ICD response to magnet placement

ICD: implantable cardioverter-defibrillator; tachy: tachycardia; brady: bradycardia; VVI: single-chamber ventricular pacemaker; bpm: beats/minute.
* After 8 hours of magnet application, the antitachycardia therapy will be reactivated even if the magnet is still in position over the ICD. At this point, to disable the antitachycardia therapy again it is necessary to remove the magnet for at least five minutes before reapplying it over the ICD.
¶ Any Boston Scientific/Guidant/CPI ICD that does not emit sound when a magnet is applied should undergo an immediate device interrogation. A stethoscope might be needed; for electronic stethoscopes, only the "Diaphragm" mode should be used since filtering in the "Bell" mode might not permit the sound to be transmitted to the earpiece.
For Boston Scientific/Guidant/CPI ICDs, if magnet mode is programmed to "ON," appropriate magnet placement disables tachy detection and therapy, and tachy therapies remain disabled for as long as the magnet remains appropriately applied. When magnet mode is enabled in these devices, the ICD will emit either a constant tone or "beep" to identify appropriate magnet placement. If the device emits a constant tone, then tachy therapy is disabled whether or not a magnet is present, and tachy therapy will not be present even after the magnet is removed. If any of these ICDs emit a "beep" (ICDs with "GDT," or "CPI," radiograph codes emit each beep with any paced or sensed R wave; ICDs with "BOS," or "BSC," radiograph code emits a beep every second), then a properly working ICD will be enabled for tachy therapy upon magnet removal. Note that the "Change Tachy Mode with Magnet" feature is present only in very few remaining "GDT," and "CPI" radiograph-labeled devices. When programmed "ON," after 30 seconds of continuous magnet application, the tachy mode will toggle; ie, it will switch from enabled when the magnet is removed (beeping with magnet correctly applied) to permanently disabled (constant tone when magnet is correctly applied) or vice versa. This mode has been phased out for most BOS/GDT/CPI ICD families, and software in programmers since October 2009 is designed to disable and eliminate this feature.
Δ Note: Medtronic ICDs will emit a "steady," tone with magnet application. However, this tone is only emitted for 10 to 30 secs, and thus cannot be used as ongoing confirmation of antitachycardia therapy suspension. In addition, Medtronic ICDs will emit an "alert" tone (either beeping or alternating high/low) if the patient alert tone is programmed on and a related alert is triggered (ie, low battery voltage, lead impedance out of range). The alert tone continues to sound as scheduled, either once every 4 or 24 hours, until the device is interrogated. The alert tone will also sound of an alert is in effect and the ICD senses a magnetic field. Thus, if an alert tone rather than a "steady" tone is heard with magnet application, the ICD should be interrogated.
◊ Microport Platinum ICDs may extend tachy inhibition for 30 seconds following magnet removal. This will only occur if charging occurred within two minutes of magnet placement.