI within hospitals, especially in critical-care areas.

(Taken from: “Electromagnetic Interference in Medical Devices: Health Canada’s Past and Current Perspectives and Activities” b Kok-Swang Tan et al, Medical Devices Bureau, Therapeutic Products Directorate, Health Canada, IEEE International EMC Symposium, Montreal, August 13-17 2001, page 1283 in the Symposium Record.)

248.  Electrosurgical equipment interferes with endoscope video monitor during operation

The problem — electromagnetic interference (EMI) from electrosurgery units transmitting noise onto real-time video images from an endoscope being used during the operation.

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

249.  Some experiences with interference to medical devices

Steve Juett provided the first “War Story” on the EMC challenges facing biomedical instrumentation in hospitals. He presented very straightforward slides illustrating the situation. The FDA has no immunity requirements for biomedical instrumentation. Not surprisingly, the myriad of telemetry links and proliferation of personal computing devices and cellphones present challenges to medical equipment used to save lives, the sensitivities of which can be in microvolts!

Finally, Steve Juett provided another story from the biomedical arena – tracing down the source of an interference problem at the hospital to a local TV station trying out its HDTV band. It took some effort to get in touch with the right individual at the TV station to resolve the problem!

(Taken from a report on the May 2001 meeting of the IEEE EMC Society Dallas USA Chapter, in the IEEE EMC Society Newsletter Issue 92, pages 8-9. Steve Juett is the Director of Biomedical Engineering at Baylor Hospital in Dallas.)

250.  Cellphone basestation interferes with hospital

I do a lot of work with shielding for MRI scanners. RF interference can ruin the images, which are time-consuming and expensive so all MRI scanners are installed in rooms with some degree of shielding.

One hospital I visited to trace an interference problem was the quickest job I ever had. The hospital had “Switch off your cellphone” warning signs all over it – and a plainly visible cellphone basestation on its roof. When they got the basestation switched off, their interference problems ceased.

(Gary Fenical of Laird Technologies, http://www.lairdtech.com, private conversation with Editor, 23rd
May 2002.
)

251.  More examples of interference with medical devices

A number of medical interference incidents listed in The “1998 EMC Encyclopaedia” from Emf-Emi Control, Inc.

  • Apnoea monitors susceptible to FM transmissions: The US FDA has reported cases where susceptible apnoea monitors used to monitor the breathing of newborns during sleep have been affected by EMI from RF broadcast sources. The apnoea monitor is designed to alarm when the newborn stop breathing. External interference has been demonstrated to mimic the rhythmic breathing patterns when the interference modulation is demodulated by an audio rectification mechanism. The effect is to fool the apnoea monitor and not alarm properly.
  • Patient monitoring system picked up EMI causing alarms not to sound. Two patients died when system failed to detect arrhythmia.
  • Paramedics could not sense heart rhythm due to excessive artifacts on CRT monitor. Patient not resuscitated.
  • External defibrillator/pacemaker stopped pacing when ambulance attendant used hand-held transmitter too close to patient.
  • Battery charger cycling at 1-Hz rate in respiration monitor, coupled to respiration circuit. Patient died with no alarm.
  • Intro-Aortic balloon pump stopped pumping when system printer was turned on.
  • Pacemaker ceased function during ambulance radio transmission.
  • Ventilator – cessation of ventilation, inoperative monitoring, error messages.
  • ESD Disabled apnoea monitors without activating an alarm.
  • Radiation therapy device – ESD caused source to turn on, display to blank, unintended gantry movement, timer failures.
  • Severe interference with heart rate and graphs of ICU patient monitor when blood-pressure monitor in use.
  • Infusion pump caused interference with patient monitors.
  • Movement of chiropractic table caused by muscle stimulator.
  • Microsurgical drill began to run when electrosurgery unit was activated.
  • Erroneous displays and latch-up of anaesthesia gas monitor during electrosurgery.
  • Intro-Aortic balloon pump stopped pumping when system printer was turned on.
  • Neonatal monitors were interfered with when placed close to similar models.
  • Respiration rate controller ceased to function when oxygen analyzer was placed on top.
  • Cellular phones interfered with incubators, infusion pumps, dialysis equipment, defibrillators. They are banned from some hospitals in Europe.
  • Reading of invasive blood pressure monitors jumped 3 to 10 mm Hg when paging transmitter on hospital roof was activated.
  • Displays of telemetry patient monitor would “flat-line” when paging company transmitted digital control information to remote sites.
  • ECG monitor in defibrillator was interfered with when emergency crew transmitted with antenna inside station wagon with defibrillator.
  • Pulse oximeter displayed saturation of 100% and pulse rate of 60 on a patient who had expired. Telemetry transceiver, part of the system, too close to oximeter.

