Magnet Effects on Pacemakers
Figure 3 details the various pacemaker responses to clinical magnet application. All pacemakers respond to a magnet by switching to an asynchronous pacing mode at a programmed atrioventricular (AV) delay and a fixed magnet rate depending on the manufacturer, device model, and the status of the battery. The programmed mode DDD switches to DOO, VVI switches to VOO, and AAI switches to AOO. The rate response feature is switched to 'OFF' on magnet application in pacemakers. In biventricular pacemakers, both the right and left ventricles continue to be paced in the above modes with magnet application so long as the device is at or above ERI. However, from below ERI voltage, this response is unpredictable.
If magnet application on a pacemaker site does not produce any response on the surface ECG pacing rate or mode, the magnet may be repositioned. If no change is still observed, the following reasons may apply: (i) a depleted pacemaker battery; (ii) the pacemaker is programmed to ignore the magnet (St. Jude, Boston Scientific, and Biotronik synchronous mode); (iii) the magnetic field does not reach the device, as in the case of those with deeper (abdominal or submuscular) implants or in very obese patients; (iv) EOL or lower battery life. Additionally, with Biotronik pacemakers in a synchronous mode (Figure 3), if the patient's intrinsic heart rate is higher than the lower rate limit (LRL) programmed, there will be no ECG response. In almost all pacemakers, removal of the magnet causes the device to revert to pacing at the normal preprogrammed rate with the exception of Sorin pacemakers as described below.
Boston Scientific pacemakers are designed to switch to an asynchronous pacing mode on magnet application if the device is not programmed to a magnet 'OFF' mode. It also has a battery test mode and an electrogram (EGM) mode to initiate ECG storage without any effect on pacing.
In the magnet response mode, Boston Scientific pacemakers are paced at a magnet rate of 100 bpm at EOL with an AV delay of 100 ms. The third pulse during the magnet test is issued at 50% of the programmed pulse width to allow the clinician to evaluate the safety margin. The magnet rate is 100, 95, and 85 beats per minute (bpm) at EOL, elective replacement near (ERN), and ERT, respectively, in Boston Scientific pacemakers.
Medtronic pacemakers do not have a magnet 'OFF' mode and asynchronous pacing can always be expected on magnet application. With the exception of EnRhythm® (Medtronic, Minneapolis, MN, USA), asynchronous pacing is preceded by three paced pulses at 100 bpm, with the pulse width of the last pacing reduced by 20%. This is known as the TMT and is used to ascertain adequate pacing capture safety margin. In Medtronic devices, the magnet rates are 85 and 65 bpm at EOL and ERI, respectively. A rate of 65 bpm is specifically reserved for providing information about battery life (ERI) with magnet application. Thus, Medtronic pacemakers cannot be programmed to pace at a rate of 65 bpm.
St. Jude Medical
St. Jude Medical pacemakers may be programmed to ignore the magnet ('OFF' mode) or have one of the following three manufacturer-programmed modes: (i) Battery test mode, in which there is a switch to asynchronous pacing immediately on magnet application; (ii) Battery test + EGM mode, in which an ECG is stored if the magnet is held over the device for less than 5 s and a switch is made to asynchronous pacing if held for >5 s; and (iii) EGM mode, in which an ECG is stored without any effect on pacing.
An additional VARIO mode is present in some older models (Microny® and Regency®, St. Jude Medical Inc., St. Paul, MN, USA). This feature was available in some Pacesetter devices, which are currently under St. Jude Medical but are being phased out. This mode is to check battery life and adequate pacing capture safety margin. It involves a repetitive cycle of 32 asynchronous events on magnet application with the first 16 paced events at 100 bpm (85 bpm at ERI) followed by 15 events at 119 bpm with successive reduction in pacing voltage and the last paced event at no output.
Biotronik devices respond differently to magnets depending on manufacturer-programmed modes that could be (i) asynchronous, (ii) synchronous (which demands pacing in programmed mode at LRL), or (iii) auto (i.e. 10 asynchronous events followed by reversion to synchronous mode). A synchronous mode is also used for a patient-triggered recording. The magnet rates for different modes vary according to the available battery life (Figure 3).
Sorin pacemakers also switch to asynchronous pacing in response to magnet application with a pulse width of 0.5 ms and rest AV delay (original programmed sensed AV delay). When the magnet is removed, the 'capture test' is initiated comprising six asynchronous pulses at magnet rate, programmed amplitude, and pulse width with an AV delay of 94 ms. This is used to verify adequate capture with programmed pulse width and amplitude. This is followed by the 'rate test' comprising two asynchronous pulses at programmed rate and AV delay to demonstrate sufficient capture at these values. Capture and rate tests are followed by pacing at the original programmed rate.
Europace. 2011;13(9):1222-1230. © 2011 Oxford University Press
Copyright 2007 European Heart Rhythm Association of the European Society of Cardiology (ESC). Published by Oxford University Press. All rights reserved.
Cite this: Clinical Applications of Magnets on Cardiac Rhythm Management Devices - Medscape - Sep 01, 2011.