DEVICE ROUNDS Device Therapy in a Patient with a Biventricular Implantable Cardioverter-Defibrillator: What Is the Tachycardia? ASHLEY CHIN, M.B.CH.B., SYAMKUMAR DIVAKARA MENON, M.B.B.S., DIEGO CHEMELLO, M.D., and CARLOS A MORILLO, M.D. From the Department of Pacing and Electrophysiology, Hamilton Health Sciences, McMaster University, Hamilton, Ontario, Canada inappropriate implantable cardioverter-defibrillator therapy, atrioventicular nodal re-entry tachycardia, supraventricular tachycardia Case Presentation A 71-year-old man with a dilated cardiomy- opathy (left ventricular ejection fraction [LVEF] < 20%) received a biventricular implantable cardioverter-defibrillator (ICD) (Boston Scientific Cognis 100-D N107, Boston Scientific, Natick, MA, USA) for primary prevention and symptomatic heart failure with left bundle branch block. His right ventricular (RV) ICD lead was implanted in the RV apex, the right atrial (RA) lead in the RA appendage, and the left ventricular lead in a lateral coronary sinus branch. He presented to the emergency room following an episode of palpitations and an ICD shock from his device 6 months postimplant. The implant settings of his ICD were as follows: ventricular fibrillation zone ≥ 240 beats/min (≤250 ms), ventricular tachycardia (VT) zone ≥ 185 beats/min (≤324 ms). VT therapy was programmed as “scan” with three bursts of 10 pulses followed by ICD shocks. Detection enhancements (Rhythm ID) was turned “on” with the sustained rate duration set at 3 minutes. The atrial fibrillation rate threshold was programmed at 170 beats/min with stability at 20 ms. Interrogation of his ICD revealed two tachy- cardia episodes. Three bursts of antitachycardia pacing (ATP) were unsuccessful in terminating the first tachycardia episode, which progressed to a successful ICD shock. The first burst of ATP of the second tachycardia episode is shown (Figure 1). Three ICD electrogram channels are recorded We declare we have no conflicts of interest. Address for reprints: Ashley Chin, M.B.Ch.B., Department of Pacing and Electrophysiology, Hamilton Health Sciences, McMaster University, 232 Barton Street East, Hamilton, Ontario, Canada L8L 2X2; e-mail: Ashley.chin1@gmail.com Received February 3, 2012; revised March 16, 2012; accepted April 2, 2012. doi: 10.1111/j.1540-8159.2012.03493.x (atrial, ventricular, and the far-field shock channels). The tachycardia had a cycle length (CL) of 298–303 ms with a 1:1 atrioventricular (AV) relationship classified in the “VT” zone (CL ≤ 324 ms). The “Rhythm ID” algorithm initially withheld ATP therapy as the rhythm was classified as a supraventricular tachycardia (SVT)—the number of atrial electrograms equaled the number of ventricular electrograms during the tachycardia (V = A) and the tachycardia ventricular electrograms correlated with the sinus rhythm ventricular electrogram template (annotated “RID+”). However, after 1 minute and 33 seconds the ventricular electrograms did not match the sinus rhythm ventricular electrogram template (the last unmatched beat annotated “RID–”) and the device delivered ATP. The first burst of ATP did not terminate the tachycardia. The response of the tachycardia to ATP is shown. The third burst of ATP resulted in termination of the tachycardia (not shown). What is the mechanism of the tachycardia? Commentary The differential diagnosis of this tachycardia includes atrial tachycardia (AT), atrioventricular nodal re-entry tachycardia (AVNRT), atrioventric- ular re-entry tachycardia (AVRT), or ventricular tachycardia (including bundle branch re-entrant VT). The ventricular-atrial (VA) or (shock lead- A) time during the tachycardia is difficult to determine but is approximately 80–90 ms and is not useful in excluding potential diagnoses. However, close inspection of the atrial electrograms before and during the first five beats of ATP reveals that the atrial electrogram CL remains relatively constant (300 ms: range 288–313 ms) despite ATP pacing at a fixed cycle length of 263 ms. This suggests that the atrial rate is not dependent on the ventricles which makes VT unlikely unless there is a dual tachycardia. A dual tachycardia is highly unlikely because of C 2012, The Authors. Journal compilation C 2012 Wiley Periodicals, Inc. PACE, Vol. 00 2012 1