Application of a Clinical Magnet over Implantable Cardioverter Defibrillators: Is It Safe and Useful?

José M. Porres, MD; Edurne Laviñeta, MD; Carlos Reviejo, MD; Josep Brugada, MD, PhD*

Pacing Clin Electrophysiol. 2008;31(12):1641-1645. 

Abstract

The growing number of implantable cardioverter defibrillator (ICD) implants mean that a high number of patients carrying these devices are attended by physicians. In an attempt to simplify their management, articles have been published on the safety of applying magnets to the ICD in order to avoid the administration of shocks during surgery. However, performance of these procedures without the supervision of expert personnel can be accompanied by serious and potentially fatal complications. We report a case where the use of a clinic magnet over an ICD caused it to switch to "end of life" in the battery indicator and lose some antitachycardia therapies.

Case Report

The case presents an 80-year-old patient with a history of anterior myocardial infarct of indeterminate age and 41% ejection fraction (EF). The patient had had, in a public street, an episode of syncopal ventricular tachycardia (VT) with a 310-ms cycle length, and had been successfully resuscitated by the emergency services by means of automatic cardioversion.

A Prizm 1852 VR implantable cardioverter defibrillator (ICD) was implanted (Guidant CPI Inc., St. Paul, MN, USA; currently Boston Scientific) with an Endotack model 0175 electrode (Guidant CPI Inc.); the threshold of stimulation was 0.7 V at 0.5 ms, wave R measured 7.1 mV and impedance 975 ohm.

Detection of VT was programmed at 170 beats/min with antitachycardia therapy and successive shocks of 41 J and detection of VF at 220 beats/min with a therapy of successive shocks of 41 J.

Three months after the implant, six episodes of VT with frequencies between 190 and 210 beats/min were adequately detected and treated with the programmed therapy.

Thirteen months later, the patient underwent aortocoronary bypass surgery. During the surgery, a clinical magnet, model 9466 (Medtronic Inc., Minneapolis, MN, USA) was placed over the generator.

The patient was later transferred to the intensive care unit for recovery.

When the ICD was checked on the following day, a warning appeared on the screen: "Device in EOL (end of life) state. In EOL some therapies are not available." The date and hour of entry into the EOL state corresponded to the time of surgery (Fig. 1A).

(A) First monitoring. EOL reading on July 12, 2007, at 10:25. (B) Monitoring by means of manual re-form of the capacitors. ERI reading on July 13, 2007, at 18:54.

After the capacitors were manually reformed, in the subsequent check the following notice appeared: "Device in ERI (elective replacement indicator) state," with the date and time of the manual reform (Fig. 1B).

A further manual reform of the capacitors determined a battery voltage of 3.20 V with a beginning of life (BOL) state. All the data of the system of electrodes had values similar to the previous follow-ups and to the implant (Fig. 2).

After the second manual reform of the capacitors. State of the BOL battery. Data from the electrode at values similar to those at the monitoring on May 9, 2007.

Interrogation of the device-stored electrograms revealed two episodes of ventricular tachycardia during surgery. Detailed analysis of the episodes revealed that episode 13 occurred at 10:24 with manual divert of the therapy, and episode 14 at 10:29 did not receive therapy (Fig. 3). The device was in an EOL state from 10:25.

After the second manual reform of the capacitors. Episodes during surgery. Episode 14 (10:29) did not receive therapy because the device had been in EOL since 10:25.

Upon examination of the electrograms of these two episodes, it was established that they corresponded to artifacts caused by electric noise due to the use of an electrocautery.

When checked at day 15, the values of the battery and electrode remained stable.

Discussion

The number of patients carrying ICDs is ever increasing, so these devices are present in numerous medical circumstances and have to be evaluated by physicians who do not have expert knowledge of all their forms of response.

A common situation arises when they must be submitted to a surgical intervention.

The guidelines recommend that in patients carrying either a pacemaker or an ICD the device should be evaluated before and after the intervention, including the determination of the rhythm of the patient, the programming, and the state of the batteries. In the specific case of a patient carrying an ICD, the device should be turned "off" immediately before the surgery and back "on" in the postoperative phase in order to avoid unexpected discharges because of the detection of spurious signals that are interpreted as ventricular tachycardia or ventricular fibrillation.[1]

When expert personnel in the management of these devices are not available, the use of a magnet has been recommended as an alternative; this rests on the belief that all models of ICD respond by inhibiting the sensing of signals and/or by inhibiting the delivering of any antitachycardia therapy during the time that the magnet remains on the ICD, and that these functions recover completely after withdrawal of the magnet. Similarly, it is claimed that the antibradycardia programming will not be influenced by the use of the magnet.

These false recommendations are repeated in some medical review articles in a categorical manner,[2] and slightly less empathically in others.[3]

The reality is very different. Only devices manufactured by Medtronic, Biotronik, and Sorin-ELA Medical respond in this way. Those produced by St. Jude Medical and Guidant Medical (Boston Scientific) behave according to the way in which they have been programmed to respond to the application of the magnet (normal-ignore in the case of St. Jude or on-off in the case of Guidant). Nominally, the programming is normal and on, respectively, but it can be modified at the discretion of the physician in such a way that the device does not modify its mode of functioning in the presence of a magnetic field. In both cases, this fact is not included in the printed notes that appear when a standard follow-up is made.