252.  Guidance on use of wireless handsets in hospitals

TETRA: The risk to medical devices from the use of TETRA handsets is comparable to that from GSM cellular phones. All personnel using TETRA handsets on hospital premises should therefore be made aware of, and follow, the local policy guidelines applicable for cellular phone systems. In the case of emergency services dealing with an on site incident, the risk of interference should be treated as secondary to the risks associated with managing the incident. Staff responsible for Trust radio communication policy should liase with local representatives of the emergency services to agree and formulate local working practices.

Outside Media Broadcasts: Ensure that a hospital representative such as the Risk, Safety Communications Manager is available to assist Media personnel with the location and operation of equipment. Media personnel using radio handsets (radio-talkback system) on hospital premises should be made aware of the hospital policy on use two-way radios for all locations in which they will be working. Ensure that any outside broadcast vehicles equipped with radio-talkback and microwave link transmitters are parked as far away as practicable from patient treatment areas or wards.

(Extracted from Medical Devices Agency Safety Notice SN 2001 (06) on January 2nd 2003. The full notice gives information on the technical basis for these warnings.)

253.  Ninety reports of medical device malfunctions due to security equipment 1998-2001

The Food and Drug Administration (FDA) received over 90 problem reports of medical device malfunctions related to EMI from magnetic field emitting security devices since 1998. The malfunctions were judged serious enough by the reporters (clinical users of these devices) to potentially cause patient injuries. Examples of malfunctions with implanted devices ranged from disturbances in the cardiac sensing operation of pacemakers, unintended firing of implanted cardiac defibrillators (ICDs), changes in drug delivery rates of infusion pumps, and over-stimulation of patients with neurostimulators resulting in severe pain or falls.

As a result, the FDA undertook a study of the EM fields emitted from the security screening systems to determine the nature of the EM fields seen by electronic medical devices worn by, or implanted in, patients passing near these screening systems. Measurements of the magnetic field emissions from security devices reveal that some security screening devices can emit fields at strengths that exceed the test level specified in some medical device standards. The FDA took action to alert users and manufacturers of active medical devices and security screening devices of the potential for interactions.

(Taken from: “Comparison of Magnetic Fields Emitted from Security Screening Devices with Magnetic Field Immunity Standards” by Jon P Casamento of the FDA’s Centre for Devices and Radiological Health (CDRH), presented at the IEEE 2002 International EMC Symposium, Minneapolis, August 19-23, pages 937-940 in the Symposium Record.)

(Editor’s note: the standards referred to in Jon’s paper were CENELEC draft standards prEN 45502, Part 2-1 for cardiac pacemakers and Part 2-2 for implantable defibrillators. The 2002 version of EN 60601-1-2 (the EMC safety standard for medical devices) includes a magnetic field immunity test of 3A/m but only at 50Hz, whereas the security screening devices he tested could emit fields of up to 1000A/m at frequencies between 200Hz and 100kHz and 3A/m up to 10MHz.)

