Implantable Cardioverter-Defibrillator Magnets
Like pacemakers, magnet behavior in ICDs can be altered by programming.
Most devices will suspend tachydysrhythmia detection (and therefore therapy) when
a magnet is appropriately placed to activate the reed switch. Some devices from
Angeion, CPI, Pacesetter (St. Jude Medical) or Ventritex can be programmed to ignore
magnet placement.
Depending on programming, antitachycardia therapy in some Guidant
and/or CPI devices can be permanently
*The
capacitor in an ICD "deforms" during inactivity, leading to increased time needed
to charge the capacitor. To mitigate the effects of deformation, all ICDs perform
a nontherapeutic charging of their capacitor (called "reforming") at a programmable
periodic interval (usually one to six months).
TABLE 35-8 -- Indications for implantable cardioverter-defibrillator placement
|
Ventricular tachycardia |
|
Ventricular fibrillation |
|
Brugada syndrome (i.e., right bundle branch block, ST-segment
elevation in V1
-V3
) |
|
Arrhythmogenic right ventricular dysplasia |
|
Long QT syndrome |
|
Hypertrophic cardiomyopathy |
|
Prophylactic use in the post-myocardial infarction patient who
has an ejection fraction less than 30% (i.e., no electrophysiologic testing is needed
to justify placement)[72]
|
|
Prophylactic use in any patient with a cardiomyopathy from any
cause who has an ejection fraction less than 35%
*
|
*Sudden
cardiac death—heart failure trial. Presented at the American College
of Cardiology Meeting, March 2004.
disabled by magnet placement for 30 seconds, and, as noted above, some patients have
been discovered with their ICD antitachycardia therapy unintentionally disabled.
[2]
In Guidant and CPI devices, if magnet mode
is
enabled and the ICD is enabled for antitachycardia therapy, the ICD will emit beeps
synchronized to R-waves, signifying adequate placement of magnet and suspension of
tachyarrhythmia detection (thereby disabling therapy). If the ICD emits a constant
tone, then antitachycardia therapy has been programmed to "off." Depending upon
programming, ICDs from Guidant and CPI can toggle between activated and inactivated
states with 30 seconds of magnet application. To re-enable therapy, the magnet must
be removed, then replaced until the constant tone reverts to beeps synchronized to
R-waves. Subsequent removal of the magnet then returns the antitachycardia therapy
to the enabled state. Any Guidant or CPI device that emits either beeps or constant
tones while a magnet is placed will have its antitachycardia therapy disabled during
the magnet session.
To ensure correct magnet placement on their devices, Medtronic
markets a device called the "Smart Magnet." This battery-powered instrument contains
a magnet and a radio-frequency (RF) receiver. All Medtronic generators broadcast
RF destined for the programmer upon magnet placement, and the Smart Magnet detects
this radiofrequency transmission and illuminates a light to indicate that RF is being
received. Medtronic recommends that the Smart Magnet be taped onto the patient.
During the occurrence of EMI, the transmission from the ICD to the Smart Magnet
is interrupted, and the ICD "found" light often turns off. In this setting, however,
the ICD remains disabled, since it is the physical presence of the magnet, rather
than the radiofrequency communication, that disables the antitachycardia therapy.
In general, magnets will not affect antibradycardia pacing mode
or rate (except ELA [rate change] and Telectronics Guardian 4202/4203 [disabled]).
Intermedics devices transiently change pacing rate (VVI mode) to reflect battery
voltage. Interrogating the device and calling the manufacturer remain the most reliable
method for determining magnet response.
