Detection of T-wave alternans using an implantable cardioverter-defibrillator

Background Microvolt T-wave alternans (TWA) increases acutely prior to ventricular tachycardia (VT) or ventricular fibrillation (VF) in computer simulations and animal models, suggesting that TWA may provide a warning for VT/VF in patients with an implantable cardioverter-defibrillator (ICD). Objectives The purposes of this study were to develop a method for analyzing TWA recorded from ICD electrograms (EGMs) and to evaluate the degree of concordance between EGM TWA and TWA recorded from the surface ECG. Methods We developed a software program to measure EGM TWA in the frequency domain and then used simulated EGMs to determine the effects of ICD signal processing, electrical noise, and variation in the EGM fiducial point on the recorded amplitude and K score (signal-to-noise ratio) of TWA. We then applied this method to analyze TWA simultaneously using both surface ECGs and ICD EGMs during incremental pacing in 25 ICD patients. Pacing modes and EGM sources were varied in repeated trials. EGMs with dynamic range adjusted to achieve a Large T wave were telemetered to a digital Hotter recorder and measured offline. ECG TWA was analyzed using a commercial system. A positive (+) ECG test had sustained alternans >= 1.9 mu V with K score >= 3. Stored EGMs were reviewed for VT/VF during a 6-month follow-up period. Results Simulations demonstrated that the EGM method accurately identified TWA >= 10 mu V. Overall, 10 (40%) patients had at least one ECG TWA+ test and 15 patients (60%) had no ECG TWA+ tests. The maximum value of TWA was greater in EGMs than in ECGs (median 64 mu V vs 2.2 mu V, P<.0001). EGM TWA was greater in ECG TWA+ tests than in ECG TWA- tests (169 +/- 175 AV vs 71 +/- 61 mu V, P<.001). Using a sustained EGM TWA threshold of 30 mu V, EGM TWA was concordant with ECG TWA in 63 (84%) of 75 analyzed tests (P<.0001) and predicted ECG TWA results with 85% sensitivity and 84% specificity. Both ECG and EGM TWA predicted VT/VF during follow-up (ECG: P=.006; EGM: P=.035). Conclusion The amplitude of TWA is at Least 10 times greater on ICD EGMs than on surface ECGs. EGM and ECG TWA have substantial concordance and comparable predictive value for spontaneous VT/VF. These observations support the hypothesis that ECG and EGM TWA detect the same electrical alternans phenomenon.