254.  Medical diathermy as a source of electromagnetic interference

Medical diathermy is used for physiotherapy, to heat tissues throughout their volume. 27MHz continuous ‘short-wave’ diathermy can use RF powers of up to 400W, but is becoming unfashionable. 27MHz pulsed short-wave diathermy is just coming into fashion and uses average RF powers of around 40W. 2.45GHz microwave diathermy is out of fashion, people being scared off by the idea of ‘microwave cooking’. There is also a technique known as Interferential Therapy which operates at 4kHz.

Electrosurgery equipment typically uses 500kHz. ‘Cutting’ typically uses 1200V and 400W, ‘Point Coagulation’ typically uses 2000V and 150W, ‘Spray Coagulation’ uses 380V and 80W, and ‘Blend’ uses 1800V and 300W (the high frequency prevents the patient from receiving a fatal shock – Editor).

There are significant levels of emissions from the diathermy and electrosurgery leads, and most theatre equipment is now designed to avoid interference from this source. ‘Bipolar’ diathermy technology reduces the interference caused; and most modern equipment uses sinusoids, which reduces the potential of harmonic emissions to cause interference problems.

A traction machine in a physiotherapy department has been seen to malfunction when a 27MHz diathermy system was switched on in the next room. The long leads associated with pacemakers make good antennas and can download large currents at 27MHz directly into the heart, damaging it. External pacemakers used during surgical operations have much longer leads than implanted pacemakers, and are a nightmare. Diathermy has also caused certain defibrillators to charge up and some pulse oximeters to give wrong readings.

(Taken from: “Surveying a hospital for electromagnetic interference” by 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. IPEM is the Institute of Physics and Engineering in Medicine, at: https://www.ipem.ac.uk. Diathermy and electrosurgery are well-known by surgeons as causes of interference problems. For more examples see Banana Skins 83, 247, 248, 251, 257, 258 and 261.)

255.  Medical device interference from mobile phones

Actual reports of serious problems are hard to come by. However, in-house tests at the University of York, and field-test studies such as that commissioned by the Medical Devices Agency have shown that many types of hospital equipment are susceptible to RF radiation, although generally only at distances of less than 2m. Victims of EMI from mobile transmitters typically include diagnostic equipment such as ECGs, EEGs, pulse oximeters and other physiological monitoring equipment; plus therapeutic equipment such as infusion pumps, ventilators and defibrillators. Physiological monitoring has a bandwidth of around 100Hz and is very sensitive – so very susceptible. For example the sensitivity of an ECG is 1mV and of an EEG is 100µV, whereas ‘Evoked potential’ monitors can be sensitive to as low as 1µV.

The type of modulation employed by the mobile transmitter can be significant. For example, an external pacemaker we tested withstood a GSM modulated signal at 30V/m field strength, but TETRA modulation caused interference at 3V/m. GSM modulates its signal at 217Hz, whereas TETRA uses 17Hz which has a greater probability of lying within the pass-band of medical equipment.

We found that a distance of 1.2 metres was required for the medical equipment we tested to be safe. For comparison: Rice and Smith (Canada) found that 10 out of 14 devices failed with a 0.6W mobile phone at distances of under 500mm; Irnich and Tobisch (Germany) tested 224 devices and recommended a safe distance of at least 1 metre; The U.K.’s Medical Devices Agency tested 178 devices and found that 4% exhibited effects with mobile transmitters at 1 metre, although only 0.1% of them had serious effects at that distance (Bulletin BD 9702).

(Taken from “Mobile communication systems and medical equipment”, by M P Robinson, I D Flintoff and A C Marvin, York Electromagnetics, University of York, IPEM conference “Practical Methods for Mitigation of EMI and EMF Hazards within Hospitals”, York, 28th January 2003. IPEM is the Institute of Physics and Engineering in Medicine at: https://www.ipem.ac.uk.)

