ICDs detect tachyarrhythmias based on rate and duration. Detection durations are programmable in number of beats or in duration, depending on ICD manufacturer, with typical nominal values ranging from 12 beats up to 3 s. If a detected tachycardia spontaneously terminates during ICD reconfirmation (typically during or just after the device charges its capacitors for high-voltage therapy), the ICD may or may not recognize that the tachycardia has terminated. In some cases, PVCs, oversensing, and/or high intrinsic rates may "fool" the ICD into believing that the tachycardia is still present, resulting in an inappropriate shock. In a preliminary analysis of the shocks in the SCD-HeFT, Poole reported that nearly 3% of all shocks delivered were for nonsustained VT.51 Wathen et al.35 reported that 34% of all VTs faster than 320 ms terminated spontaneously after being detected using a programmed detection duration of 18 beats, and Gunderson et al.30 has suggested that longer detection durations may provide reduction in inappropriate detection and therapy for self-terminating VT. ICD manufacturers have made some improvements to reconfirmation algorithms over the years, including analysis of intervals during capacitor charging, to make algorithms less prone to confirm the presence of tachyarrhythmia based on one or two fast intervals. Figure 10 presents examples of two spontaneously terminating VT episodes, one that received a shock due to premature ventricular contractions during the VT reconfirmation period and one where an improved reconfirmation algorithm aborted therapy successfully.
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