Clinical Applications of Magnets on Cardiac Rhythm Management Devices

Sony Jacob; Sidakpal S. Panaich; Rahul Maheshwari; John W. Haddad; Benzy J. Padanilam; Sinoj K. John

Disclosures

Europace. 2011;13(9):1222-1230. 

In This Article

Magnet Effects on Implantable Cardioverter Defibrillators

Figure 4 shows various ICD responses to clinical magnet application. In general, magnet application suspends anti-tachycardia therapy without any effect on the pacing mode. Implantable cardioverter defibrillator models from St. Jude Medical and Boston Scientific, however, have additional programmable features to ignore the magnet or respond differently to a magnet. All biventricular ICDs behave like any other ICDs of the corresponding manufacturer. In most cases, anti-tachycardia therapy resumes with removal of the magnet. However, in some instances, magnet removal may or may not re-enable anti-tachycardia therapy. Specific care should be exercised in Guidant/Boston Scientific ICDs since some of the older models (Table 1) are equipped with circuitry that enables the magnet to permanently programme the anti-tachycardia therapy to 'OFF'. In addition, magnet application on Sorin ICDs changes the pacing rate without altering the pacing mode (discussed below). Rate response of ICDs is not influenced by magnet application.

Figure 4.

Response of implantable cardioverter defibrillators (ICDs) to magnet placement. In general, on magnet application, tachycardia detection is suspended in all ICDs and hence no anti-tachycardia therapy (Tachy Tx) occurs. The first step shows the programmed response of the ICD to the magnet. The ICD may be programmed to ignore the magnet only in Boston Scientific and St. Jude devices. There is no effect on bradycardia therapy (Brady Tx) function of ICDs, except in Sorin ICDs [pace at 96 beats per minute (bpm) at BOL to 80 bpm at ERI, without any pacing mode change (Δ)]. Removal of the magnet re-enables anti-tachycardia therapy in most of ICDs (except Guidant older devices that have been programmed to 'change tachy mode with magnet'). Note the various audio responses seen in ICDs upon inhibition/suspension of anti-tachycardia therapy (featured only in Boston Scientific and Medtronic ICDs). In Boston Scientific ICDs: Long line, long constant tone: tachy mode is programmed to 'OFF'. Dotted line, initial beeping tones: anti-tachycardia therapy is inhibited for as long as magnet is secured over the ICD; these beeps correspond to the R waves on the electrocardiogram or in newer devices, and beep once per second (each beep is 100 ms long). Dotted line plus long line, initial beeping tones change to long constant tone after 30 s: anti-tachycardia therapy is disabled ('tachy mode' is now programmed to 'OFF') and the magnet can be removed. If the anti-tachycardia therapy had been permanently disabled (i.e. 'tachy mode' had been programmed to 'OFF'), the magnet will have to be re-applied for 30 s or more until the long constant tone reverts to beeping tones. Long line plus dotted line, long constant tone changes to beeping tones after 30 s: 'tachy mode' is programmed to 'ON' (anti-tachycardia therapy will be re-enabled once the magnet is removed). In Medtronic ICDs: Audio tones are heard only if the 'alert tone' (last for 10 s) feature is programmed 'ON'. Long line, long constant tone (all clear tone; all programmed alert parameters are within normal limits).Dotted line, low-urgency tone: 0.5 s 'ON' and 0.5 s 'OFF' for a total of 10 s (one or more of the low-urgency alert parameter needs attention). Dotted line plus line, high-urgency tone: 0.5 s high-frequency tone followed by 0.5 s lower-frequency tone for a total of 10 s (one or more of the high-urgency alert parameter needs attention). If a low- or high-urgency tone is heard, device interrogation is warranted for possible electrical reset. St. Jude ICDs from Atlas® and Epic® II models have a vibration alert feature that warrants possible device interrogation. However, there is no vibration in response to a magnet in these devices.

