Potential hazards of fixed gain sensing and arrhythmia reconfirmation for implantable cardioverter defibrillators

Appropriate sensing of ventricular tachycardia (VT) and ventricular fibrillation (VF) is of paramount importance for safety of patients with implanted cardioverter defibrillators (ICDs). Recently, the GuardianR ATP 4210, a new third generation ICD that uses programmable but fixed sensing during sinus rhythm and doubles its sensitivity settings when VF is detected, to a maximum programmable sensitivity of 1 mV, has been tested in phase I and II clinical trials. A reconfirmation algorithm of this ICD confirms the presence of VT or VF prior to therapy. This case report describes undersensing of VF in a patient with the GuardianR ATP 4210 at the maximum programmed sensitivity of 1 mV. Inappropriate episodes of asystole and prolonged bradycardias were also observed in this patient due to shortcomings in the reconfirmation algorithm design. Reoperation was required, with positioning of a new endocardial sensing lead to correct the undersensing of VF. This, however, did not correct asystolic pauses following antitachycardia pacing or spontaneous tachycardia termination prior to therapy. This case report highlights the hazards of fixed gain sensing for implantable ICDs and a potential limitation of a specific tachyarrhythmia reconfirmation algorithm used in this device.