PACEMAKER/ICD PROBLEM OF THE MONTH Inappropriate implantable cardioverter-defibrillator therapy during exercise: What is the mechanism? Dirk Vollmann, MD, Lars Lüthje, MD, Markus Zabel, MD From Abteilung für Kardiologie und Pneumologie, Georg-August-Universität Göttingen, Göttingen, Germany. Case summary A 48-year-old male patient with dilated cardiomyopathy, severely reduced left ventricular ejection fraction, and a history of ventricular tachycardia (VT) presented after ex- periencing shock discharges of his dual-chamber implant- able cardioverter-defibrillator (ICD) during moderate exer- cise. Four weeks earlier, his present device (Lexos DR, Biotronik, Berlin, Germany) had replaced the ICD (Jewel AF, Medtronic, Minneapolis, MN, USA) that had been implanted in 1999. The ICD lead (model 6942, Medtronic) and right atrial electrode (model 6940, Medtronic) were left in place because no evidence of lead dysfunction was present. ICD programming at discharge was similar to that of the explanted device: DDD mode 50 –130 bpm, VT detection window 350 –270 ms, ventricular fibrillation de- tection zone 270 ms. Device interrogation during the unscheduled fol- low-up revealed that recurrent VT episodes had been detected since the ICD had been replaced. Starting 7 days after hospital discharge, VT detection had triggered mul- tiple trains of antitachycardia pacing therapies and two high-voltage shock discharges. Figure 1 shows stored intracardiac electrograms and marker annotations of an episode that triggered antitachycardia pacing and finally resulted in shock discharge. Ventricular electrode param- eters at the time of device interrogation were comparable to the values measured during device replacement: R- wave amplitude 13 mV, pacing impedance 549 , pacing threshold 0.5 V at 0.5 ms. Ventricular sensitivity was programmed to 0.8 mV. What is the underlying reason for ICD therapy in this patient, and how can it be prevented in the future? Commentary The electrogram strip shown in Figure 1 reveals inappro- priate VT detection during sinus tachycardia, caused by double sensing of the ventricular signal. This episode trig- gered antitachycardia pacing that induced a true rapid VT and thereby caused a shock discharge. Ventricular oversensing is not uncommon in ICD recipients and may lead to potentially life-threatening events. 1 Although the overall incidence is low, several reports have described R-wave double counting as a typical complication in early- generation devices placed for cardiac resychronization. 1–3 In these devices, double sensing is related to extended bipolar sensing fields between the tips of the right ventricular and left ventricular electrodes. In the present case, double sensing of ventricular signals occurred in an ICD that uses true bipolar right ventricular sensing. With such a small sensing field, R-wave double counting appears to be rare. 4,5 KEYWORDS Implantable cardioverter-defibrillator; Double sensing; Therapy (Heart Rhythm 2009;6:718 –719) Address reprint requests and correspondence: Priv. Doz. Dr. med. Dirk Vollmann, Universitätsmedizin Göttingen, Georg-August Universität, Herz- entrum Göttingen, Abteilung Kardiologie und Pneumologie, Schwerpunkt Klinische Elektrophysiologie, Robert-Koch-Strasse 40, 37075 Göttingen, Ger- many. E-mail address: d.vollmann@med.uni-goettingen.de. Figure 1 Stored intracardiac atrial and ventricular electrograms and marker annotations of a tachycardia episode that triggered antitachycardia pacing and finally resulted in a shock discharge by the ICD. 1547-5271/$ -see front matter © 2009 Heart Rhythm Society. All rights reserved. doi:10.1016/j.hrthm.2009.01.024