Pacemakers and magnets: An arranged marriage

Abstract

Multi-programmable heart rhythm devices are so prevalent today that its easy to forget the brief but amazing history of cardiac pacing, which started only about 50 years ago, in the late 1950s. Pacemakers became implantable in the 1960s; before that they were external devices powered by AC current or by batteries. Once these devices became implantable, communication with the device became an issue. Before the development of telemetry communication with pacemakers in the 1970s, magnets were routinely used to assess battery status or to change the pacing rate. In the mid 1980s, when the implantable cardioverter-defibrillator (ICD) became available, communication with these early devices was very limited. These devices came from the manufacturer with a preset rate cutoff for tachycardia detection, no pacing therapies, and only one shock strength. Magnets were used to turn the device on and off and to assess charge time to charge the capacitors, thereby to reform the capacitors periodically to facilitate quick charging in case of needed shocks. Additionally, determination of battery longevity and the decision to replace the ICD depended on a predetermined capacitor charge time specific to the device. In modern devices, magnets interact with pacemakers and ICDs by closing the reed switch. The reed switch consists of 2 flat, ferromagnetic reeds, separated by a small gap, enclosed in a glass capsule filled with an inert gas. The magnetic switch is designed to close when it is exposed to an approximately 10-Gauss magnetic field. Closure of the magnetic reed switch disables sensing, resulting in temporary asynchronous pacing in a pacemaker or suspending therapies in an ICD. The switch is reactivated when the magnetic field is removed. Certain ICD models can be deactivated and reactivated by continuous, sustained application of a magnetic field for a few seconds. The ability to close the magnetic switch by applying a magnet over pacemakers or ICDs is necessary in certain circumstances when the telemetric programmer or the expertise to use it are not readily available. Magnet application can also be used to trigger specific features in some devices, such as electrogram storing capability or event markers. Therefore, magnet use to activate certain functions, or to deactivate an ICD, remains an important feature for both pacemaker and ICD follow-up care in the modern age because of the conveniences it offers. So, although we have come a long way in the development of pacemakers and defibrillators, magnets have served many important functions along the way. Absent attainable alternatives at the time, this marriage was arranged out of necessity, and it has endured because of convenience. Static magnetic fields in the normal environment or even in most industrial environments are unlikely to be strong enough to close the magnetic switch because the magnetic field strength dissipates quickly as distance from the source increases. 1 However, cardiac devices can be affected if patients are close enough to some objects that can generate strong magnetic fields, such as stereo speakers or bingo wands. 2,3 Inadvertent application of a magnet over pacemakers or ICDs is not uncommon because of the increasing number of objects that contain or are made of magnets. Even a small magnet can affect pacemakers or ICDs if it is strong enough and placed close enough to the device.