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ICD Arrhythmia Detection and Discrimination: Are We There Yet?

Implantable cardioverter defibrillator (ICD) therapy is beneficial to patients who have experienced life-threatening ventricular arrhythmias (secondary prevention) and those at risk for ventricular arrhythmias due to the presence of structural heart disease (primary prevention).1-5 The DAVID trial suggested that dual-chamber ICDs programmed to “force” ventricular pacing result in an increase in morbidity and mortality.6 The increased costs and added complexity of dualchamber ICDs, along with results from DAVID and other trials, have lead to the recommendation that, in the absence of a pacing indication, single-chamber ICDs should be implanted and programmed to permit intrinsic ventricular activation and avoid right ventricular pacing.7 ICD technology has evolved to allow more accurate rhythm detection to avoid inappropriate shocks. There are, however, differences in the sensitivity and specificity of ventricular tachycardia (VT) detection dependent on the specific device programming, the type of device (single vs dual chamber), and the clinical arrhythmia. Furthermore, expectations related to the ability of the device to detect and provide therapy are different, dependent on the clinical arrhythmia. For example, ICD detection of ventricular fibrillation (VF) must have high sensitivity, as the consequences of underdetection are potentially fatal.8 In contrast, detection algorithms for ventricular tachycardia (VT) must balance the risks of underdetection with the painful, psychologically troubling, and potentially proarrhythmic consequences of inappropriate therapy.9-17 A highly sensitive detection algorithm is appropriate for faster tachycardias because the risks of underdetection are high and the probability of rate-zone overlap with supraventricular tachycardia (SVT) is low. On the other hand, a highly specific algorithm is appropriate for hemodynamically stable, slower VT because the risks of underdetection are low and the probability of rate-zone overlap with SVT is high. Historically, when tiered-therapy ICDs detected VT by rate criteria only, inappropriate therapy for SVT occurred in 45% of patients.8 The problem of inappropriate therapy was greater for tiered-therapy ICDs because the probability of rate overlap between the target VT and SVT was greater. Pacing therapies delivered during SVT could potentially induce VT; or could induce atrial fibrillation, which in turn could be sensed as VT and treated with pacing, reinitiating VT.