Pacemaker Magnets

Despite the often-repeated folklore, most pacemaker manufacturers warn that magnets were never intended to treat pacemaker emergencies or prevent electromagnetic interference effects. Rather, magnet-activated reed switches were incorporated to produce pacing behavior that demonstrates remaining battery life and, sometimes, pacing threshold safety factors.

Placement of a magnet over a generator might produce no change in pacing since not all pacemakers switch to a continuous asynchronous mode when a magnet is placed. Also, not all models from a given company behave the same way. Only about 60% of pacemakers have high-rate (80 to 100 beats/min) asynchronous pacing with magnet application. About 25% switch to asynchronous pacing at program rate, and 15% respond with a brief (60 to 100 beats) asynchronous pacing event. Possible effects of magnet placement are shown in Table 35-4 .^{[23] [24] [25]} In some devices, magnet behavior can be altered by programming, and magnet behavior can be completely eliminated by programming in some devices.

For all generators, calling the manufacturer remains the most reliable method for determining magnet response and using this response to predict remaining battery life (phone numbers for the device manufacturers are shown in Appendix 2 ). As battery voltage falls, the magnet response can be used to detect the following:

IFI (Intensified follow-up required)—the device must be checked frequently (approximately every 4 weeks for most models);

ERI (Elective replacement indicator)—the device is nearing the end of its useful life and should be electively replaced;

EOL (End of life)—the device has insufficient battery power remaining and should be replaced immediately.

On application of a magnet, some devices perform a threshold margin test (TMT). In this test, one or more of the pacemaker pulses is reduced in amplitude, pulse width, or both, in an attempt to gauge the safety margin for pacing voltage. Loss of capture on these TMT pulses indicates an inadequate safety margin for pacing ( Fig. 35-3 ). Some devices from St. Jude Medical (formerly Pacesetter) with the Siemans "vario" engine reduce the ventricular pacing energy over 16 cycles to demonstrate the pacing threshold. As a result, many pacing cycles can take place at insufficient energy for ventricular capture, which can produce periods of asystole while the magnet is applied.^[26]

Occasionally, a pacemaker mediated tachycardia (PMT) can ensue upon removal of the magnet from a dual chamber pacemaker ( Fig. 35-4 ). These PMTs result from retrograde-conducted P waves during asynchronous ventricular pacing, most commonly when the magnet rate is lower than the patient's intrinsic rate. These retrograde P waves are then "tracked" after magnet removal by a dual chamber device (DDD, VDD modes only), resulting in pacing at the upper tracking rate. Each paced ventricular cycle results in another retrograde P wave, producing an endless loop tachycardia. Should this behavior be observed, it can be subverted by reapplication, then removal of the magnet. Some devices can be programmed to recognize these

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Figure 35-3 A threshold margin test (TMT), demonstrating inadequate safety margin for pacing. Application of a magnet to some pacemakers produces asynchronous pacing in which one or more pacing stimuli are emitted with a reduced ventricular pacing voltage, pulse width, or both. This sequence of the TMT is used to determine, without a formal pacemaker interrogation, the adequacy of the pacing energy settings. In this electrocardiographic strip from an Intermedics device, the patient was being paced in the VVI mode (ventricular pacing in the inhibited mode for a single chamber) at a rate of 70 beats/min, which is equal to 857-msec intervals. On application of the magnet, this pacemaker produced four intervals (i.e., five pacing stimuli) of asynchronous pacing at a rate of 90 beats/min (667-msec intervals), demonstrating adequate battery voltage for this device. At the fifth pacing stimulus after magnet application, the pacemaker performs a TMT by reducing the stimulus pulse width to 50% of the programmed value (equal to 50% of programmed pacing energy). The failure of this stimulus to produce a ventricular systole (i.e., failure to capture) demonstrates a dangerously low safety margin for ventricular pacing, because pacing pulse width should be at least three and usually four times the threshold for capture. After five initial stimuli, Intermedics pacemakers then pace asynchronously at the programmed lower rate (70 beats/min in this case) for 60 additional cycles. On completion of these 64 cycles (65 stimuli), Intermedics pacemakers return to programmed values and ignore the magnet.

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Figure 35-4 A pacemaker-mediated tachycardia (PMT) follows magnet removal from a pacemaker. This patient had a dual chamber pacemaker implanted for atrioventricular (AV) nodal disease, and she was pacemaker dependent for ventricular activity. She had a sinus rate of 75 beats/min before the magnet application with appropriate ventricular pacing (strip not shown). Her programmed AV delay is 200 msec. With magnet application, her pacemaker produced asynchronous AV sequential pacing (DOO mode) at a rate of 60 beats/min. This strip is from an electrocardiographic recorder that enhances the pacemaker artifact with small, downward arrows. Because the asynchronous magnet rate of this device is lower than her intrinsic atrial rate, many of the atrial pacing stimuli were applied during an atrial refractory period (i.e., functional noncapture). A consequence of atrial noncapture can be retrograde AV nodal conduction, with depolarization of the atria after the depolarization of the ventricles. The retrograde P waves are shown with the upward arrows. While the magnet is applied, this retrograde depolarization of the atria is ignored. Shortly after the magnet was removed (open arrow), there was a paced ventricular event, followed by retrograde AV nodal conduction. With the ensuing depolarization of the atria from this retrograde conduction, the pacemaker sensed an atrial event and responded by pacing the ventricle 200 msec later. Another retrograde P wave appears, and each pace in response to a retrograde P wave created yet another ventricular pace. The result is a PMT at the upper tracking rate (programmed here to 130 beats/min) of the pacemaker. PMT from retrograde AV nodal conduction can occur in any DDD or VDD device with magnet removal, with a premature ventricular contraction, or with a noncaptured atrial pace. Treatment of this PMT entails reapplication of the magnet. Some pacemakers can be programmed to eventually break PMT by delaying one AV cycle when pacing at the upper tracking limit.

The patient with a dual-chamber device that has detected a high atrial rate and "mode-switched" to prevent UTR pacing could have the mode-switch reset upon application and removal of the magnet. These patients will then undergo pacing at the upper tracking rate until criteria are met to return to the mode-switch mode. Distinguishing the PMT from retrograde P waves from the UTR pacing due to high atrial rates (prior to mode-switch entry) can be very difficult. In general, though, mode-switch due to high atrial rates will take place within 10–15 seconds, and PMT from retrograde P waves is quite persistent.

For generators with programmable magnet behavior [Guidant Medical, CPI, Pacesetter, St Jude Medical, Telectronics, Biotronik, others], only an interrogation with a programmer can reveal current settings. Many manufacturers publish a generator reference guide, although not all of these guides list all magnet idiosyncrasies.