Determination of the Upper Limit of Vulnerability Using Implantable Cardioverter-Defibrillator Electrograms

Abstract

Background-The upper limit of vulnerability (ULV) correlates with the defibrillation threshold and can be determined with 1 episode of ventricular fibrillation (VF). To automate the ULV in an implantable cardioverter-defibrillator (ICD), the most vulnerable intervals must be identified from an ICD electrogram rather than the latest-peaking surface T wave (T peak ). We hypothesized that the recovery time (T R ), defined as the maximum derivative (dV/dt) of the T wave of the shock electrogram, correlates with the most vulnerable intervals. Methods and Results-We determined ULV, defibrillation threshold, and the most vulnerable intervals in 25 patients at ICD implantation. The ULV was the weakest T-wave shock that did not induce VF. The most vulnerable intervals were the ones associated with the strongest shocks that induced VF. Telemetered shock electrograms were stored on digital tape and differentiated offline to measure T R . T peak and T R were highly correlated (T peak ϪT R ϭϪ2Ϯ11 ms; ϭ0.80, PϽ0.001). At least 1 most vulnerable interval timed between Ϫ20 ms and ϩ20 ms relative to T peak in all patients and between Ϫ40 ms and ϩ20 ms relative to T R in 96% of patients. Conclusions-The

Automated Vulnerability Testing Identifies Patients With Inadequate Defibrillation Safety Margin

“…Proof-of-concept studies were performed to develop a system that could be implemented in the ICD or programmer and would identify the vulnerable zone by automated analysis of the far-field electrogram of ICD. 16,17 As in those studies, the automatically determined timing, T R , was close to the timing determined from the surface ECG, T peak . The present study establishes the clinical feasibility of using the automatically identified vulnerable zone in an automated system of ICD implant testing, the AVSM system.…”

“…Although T R does not precisely correspond to the peak of the surface T wave, studies have shown that it can be processed automatically by an ICD and used as a reliable marker for the timing of the peak of the vulnerable zone. 16,17 A new method for ICD testing has been developed in which vulnerable period timing is automatically determined by calculating T R from the far-field ICD electrogram. T R is used as the timing marker for the vulnerable period, and a sequence of T-wave shocks are delivered at a predetermined energy.…”

“…Determination of T R has been described in detail previously. 16,17 T R is the peak of the derivative of the far-field intracardiac T wave and is calculated numerically after the electrogram is filtered. Because numerical differentiation amplifies high-frequency signal components, conventional numerical peak detection on a differentiated signal generally produces unstable results.…”

Should You Use DFT or ULV to Assess Defibrillation Efficacy?

“…22,23 For a single-coil shock vector, a 4-shock scan should be delivered at À40 ms, À20 ms, 0 ms, and 120 ms relative to the peak of the T wave. 22 For a dual-coil shock vector, a 4-shock scan should be delivered at À20, 0, 120, and 140 ms relative to the peak (see Fig.…”

“…6). 3,[22][23][24] Measurements from the endocardial electrograms Vulnerability testing would be more efficient if ICDs selected timing intervals for T-wave shocks automatically. Because ICD electrograms typically have biphasic T waves, their peaks are difficult to measure.…”