"Two for the Price of One": A Single-Lead Implantable Cardioverter-Defibrillator System with a Floating Atrial Dipole

Abstract

In patients known to be a high risk for sudden cardiac arrest, implantable cardioverter defibrillators (ICD) are a proven therapy to reduce risk of death. However, in patients without conventional indications for pacing, the optimal strategy for type of device, dual- versus single-chamber, remains debatable. The benefit of prophylactic pacing in this category of patients has never been documented. Although available atrial electrograms in a dual chamber system improve interpretation of stored arrhythmia events, allow monitoring of atrial fibrillation and may potentially reduce the risk of inappropriate shocks by enhancing automated arrhythmia discrimination, the use of dual-chamber ICDs has a number of disadvantages. The addition of an atrial lead adds complexity to implantation and extraction procedures, increases procedural cost and is associated with a higher risk of periprocedural complications. The single lead pacing system with ability to sense atrial signals via floating atrial electrodes (VDD) clinically became available in early 1980's but did not gain much popularity due to inconsistent atrial sensing and concerns about the potential need for an atrial lead if sinus node fails. Most ICD patients do not have indications for pacing at implantation and subsequent risk of symptomatic bradycardia seems to be low. The concept of atrial sensing via floating electrodes has recently been revitalized in the Biotronik DX ICD system (Biotronik, SE & Co., Berlin, Germany) aiming to provide all of the potential advantages of available atrial electrograms without the risks and incremental cost of an additional atrial lead. Compared to a traditional VDD pacing system, the DX ICD system uses an optimized (15 mm) atrial dipole spacing and improved atrial signal processing to offer more reliable atrial sensing. The initial experience with the DX system indicates that the clinically useful atrial signal amplitude in sinus rhythm remains stable over time. Future studies are needed to determine reliability of atrial sensing during tachyarrhythmias, particularly atrial fibrillation as well as clinical utility and cost-effectiveness of this technology in different populations of patients.

Keywords: DX ICD System; Floating Atrial Electrodes; Implantable Cardioverter-Defibrillator; VDD Pacing.

Figure 1. The Biotronik Linox Smart DX active fixation lead. The atrial dipole is mounted 15 -17 cm from the tip of the lead. Courtesy of Biotronik

Figure 2A. A chest X-ray images showing placement of the Biotronik Linox Smart DX active fixation lead in the apex

Figure 2B. A chest X-ray image showing placement of the Biotronik Linox Smart DX active fixation lead in the septum

Figure 3. The schematic shows atrial signal processing in the Biotronik DX ICD system, which includes a dedicated atrial input stage with up to 4-fold signal amplification and noise filtering. Courtesy of Biotronik

Figure 4. Rhythm discrimination criteria employed in the SMART Detection algorithm. A and V denote atrial and ventricular, respectively. Courtesy of Biotronik

Figure 5A. An examples of stored arrhythmia events during supraventricular tachycardia

Figure 5B. An examples of stored arrhythmia events during ventricular tachycardia

Figure 5C. An examples of stored arrhythmia events during atrial flutter