Inappropriate Shock Therapy Delivered by a Subcutaneous Implantable Cardioverter Defibrillator

Abstract

Inappropriate shock therapy by implantable cardioverter defibrillators (ICDs) is an important cause of morbidity and mortality. When inappropriate shocks occur, it is imperative the cause be identified and remedied. This Case illustrates an unusual cause of inappropriate subcutaneous ICD (S-ICD) shocks. Mr X is an 18-year-old Tokelauan with repaired coarctation, dextrocardia, situs solitus and bicuspid aortic valve. He has mild heart failure symptoms, with left ventricular ejection fraction 33%. Following a community cardiac arrest, an S-ICD was implanted using the two-incision technique. This was programmed to two therapy zones using the secondary vector: 80J shocks, with VT zone 200bpm and VF zone 221bpm. Eleven days after implant he had two ICD shocks. Device interrogation showed ten episodes of oversensing non-physiological artefact: two resulting in shock therapy. One of these episodes serendipitously occurred during VT, which was beneath the detection zone at 160bpm. There was no history suggesting external electromagnetic noise. Air bubbles within the header due to a compromised integrity of the seal plug was thought to be the most likely explanation for the artefact. To mitigate this, the sensing vector was changed from secondary to alternate, which uses the A and B electrodes for sensing— excluding the generator from the sensing circuit. No further artefact was seen, including with exercise provocation. The mechanism for inappropriate S-ICD shock can differ from traditional transvenous ICD systems. Inappropriate shocks as the result of artefact caused by air within the header can be mitigated by using the alternate sensing vector.Figure 2Sensing vectors for S-ICD.View Large Image Figure ViewerDownload Hi-res image Download (PPT)