Minimizing Inappropriate or “Unnecessary” Implantable Cardioverter-Defibrillator Shocks

Abstract

Life-saving shocks are the raisons d'etre of implantable cardioverter-defibrillators (ICDs). Paradoxically, shocks also cause much of the morbidity associated with ICDs. Consistently, shocks reduce quality of life,1,2 and rarely, they may cause proarrhythmia.3,4 Additionally, shocks have been reported to be associated with excess mortality.5 Experts disagree about whether shocks are responsible for this excess mortality,6 but they do agree that ICDs should deliver the fewest shocks necessary to protect patients from ventricular tachycardia (VT) or ventricular fibrillation (VF). Minimizing ICD shocks requires a comprehensive approach, beginning with patient selection,7 general medical care (eg, preventing electrolyte abnormalities), and general cardiac care (preventing ischemia and treating heart failure). It includes the use of antiarrhythmic drugs and catheter ablation both to prevent VT or supraventricular tachycardia (SVT) and to control the ventricular rate in atrial fibrillation (AF) as well as the appropriate choice of hardware and device programming. This review focuses on ICD features and programming to minimize shocks. Figure 1 summarizes the sequential processes required for an ICD to deliver a shock. The sense amplifiers convert the continuous electrogram (EGM) from the tip of the right ventricular (RV) lead into a series of instantaneously sensed events representing ventricular depolarization. This series of sensed events is processed by ICD algorithms, including those for initial detection; SVT-VT discrimination; and in some models of ICDs, enhancements to minimize oversensing. If the rhythm is classified as a VT, antitachycardia pacing (ATP) can be delivered. The ICD then performs a redetection process, which results in determination that the device-defined VT either has terminated or persists. If VT is redetected, the process iterates until all programmed ATPs are delivered. The ICD then charges the high-voltage capacitors, during which time a final round of ATP may be delivered. After capacitor …