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Other functions of implantable defibrillators
In current models of implantable defibrillators there are a number of features that are not directly related to the analysis of or the delivery of therapy for ventricular arrhythmias. All models now have pacing modes similar to those in single- or dual-chamber pacemakers. All models routinely store electrograms for sensed arrhythmias, a feature that is extremely helpful during follow-up for analysis of the therapies delivered and for detection of many malfunc-tions that may occur in the device . Information about battery voltage, lead impedance, and the time needed to charge the capacitor is stored for later analysis. Some models can detect atrial arrhyth-mia and deliver the appropriate therapy (shock or antitachycardia pacing). A dedicated atrial defibrillator has been developed and tested in limited clinical trials in patients with atrial fibrillation, but it is not yet available as a separate unit.12,13 The most recent major innovation in implantable cardioverter–defibrillators is implementation of biventricular pacing to achieve cardiac resynchronization in patients with advanced congestive heart failure and intraventricular conduction delays, especially left bundle-branch block.
Clinical trials
From the time of its clinical introduction, the implantable cardioverter–defibrillator has been shown to recognize ventricular fibrillation and terminate the arrhythmia by delivering shocks. In early, uncontrolled studies, the delivery of a shock was assumed to represent a life saved. This analytic approach overestimated the benefit of a defibrillator, because not all shocks are appropriate, not every arrhythmia would prove fatal if not terminated, and death may still soon occur from other cardiac causes. Randomized clinical trials were therefore conducted to evaluate the effect of implanted defibrillators on mortality. Secondary-prevention trials in which the subjects enrolled were survivors of cardiac arrest or had sustained ventricular tachycardia were conducted to compare the effect of defibrillator therapy and antiarrhythmic-drug therapy on mortality. The enrollment of an untreated control group was considered unethical. Primary-prevention trials in which the subjects enrolled were high-risk patients without a history of sustained ventricular arrhythmias compared the effects of no treatment.