65 yo M PMH of HTN, CAD, CHF (EF <30%), AICD wheeled into the cardiac room saying that he is getting shocked every 30 seconds. Vitals: T 36 HR 45 BP 205/100 RR 30 O2 sat 100% RA. Pt is placed on a monitor, EKG done which shows atrial fibrillation with slow ventricular rate.  A magnet was placed on the patient. No AICD firing noted on monitor and rhythm strip, despite patient screaming in agony that he is still getting shocked. What’s going on?

Pt is experiencing a “phantom shock,” a term which implies that the patient perceives an ICD shock, even though the memory of the ICD indicates that no therapy (shock) was delivered. Pts usually respond well to benzos to treat anxiety.

How does a magnet affect an AICD? or a pacemaker?

A magnet placed on the AICD  turns off the defibrillator function of the ICD (no shock will be administered). Only do this if you are sure that the AICD is administering inappropriate shocks or if you are not sure whether the AICD is giving effective shocks and you want to take over using external paddles.

A magnet placed on the pacemaker stops it from sensing the patient’s activity and sets the pacemaker into asynchronous mode (manufacturer’s preset settings independent of patient’s rate/rhythm).


What causes inappropriate shocks?

1. Inappropriate sensing of supraventricular tachycardia

2. Lead damage:

     a) fractures: usually occurs after many years (check chest XR)

     b)  insulation defects: usually occurs after many years (difficult to appreciate on XR)

     c) Twiddler’s syndrome (migration): usually occurs early, prior to scar formation of subcutaneous pocket and is caused by conscious or subconscious manipulation of pulse generator within subcutaneous pocket by the patient, resulting in dislodgement and/or retraction of leads, leading to loss of device function and migration

3. Artifict

External: environmental artifacts i.e, patient is in the shower and flow of water is interpreted as cardiac activity

Internal: caused by patient motion i.e, repetitive arm motions (playing tennis, painting) interpreted as cardiac activity

In this case the patient was found to have a fracture of his lead causing the inappropriate shocks that he felt prior the magnet was placed.


 Thank you Dr Cabot for this interesting case!




June 2024