Effect of Programmed Number of Intervals to Detect Ventricular Fibrillation on Implantable Cardioverter-Defibrillator Aborted and Unnecessary Shocks

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Detection of self-terminating arrhythmias by implantable cardioverter-defibrillators (ICDs) causes unnecessary battery depletion and unnecessary shocks. Our goal was to estimate the effect of the programmed number of intervals to detect (NID) ventricular fibrillation (VF) on ICD temporal episode rate, unnecessary shocks, and delay in detection of VF.

Methods and Results

We analyzed 773 ICD-detected VF episodes in 875 patients. The number of intervals to detect VF was programmed to 12 of 16 (NID 12) in 305 patients and 18 of 24 (NID 18) in 570 patients. For patients with NID 12, we estimated the increase of mean cumulative episode rate at 6 months since implant and decrease in detection time for VF compared with a hypothetical NID 18. For patients with NID 18, we estimated the decrease of mean cumulative episode rate and unnecessary shocks compared with a hypothetical NID 12.

Methods and Results

Patients with NID 12 had a 17% increased episode rate resulting in unnecessary capacitor charging for self-terminating arrhythmias. Patients with NID 18 had a 22% decreased episode rate. In patients with NID 12, hypothetical NID 18 would have delayed detection of 273 VF episodes in 1.8 seconds. In patients with NID 18, hypothetical NID 12 would have resulted in inappropriate delivery of 14 aborted shocks in 10% of patients with episodes.


In patients with self-terminating device-detected VF, increasing the number of intervals to detect VF from 12/16 to 18/24 results in a clinically significant decrease in ICD detections and fewer unnecessary shocks with minimal incremental delay in VF detection.

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