Undetected Ventricular Fibrillation in Transvenous Implantable Cardioverter-Defibrillators

Abstract

Background The purpose of this study was to prospectively analyze redetection problems after unsuccessful shock with different lead systems and devices. Methods and Results We prospectively analyzed detection and redetection characteristics among transvenous implantable cardioverter-defibrillators (ICDs) using standard bipolar and integrated bipolar sensing. Monophasic and biphasic ICDs were included. Subthreshold shocks were intentionally delivered, and redetection of ventricular fibrillation (VF) was assessed before discharge and at 1, 3, 6, and 12 months later. Sensing of VF resulting from antitachycardia pacing and low-energy cardioversion (≤2 J) also was analyzed. Before inclusion in the study, each patient underwent subthreshold shock testing at three different time intervals. Among the 160 ICDs with standard bipolar sensing, 530 VF inductions were analyzed. After the failed shocks, undersensing was more frequent (3% versus 20%, P <.01) but did not remarkably prolong redetection (3.1±0.8 versus 3.3±1.1 seconds). Among the 201 ICDs with integrated bipolar sensing, 80 were connected to a CPI device (60 Ventak 1600–Endotak 60 series and 20 PRx II 1715–Endotak 70 series) and 121 to the Ventritex defibrillator (91 Endotak 60 series, 14 TVL systems, and 16 Endotak 70 series). After 252 failed shocks, redetection was prolonged with the CPI system (3.1±1.4 versus 4.6±3.6 seconds, P <.05) but did not change after 396 failed shocks with the Ventritex ICD (5.4±1.9 versus 4.9±2.2 seconds). This may reflect different nominal settings for detection and redetection. In 9 of 121 patients with Ventritex and 1 of 80 with the CPI ICDs, the devices failed to redetect VF. However, redetection malfunction was never observed in patients with integrated bipolar systems with >6-mm electrode separation. After antitachycardia pacing in 1 patient and a 2-J shock in 1 patient, ventricular tachycardia turned into VF, which was undetected. Both patients used the Endotak 60 series–Cadence combination. None of the patients showing VF undersensing had sudden death at follow-up. Only 3 of the 12 patients with sensing malfunction were on antiarrhythmia drugs at the time of testing. Analysis of endocardial electrograms showed that failure to redetect VF is not associated with a uniform reduction but with a rapid and repetitive change of electrogram amplitude. Conclusions Standard bipolar sensing redetects VF more effectively than integrated bipolar sensing. Endocardial electrogram analysis provides insights into the understanding of the mechanism of undersensing, and certain lead-device combinations result in a higher occurrence of VF undersensing. The clinical relevance of this phenomenon remains unknown.