(Also see: M P Robinson, I D Flintoft and A C Marvin, ‘Interference to medical equipment from mobile phones’, J. Med. Eng. Technol. vol. 21, p. 141, 1997. M L Rice and J M Smith, ‘Study of electromagnetic interference between portable cellular phones and medical equipment’, Proc. Canadian Med. Biol. Eng. Conf. p330, 1993. Steve Smye, ‘Assessing the risk to medical equipment of interference from mobile phones’, EMC York ‘98 Conf. Proc., July 1998, “Electromagnetic compatibility of medical devices with mobile communications”, Bulletin MDA DB 9702 March 1997 from the U.K. Medical Devices Agency, “Safety Notice SN 2001 (06)”, the U.K. Medical Devices Agency.)

256.  MRI scanners as a source of electromagnetic problems

Magnetic Resonance Imaging (MRI) uses very powerful static magnetic fields, up to 3 Tesla in the U.K., but systems with up to 8 Tesla are available and there is a trend towards using more powerful fields. This magnetic field can accelerate ferromagnetic objects with serious consequences. A patient was struck by an oxygen bottle while being placed in the magnet bore. Parts of a fork lift truck weighing 800 pounds were accelerated by the magnet, striking a technician and resulting in serious injury. A pair of scissors was pulled out of a nurse’s hand as she entered the magnet room, hit a patient, causing a head wound. Dislodgement of an iron filing in a patient’s eye during an MRI exam resulted in vision loss in that eye.

Implantable medial devices such as stents, clips, prostheses, pacemakers and neuro-stimulators are all potential hazards in an MRI scan, and devices should be tested for MR compatibility. It is known that pacemakers can be very sensitive to static magnetic fields of the order of 1 milliTesla. Monitoring equipment such as ECG, heart-rate, blood pressure, blood oxygen monitors are also of concern. MRI scanners also use intense RF fields, with most U.K. systems operating at 42.6, 63.9 or 127.8MHz.

(Taken from: “Electromagnetic fields in the hospital environment”, by Jeff W. Hand, Director, Radiological Sciences Unit, Hammersmith Hospitals NHS Trust, London, IPEM conference “Practical Methods for Mitigation of EMI and EMF Hazards within Hospitals”, York, 28th January 2003. IPEM is the Institute of Physics and Engineering in Medicine: at https://www.ipem.ac.uk.)

257.  The hospital EM environment often exceeds IEC immunity standards for medical devices

E-M fields in hospitals. Broadband RF field measurements in the hospital environment have found that E fields can be up to 30V/m. The strongest sources included electrosurgical units, hand-held radios and VDUs. Power frequency magnetic field measurements in the hospital environment have found H fields up to 5A/m.

The strongest sources included power lines and supplies, patient monitoring equipment, VDUs and electrosurgical units. 63% of all E-field measurements and 7% of all H-field measurements made in the hospital environment exceeded proposed IEC immunity requirements for medical devices.

(Also taken from: “Electromagnetic fields in the hospital environment”, by Jeff W. Hand, Director, Radiological Sciences Unit, Hammersmith Hospitals NHS Trust, London, IPEM conference “Practical Methods for Mitigation of EMI and EMF Hazards within Hospitals”, York, 28th January 2003. IPEM is the Institute of Physics and Engineering in Medicine: at https://www.ipem.ac.uk.)

About The Author

Keith Armstrong

After working as an electronic designer, then project manager and design department manager, Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Over the last 20 years, Keith has presented many papers, demonstrations, and training courses on good EMC engineering techniques and on EMC for Functional Safety, worldwide, and also written very many articles on these topics. He chairs the IET’s Working Group on EMC for Functional Safety, and is the UK Government’s appointed expert to the IEC committees working on 61000-1-2 (EMC & Functional Safety), 60601-1-2 (EMC for Medical Devices), and 61000-6-7 (Generic standard on EMC & Functional Safety).

Related Posts

Leave a Reply

Your email address will not be published.

X