Inappropriate S–icd Shock Related to Atrio–ventricular Block: When the True Culprit Is Not Overt

Abstract Subcutaneous implantable cardiac defibrillator (S–ICD) is a well–established alternative to transvenous defibrillator in patients with an indication for an ICD without the concomitant need of pacing. Despite the overall excellent performance of S–ICD, oversensing of cardiac and non–cardiac signals is still the main cause of inappropriate shocks (IS). We present a case of IS due to F–wave oversensing and QRS morphological variability related to compete atrio–ventricular (AV) block. A 77–year–old man was admitted to the Emergency Room reporting two episodes of defibrillator shock, identified by remote monitoring as IS due to F–wave oversensing during paroxysmal atrial flutter (Figure 1). His past medical history included paroxysmal atrial fibrillation and post–ischemic left ventricular (LV) dysfunction with left bundle branch block, treated in 2012 with cardiac resynchronization therapy and implantable defibrillator (CRT–D) with subsequent normalization of LV ejection fraction (EF). The LV stimulation was later deactivated due to phrenic capture unavoidable with device reprogramming; afterwards, the occurrence of aortic valve and lead endocarditis resulted in CRT–D system extraction. At that time, considering the persistence of normal LVEF even without LV stimulation, an S–ICD was then implanted in primary prevention. The ECG at the time of admission presented with a complete AV block (Figure 2A), not clearly evident at the remote monitoring report. Few hours later a new episode of atrial flutter occurred, reveling a variable QRS morphologies of ventricular rhythm (Figure 2B). A more careful analysis of device memory reveals a previous episode of AV block with variable QRS morphologies leading to SMART Pass filter disabling (Figure 3). A transvenous ICD with left bundle branch area stimulation was then successfully implanted and the patient was discharged at home after two days. To the best of our knowledge, this is the first described case of inappropriate S–ICD shock related to AV block. The unpredictable QRS morphology of an escape rhythm and the inability of S–ICD to store diagnostic data regarding bradyarrhythmia may limit our knowledge on the behavior of S–ICD in case of AV block. The occurrence of an IS should always prompt careful analysis of all available S–ECGs. Previous events of QRS morphology change and/or disabling of the SMART Pass filter should be regarded with suspicion, even if an AV bock is not overt evident.