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

BACKGROUND: 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. OBJECTIVES: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: A simplified ULV testing protocol, using a single electrogram channel, accurately separates low from high defibrillation threshold testing patients.

Efficacy of tuned waveforms based on different membrane time constants on defibrillation thresholds: primary results from the POWER trial.

BACKGROUND: The efficacy of tuned defibrillation waveforms versus the nominal fixed-tilt waveform has been previously studied. However, the optimal membrane time constant for tuning was not known. The POWER (Pulsewidth Optimized Waveform Evaluation tRial) trial was designed to determine the optimal membrane time constant for programming "tuned" biphasic waveforms. METHODS: This acute, multicenter study included 121 implantable cardioverter-defibrillator/cardiac resynchronization therapy defibrillator patients who were randomized at implant to any two of the three membrane time constant waveforms (2.5, 3.5, and 4.5 ms). Fixed pulse widths were programmed using the measured high voltage shock impedance. The defibrillation threshold (DFT) estimates were obtained using a hybrid protocol starting with an upper limit of vulnerability estimate followed by a step-up/step-down ventricular fibrillation induction process. RESULTS: DFT voltage was significantly lower using 3.5- and 4.5-ms waveforms as compared to the 2.5-ms waveform (P = 0.004 and 0.035, respectively). DFT voltage with both 3.5- and 4.5-ms waveforms was ≤ that obtained with the 2.5-ms waveform in 78.5% of the cases. The mean difference in DFT voltage using the 3.5-ms waveform and the 4.5-ms waveform was not significant (P = 0.4). However, the 3.5-ms waveform gave a lower DFT than the 4.5-ms waveform in 19 patients although the reverse was true in only nine (P = 0.02 not significant for multiple comparisons). CONCLUSIONS: The use of a 3.5- or 4.5-ms time constant-based waveforms had lower DFTs when compared to the 2.5-ms waveform. This study suggests that the first defibrillation attempt at implantation should be with 3.5- or 4.5-ms time constant-based waveforms. The 3.5-ms-based waveform trended toward the best choice.