The influence of magnetic fields on the ICDs has given rise to many publications.

According to public organizations[4] and the manufacturing companies,[5-7] it is still considered that an ICD in a patient represents a contraindication against conducting a magnetic resonance imaging (MRI) scan, even though a high degree of safety has been shown in these patients in the realization of both cardiac and noncardiac MRI series using magnetic intensities between 0.5 and 1.5 Tesla.[8,9] In these series, neither the disturbances nor the changes in the state of the batteries and times of charge, nor changes in the thresholds of stimulation, sensing, and impedances of short or long term have been encountered, leading to the conclusion that with the necessary precautions and adequate monitoring, the devices can be subjected to very strong magnetic fields without any problems.

However, isolated references reflect the possibility of occasional dysfunctions.[10-12] Roguin et al.[10] analyzed the effect of magnetic intensities of 1.5 Tesla over ICDs from different manufacturers. The results showed a temporary battery depletion in Guidant models 1850, 1851, 1860, and 1861 that returned to normal after 1 to 2 weeks; however, Nazarian et al.[9] performed a knee MRI scan on a patient with a Guidant Ventak Prizm DR 1851 and reported no battery changes.

In the case of Anfinsen et al.,[10] after an MRI exploration with 0.5 Tesla, a similar complication appeared, with a change in the voltage of the battery to EOL, and they attributed this to excessive duration of charge.

The remarkable and novel aspect of our case is that this dysfunction appears with a procedure considered to be consistently safe (application of a clinical magnet) and with a very low magnetic intensity (0.09 Tesla according to the manufacturer of the magnet).

Conclusion

The routine use of magnets to inhibit therapies in patients carrying an ICD during surgery without pre- or postprocedure supervision cannot be regarded as a useful and safe procedure.

Therapy will be inhibited only depending on the model of the device and on its programming.

Permanent changes can appear in the devices, and this leads to their subsequent malfunction, with serious risk to the life of the patient.


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Sidebar: A Note From the Manufacturer

Boston Scientific CRM recommends using a programmer to temporarily deactivate tachy therapy to avoid inappropriate shocks associated with oversensing of noise generated by surgical tools. However, when programmed to respond to magnet application, positioning a magnet over a Boston Scientific CRM ICD during surgery will also deactivate therapy.

In this case study, unintended movement of an applied magnet allowed the magnetic switch in the ICD to open momentarily, permitting the device to charge in the presence of sensed electrocautery noise (episode 13). Because the magnet remained in the vicinity of the device, its magnetic field interfered with (slowed) charging, and a charge time replacement indicator was triggered even though significant battery energy was available. Proper positioning of the magnet over the device was restored before shock delivery, and device memory recorded a manual divert of therapy. Note that when the magnet was moved away from the device to reactivate therapy, both charge time and the replacement indicators returned to normal.

Conclusion: If a programmer is not available, manual inhibition of therapy using a magnet can be a useful and convenient tool during surgery. However, to ensure uninterrupted inhibition of therapy, care must be exercised to maintain proper magnet positioning over the implanted device.

Daniel J. Tich

Boston Scientific Corporation

Daniel.Tich@bsci.com

Sidebar: Editor's Note

St. Jude Medical and Medtronic were also asked about the response of their internal cardioverter defibrillators to magnet application.

SJM devices do not use charge time as a surrogate for ERI and as such would not present an ERI message as a result of extended charge times. ERI criteria for all SJM ICDs are based on battery voltage. SJM indicated that their usual recommendation to ensure that an ICD will not detect, charge, and potentially deliver therapy when a patient undergoing surgery is exposed to electrocautery is to program the tachy therapies off prior to the procedure and reprogram them on afterward. As an alternative, a magnet could be applied but there would be no guarantee that the magnet would be in or remain in the proper location to activate the magnetic switch and appropriately inhibit therapies. (I acknowledge the input of Larry Selznick of St. Jude Medical— LSelznick@sjm.com )

The application of a magnet to Medtronic devices does not affect charge time and consequently would not result in an ERI message when the device was interrogated. If a magnet slipped out of position when electrocautery was in use, it is likely that electromagnetic interference would be sensed, detected, and a charge initiated. If the magnet was reapplied before charging was completed, detection would shut off and any future events would start a new episode. Any accumulated charge would be stored in the capacitor and slowly bleed off. Medtronic provided three options to avoid oversensing electromagnetic interference (such as electrocautery) during surgery. Option 1: temporarily suspend detection by placing a ring magnet over the body of the device. Option 2: temporarily suspend detection by placing a Medtronic Smart Magnet™ directly over the ICD (this device has an indicator light to confirm position). Option 3: program VT/VF detection OFF using a programmer prior to the procedure and reprogram detection ON once completed. Medtronic adds the caution that continuous monitoring of the rhythm is required for all the three options. (I acknowledge the input of Elizabeth Olsen of Medtronic—elizabeth.l.ol sen@medtronic.com.)

Stephen C. Vlay, M.D.

PACE Case Report editor

svlay@notes.cc.sunysb.edu

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