Excerpt

A 61-year-old female presented 14 months after implantation of a secondary prevention subcutaneous implantable cardiac defibrillator (S-ICD) with an inappropriate shock, while reaching for a mop in her house. The S-ICD was initially implanted after a documented ventricular fibrillation (VF) arrest complicating severe obstructive urosepsis. Secondary vector electrogram (Fig. 1) revealed cardiac oversensing from myopotential generation. Despite changing the sensing vector to alternate vector, she had further inappropriate shocks due to postural-related R amplitude reduction resulting in T wave oversensing (Fig. 2). She underwent a fluoroscopic study (Fig. 3) which demonstrated movement of the pulse generator away from the chest wall when she abducted her shoulder, corresponding with generation of myopotential. She underwent a device pocket revision and myopotential was no longer reproducible (Fig. 4). Postural-related T wave oversensing was not reproducible as well. Her sensing vector was reprogrammed to the primary vector. Inappropriate shocks remain an issue with S-ICD with cardiac signal oversensing being the predominant mechanism [1]. This is usually managed non invasively via a change in sensing vector [2]. However, in our case, this resulted in further inappropriate shock. Fluoroscopy was clinically useful to demonstrate significant pulse generator movement, which was not picked up on routine CXR as the position of the device appeared normal at rest. The utility of routine fluoroscopy should be further studied in patients with myopotential generation complicating S-ICD sensing.

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