There is much concern regarding the effects of magnets on pacemaker devices. Since there is no known insulator for magnetic flux, it is impossible to prevent this force from reaching pacemakers. As a result, current pacemaker design accommodates passive magnetic interference with minimal disruption to the device's function. Pacemakers have a preset default rate based on the patient's requirements at rest. This means that, in the presence of a strong static magnetic field (in excess of 20 gauss), the device will seek the default rate. The presence of a strong magnetic field should not cause the device to turn off or inhibit this default function.

Because magnetic flux is poorly conducted in air, as well as human tissue, a large magnet would have to be very close to the pacemaker to create a problem. A reasonable field strength test can be done with a standard quartz watch, which generally has a tolerance of 10 gauss. To create a field strong enough to stop the quartz movement on a watch, a 1/2" dia. by 1/8" thick rare-earth magnet must be placed directly on the face of the watch. Once the magnet is removed, the watch, like the pacemaker, should return to its normal function without sustaining any permanent damage.

The warnings posted near devices such as magnetic imaging equipment are to warn pacemaker and defibrillator patients of the risk of entering an active Electromagnetic Field (EMF), which is a far more serious issue, and very different from the drive force created by a permanent magnet. EMFs, both naturally occurring and man made, have been blamed for everything from health problems to power-grid black-outs, and are an ever-present threat to the function of microelectronic devices.

EMFs differ from magnetic fields in that they alternate or modulate, and in so doing are able to transmit energy through induction. On a global scale, the earth's EMFs are induced into long runs of electrical utility transmission wire, especially those running north-south. They have been observed at certain times to induce in excess of two million volts across the 1200 km transmission line between the generating station at James Bay and the electrical distribution center north of Montreal. Needless to say, this has created some long, chilly nights for distribution engineers, who wondered why Montreal got dark when the Northern Lights shone brightest.

On the pacemaker level, the short wires contained within the human body are fairly secure from natural forces, but require caution near some of the manmade devices, especially those that operate at high frequencies, such as Magnetic Resonance Imaging (MRI) devices. Over the last decade, appliances such as home computers, televisions and microwaves have become subject to ultra-strict Electromagnetic Interference (EMI) regulations, which have made the world a safer place for cordless phones and Walkmans®, as well as for pacemaker patients. But environments such as radio transmitter rooms, high-frequency welding equipment and especially the powerful magnetrons of MRI equipment remain very dangerous. High-frequency electromagnetic radiating devices create three possible threats:

  1. inducing a destructive voltage level into the device, which could cause permanent circuit failure,
  2. inducing a voltage greater than the operating voltage of the device, which masks the pacemaker's signal, and
  3. inducing a pulse sequence into the heart that is not supplied by the pacemaker, but the outside source.

None of these conditions will be created in the presence of a fixed-pole magnetic field such as that generated by a rare-earth magnet. Our advice to pacemaker and defibrillator patients is to exercise a modest degree of caution when handling large permanent magnets, keeping them from coming into direct contact with the implant area. If any rhythmic change is experienced, move the magnet away from the implant area.

— M.O.
11/98


 
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