Effects on equipment

Implanted Medical Devices and EMFs

Implanted medical devices are pacemakers, implanted defibrillators and similar. People with these devices may be particularly concerned about interference from EMFs, so we give quite a lot of detail here. For a simpler summary and for information on how EMFs interfere with other equipment, see here.

Advances in medical technology over the past 50 years have resulted in an increased number of patients who have active implantable medical devices (AIMDs). There are no longer the constraints there used to be on these patients and many are able to live active, full lives, including returning to full-time work. There are however electric and magnetic fields present in the environment, which can potentially interact with these devices.

• What are Active Implantable Medical Devices?
• Pacemakers and Intra-cardiac Defibrillators
• Electric and Magnetic Field Interference
• Advice to patients

An active implantable medical device (AIMD) is any medical device which is intended to be totally or partially introduced, surgically or medically, into the human body, and which is intended to remain after the procedure. Some of the more common of these are:

• Pacemakers
• Implantable cardioverter defibrillators (ICDs)
• Neurostimulators
• Cochlear implants
• Insulin pumps

All usually comprise of a small battery-powered box with electronic circuitry and leads, electrodes and/or sensors, which detect a biological function or deliver a stimulus. Pacemakers and ICDs are by far the most common types of AIMDs and these are designed to sense the heart’s electrical activity and deliver an appropriate form of electrical stimulation to the heart when necessary.

Pacemakers and Intra-cardiac Defibrillators

The heart produces electrical signals or action potentials, which need to be reliably detected by pacemakers’ and ICDs’ sensing leads. The heart has its own natural pacemaker, called the Sino-Atrial Node (SA). Pacemakers and ICDs are needed when problems arise affecting the heart’s natural cardiac cycle.

Pacemakers are designed to stimulate the heart when either the heart’s own pacemaker paces at an inappropriate rate or the heart's electrical conduction system prevents the propagation of electrical impulses from the native pacemaker to the lower chambers of the heart. Pacemakers are needed to solve a number of problems with the hearts cardiac cycle and therefore have to be programmable and function in a number of different modes.

ICDs are designed to detect and treat ventricular fibrillation of the heart by sensing the heart’s electrical activity. ICDs deliver a shock to the heart that is designed to stop the heart’s activity, allowing it to restart itself at the correct pacing.

The electrical sensitivities of the atria and ventricles vary, but are both very low. The atrial sensitivity for electrical stimulation is 0.25-1.6 mV and ventricular is 0.75-4.0 mV, both of which are so low that EMFs realistically can induce potentials in sensing leads. The sensitivity setting varies and is dependant on the patient not the device itself. The sensitivity to external interference varies greatly between devices; however it is clear that unipolar sensing is more sensitive than bipolar. Unipolar sensing has only one lead attached to the atrium or ventricular wall, as apposed to bipolar, which is attached to both.

The devices themselves are usually tested to EMC standards that are designed to prevent damage to the device itself, and not the influence of external fields. Once implanted the sensing of the devices can be tuned to that which is required by the patient. The more sensitive the setting of the device the more susceptible to interference the implanted system can be. Therefore, it is not only the model of device that is important; it is also the clinical configuration (including lead location and number, and sensitivity setting of the device).

Electric and Magnetic Field Interference

EPRI produced a report in 2004 summarising our current state of knowledge about interference to implanted cardiac pacemakers and defibrillators from electric and magnetic fields in the frequency range of 1 hertz (Hz) to 3 kilohertz (kHz). These found that electric and magnetic fields could alter the function of pacemakers and ICDs, but electric fields appear to be the most likely source of interference. The magnitude or intensity of the magnetic field required to alter the function of these devices varies widely with frequency and waveform. Their summary figures are:

Electric fields: sensitivity reported at levels ranging from 1.5 kV/m upwards, though some units are immune at 20 kV/m

Magnetic fields: 100 µT to 1.2 mT may produce interference

There have been very few studies investigating the effects of external EMFs on pacemaker and ICD function. Recently Trigano et al., (2005) [link to abstract at bottom of page] examined the risk of interference by magnetic fields with permanent pacemakers. These investigated the effects of 100 µT on various patients with differing models of pacemakers. All pacemakers were set to their maximum sensitivity settings and subjects walked through the uniform magnetic field. During the tests only a small incidence of interaction with the magnetic field was observed. A switch to asynchronous mode was observed in three patients with devices set to unipolar sensing configuration. In one patient pacing inhibition occurred after a sustained mode switch. The conclusions drawn from this large study are that magnetic fields have a low incidence of interactions at levels up to the ICNIRP public magnetic field reference levels.

Electric fields can induce currents in the body that can interfere with the proper operation of pacemakers and ICDs. Electric field interference with pacemakers has been observed over a range of magnitudes. Some models show inappropriate behaviour at levels as low as 1.5-2 kV/m. However, modern duel-chamber pacemakers appear unaffected at levels as high as 20 kV/m.

Most modern pacemakers are designed to revert to fixed rate or asynchronous pacing when external electric and magnetic fields interfere with their electronic sensing capability. This is not considered clinically significant and does not cause adverse health effects. EMI can potentially result in a range of responses from the pacemaker, however this is dependant on the characteristics of the signal. Possible interference outcomes can include:

• Switch to asynchronous pacing or fixed-rate pacing
• Rate increase
• Erratic pacing
• Single beat inhibition (i.e. a single beat is missed by the pacemaker)
• Total inhibition

These responses are on the whole temporary and the pacemaker will revert to normal operating parameters once removed from the stimulus.

Advice to patients

Medtronic (manufacturers of AIMDs) produced advice to patients that high voltage powerlines (400 kV) may cause asynchronous pacing and suggest a distance of 25 feet between the patient and line should be maintained. The USA FDA released advice to patients with pacemakers and ICD, not to lean or linger close to sources of EMFs. But the UK Department of Health, Medicines and Healthcare products Regulatory Agency (MHRA), formerly the Medical Devices Agency, does not consider the transmission line electric or magnetic fields constitute a significant hazard.

All pacemaker and ICD patients are informed of the potential problems associated with exposure to EMFs. Interference of electric and magnetic fields is on the whole temporary, and moving away from the source will alleviate the response. Advice to patients with AIMDs is to consult with their cardiologist and the device manufactures if in doubt about EMF interactions.

The power system is not the only source of possible interference; patients should also be aware of shop security devices, for example.

CENELEC are in the process of producing a standard for the ‘Assessment, measurement and calculations of human exposure at the workplace bearing active implantable medical devices in electric, magnetic and electromagnetic fields with frequencies from 0 to 100 GHz’. This standard will allow companies to manage employees with AIMDs.

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