Undersensing during ventricular tachyarrhythmias in a third-generation implantable cardioverter defibrillator: diagnosis using stored electrograms and correction with programming.

Third-generation implantable cardioverter defibrillators with stored electrograms allow diagnosis of various sensing problems that may lead to an inappropriate device response. Undersensing of ventricular tachyarrhythmias is a potentially serious problem, as it may lead to failure to deliver therapy. To determine the incidence of this problem, we reviewed 98 patients with Ventritex Cadence defibrillator systems and found 2 patients in whom defibrillation therapy was delayed or aborted because of undersensing during induced ventricular tachyarrhythmias. In both cases, examination of stored electrograms revealed variation in electrogram amplitude, which presumably resulted in failure of the autogain feature to increase its sensitivity enough to count each complex. During charging, criteria for redetection of sinus rhythm were met because of this undercounting, leading to failure to deliver defibrillation therapy. This problem was detected in both patients 4-6 weeks following device implant during device testing, and both patients had been started on antiarrhythmic drug therapy prior to this testing. Programming the sinus redetection parameter from nominal to slow, increasing the number of beats necessary to confirm resumption of sinus rhythm, corrected the problem in both patients. Device testing in the electrophysiology laboratory, routinely postoperatively and following initiation of antiarrhythmic drug therapy, and the ability to retrieve stored electrograms are useful in detecting such sensing anomalies.