Implantable defibrillators have a built-in tiny reed switch that is activated whenever a magnet is placed over the device. The presence of a magnetic field near the reed switch causes it to close and tells the device to temporarily suspend tachycardia detection and therapy.
For as long as the magnet is over the device, no shocks are delivered to the patient in case of ventricular tachycardia (VT) or fibrillation (VF). Once the magnet is removed, the switch opens again, restoring the device’s ability to deliver shocks.
This is a particularly useful feature during surgical procedures where electrocautery is used. Electrocautery and certain other types of surgical tools may cause the ICD to deliver unnecessary shocks due to oversensing.
Figure: magnetic reed switch.
Last week, Boston Scientific announced that the reed switch in some of their ICDs might get permanently stuck. It goes without saying that a jammed reed switch in your ICD would not be good, since it would preclude the delivery of potentially life-saving shocks.
Boston Scientific says that the affected ICDs were manufactured during 2006 and 2007 and include the CONTAK RENEWAL models 3 and 4, and the VITALITY DR HE. Affected devices are no longer available for implant, but there are still about 34,000 people out there with these devices.
To know if you have one of these devices, go to Boston Scientific’s online “Device Lookup Tool” and enter your device’s model and serial numbers. These numbers can be found in your medical device ID card.
Boston Scientific says that the risk of harm is remote and that no deaths or injuries have been reported. A PDF of this current advisory can be downloaded here.
Now, the good news:
Boston Scientific ICDs (and CRT-Ds) can be programmed to recognize a magnet or to ignore it altogether. So, in the unlikely event that your device’s reed switch gets stuck, your doctor can program the ICD to ignore the defective switch and the device will no longer inhibit tachycardia therapy.
Also, it’s good to know that the ICD will emit audible tones (synchronized to your heart beat) whenever the reed switch is closed or in the event it needs to tell you something is wrong. So, if the device is beeping, have it checked out right away.
More information, including copies of the letters to physicians and patients can be viewed and downloaded from the Heart Rhythm Society's web site.
Image from here.