Inappropriate implantable cardioverter-defibrillator (ICD) shock therapy is painful, stressful, and typically occurs unexpected in conscious patients and may be related to a less favourable prognosis. In our institution, we have observed four cases of multiple inappropriate ICD shocks during reset to VVI backup mode. All four patients were implanted with a St Jude Medical ICD since 2010. The reset to VVI backup mode happens as a 'safety' response when the ICD encounters errors in the software or hardware often due to electromagnetic interference. The ICD then operates in a simple mode, with only a ventricular fibrillation (VF) zone starting at 146 b.p.m., with shock therapy only and changes in sensitivity settings making the ICD more sensitive. In all cases, the reason for the multiple inappropriate shocks was that the VF zone was reached due to exercise-induced sinus tachycardia or due to oversensing during sinus rhythm. The VVI backup mode has to balance between protection from failure of ICD therapy during life-threatening ventricular arrhythmias and from inappropriate shocks. It seems the non-programmable parameters in VVI backup mode of St Jude Medical ICDs carry an unacceptable high risk of inappropriate shocks during normal rhythm as illustrated by our four cases. A higher VF zone comparable with the zones chosen by the other manufacturer would give a better balance, since it is very unlikely that a patient will need shock therapy urgently for slow ventricular tachycardia.
Keywords: Detection; Implantable cardioverter-defibrillator; Inappropriate shock; Mechanism; VVI backup mode; Ventricular fibrillation zone.
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