Reducing ICD Test Shocks with a Simplified Upper Limit of Vulnerability.

Abstract

Determination of an adequate defibrillation safety margin or defibrillation threshold can be a challenge in patients with implantable defibrillators (ICDs). The upper limit of vulnerability (ULV) has been shown to be highly correlated with measured defibrillation thresholds. The peak of the latest peaking monophasic T wave measured from the pacing spike of the induction train of S1 via the surface electrocardiogram (ECG) is generally accepted to approximate the vulnerable period of the cardiac cycle. The purpose of this study was to determine whether a single electrogram-derived coupling interval could provide an accurate determination of the vulnerable period for a simplified test method to approximate the defibrillation safety margin. We used a single electrogram-derived coupling interval for the timing of the T shock. We compared the measured intracardiac electrogram coupling interval to the latest peaking T wave on the surface ECG. A total of 72 patients were studied: single-chamber ICD (n = 28), dual-chamber ICD (n = 26), or cardiac resynchronization therapy-defibrillator (n = 18). The coupling intervals were greater on the electrograms versus the surface ECG: 365 ms ± 27 versus 347 ms ± 26 (P < 0.0001). Almost all of the patients tested, 69/72 (96%), were indeed defibrillated with the T-shock energy that failed to induce ventricular fibrillation (VF). Only three (4%) of the patients failed the hypothesis when 500 V failed to induce VF but subsequently also failed to defibrillate a forced induction of VF. A simplified ULV testing protocol, using a single electrogram channel, accurately separates low from high defibrillation threshold testing patients.