Case Report
We present the case of a 30-year-old man with a subcutaneous implantable cardioverter defibrillator (S-ICD) who suffered sudden out-of-hospital cardiac arrest. During resuscitation, the patient received inappropriate shocks due to oversensing of chest compressions. The rhythm on the EMS monitor showed asystole throughout the period of resuscitation. 

Discussion
Inappropriate shocks are most often induced by supraventricular tachycardia and T-wave oversensing. T-wave oversensing develops because of high T-wave amplitude, which the S-ICD interprets as another QRS complex. This leads to double counting. In such cases, the heart rate reaches the cut-off value for the Shock Zone and the S-ICD delivers the shock. The inappropriate shocks in the patient presented here were due to oversensing to QRS artefacts that developed during chest compression. The S-ICD device includes a function termed Smart Pass, which reduces oversensing of lower frequency signals, such as T waves or the double counting of wide QRS complexes. The Smart Pass is automatically turned off in the case of a slow heart rate (˂43 beats per minute) or a low amplitude (˂0.5 mV). The main role of the Smart Pass function is to decrease the risk of inappropriate treatment of fine ventricular fibrillation. Analysis of the patient's S-ICD recording demonstrated that this function was off during the resuscitation. This led to an increase in the S-ICD's sensitivity and to double counting of wide QRS artefacts. With chest compressions at a rate of 110–120 per minute, the cut-off value for the Conditional Shock Zone and the Shock Zone of 200 and 230 beats per minute respectively was reached. Once this cut-off value was exceeded, the S-ICD delivered a shock.

Conclusion
Inappropriate shocks and the specificities of magnet placement to inhibit shock delivery may decrease the quality of resuscitation.

6 - Bystander hit by leakage current from S-ICD