Implantable cardioverter defibrillators have either an auditory or a vibratory alert feature; only Medtronic and Boston Scientific ICDs provide an auditory confirmation of the suspension of anti-tachycardia therapy on magnet application. If a tone is not heard, a stethoscope may be used to identify auditory response, particularly in obese patients and in those with submuscular implantations. No tones are heard in St. Jude Medical, Biotronik, and Sorin ICDs on magnet application. In Medtronic or Boston Scientific devices, if no tone is heard even after repositioning the magnet, the following reasons may apply: (i) a depleted battery; (ii) all the alerts are programmed to remain 'OFF', as in Medtronic devices; (iii) the device is programmed to ignore the magnet, as in Boston Scientific devices, and/or (iv), the magnetic field does not reach the device due to sheer depth, as in the case of those with deeper (abdominal or submuscular) implants or in very obese patients.

Boston Scientific

Boston Scientific ICDs (if not programmed to ignore the magnet) respond to clinical magnets in a complex fashion, which depends on the presence of another specific programmable mode called 'change tachy mode with magnet' found only in certain models (Table 1). If this feature is programmed 'ON', magnet placement can permanently disable tachycardia detection and anti-tachycardia therapy. This may be useful in surgical situations in which magnet placement interferes with the sterility of the surgical field (e.g. upper-torso surgery), but the tachycardia therapies must be reprogrammed to 'ON' after the procedure. The anti-tachycardia therapy can be re-enabled by re-applying the magnet for ≥30 s. However, if the 'change tachy mode with magnet' feature is absent, as in more recent models, magnet placement only inhibits anti-tachycardia therapy for as long as the magnet is placed over the device and anti-tachycardia therapy resumes with removal of the magnet.

Specific tones (Figure 4) indicate temporary or permanent suspension of anti-tachycardia therapy.[33] A long constant tone on initial magnet placement is a warning that the device's 'tachy mode' is programmed to 'OFF' (and the anti-tachycardia therapy had been suspended irrespective of magnet placement in this device), the reason for which needs to be ascertained before proceeding further. If a beeping tone is heard for more than 30 s, it is indicative of anti-tachycardia therapy inhibition that persists only for as long as the magnet is held over the device. These beeps correspond to the R waves on the ECG or, in some cases, simply beep once per second. If the beeping tones change to a long constant tone after the magnet has been held over the patient's device for 30 s, it indicates that the 'change tachy mode with magnet' feature is present in that device model. In this case, if the magnet is then removed the anti-tachycardia therapy remains disabled. In order to re-enable the anti-tachycardia therapy, the magnet must be re-applied over the device for ≥30 s until the constant tone reverts to beeping tones.

Medtronic

All Medtronic ICDs suspend anti-tachycardia therapy on magnet application since the magnet 'OFF' mode is absent in these devices. The anti-tachycardia therapy remains suspended as long as the magnet is secured over the device site. Audio confirmation of therapy suspension is only heard if the 'device alert' functions are programmed to 'ON'.

Generally, anti-tachycardia therapy resumes after the magnet is removed. However, it should be confirmed by re-applying the magnet to ascertain any electrical reset that might have occurred as a result of external interference. Absence of electrical reset is indicated by the presence of a long constant tone. A high–low alternating tone, on the contrary, is a high-urgency alert indicating conditions such as electrical reset or ERI battery voltage.

St. Jude Medical, Biotronik, and Sorin

All ICDs from these companies suspend their anti-tachycardia therapy, with the exception of St. Jude Medical ICDs that can be programmed to ignore the magnet. The bradycardia pacing function remains unaffected during external magnet application except in Sorin ICDs. On magnet application, Sorin ICDs are designed to pace at 96 bpm at BOL to 80 bpm at ERI without any change in the programmed mode and AV delay. AV delay may be auto (Paradym®/Ovatio®) or at rest (all other models). Removal of the magnet re-enables anti-tachycardia therapy. ICDs from Biotronik and Sorin also have audible tones (these, however, are not altered by the presence of an external magnet) to alert the patient towards possible device interrogation. The St. Jude ICDs have an alternative vibratory alert mechanism in the models from Atlas® and Epic® II. In Lumax® ICDs from (Biotronik Inc., Oregon, USA) the anti-tachycardia therapy is disabled only for 8 h on constant magnet application, after which the ICD automatically resumes anti-tachycardia therapy. Therefore, to disable anti-tachycardia therapy for >8 h, the device needs to be reprogrammed or one may simply remove and re-apply the magnet for an additional 8 h of disabled anti-tachycardia therapy.

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