Treatment of atypical pacemaker-mediated tachycardia with ablation of the retrograde atrioventricular nodal pathway.

A 25-year-old runner received a single-lead, VDD pacemaker after ablation of AV nodal reentrant tachycardia complicated by intermittent AV block. The rate-adaptive AV delay algorithm (RAAV), which shortens the sensed AV interval (SAV) at faster atrial rates, was programmed to provide a physiologic SAV with exercise. She developed repetitive, atypical, long-RP pacemaker-mediated tachycardia (PMT) because the RAAV shortened the antegrade SAV and retrograde conduction occurred over the slow AV nodal pathway. PMT was refractory to usual programming solutions. Using high-density electroanatomic mapping, we were able to ablate the retrograde limb of PMT without further damaging AV conduction.

Mechanisms of Undersensing by a Noise Detection Algorithm That Utilizes Far-Field Electrograms With Near-Field Bandpass Filtering.

BACKGROUND: Implantable cardioverter defibrillators (ICDs) must establish a balance between delivering appropriate shocks for ventricular tachyarrhythmias and withholding inappropriate shocks for lead-related oversensing ("noise"). To improve the specificity of ICD therapy, manufacturers have developed proprietary algorithms that detect lead noise. The SecureSenseTM RV Lead Noise discrimination (St. Jude Medical, St. Paul, MN, USA) algorithm is designed to differentiate oversensing due to lead failure from ventricular tachyarrhythmias and withhold therapies in the presence of sustained lead-related oversensing. METHODS AND RESULTS: We report 5 patients in whom appropriate ICD therapy was withheld due to the operation of the SecureSense algorithm and explain the mechanism for inhibition of therapy in each case. Limitations of algorithms designed to increase ICD therapy specificity, especially for the SecureSense algorithm, are analyzed. CONCLUSION: The SecureSense algorithm can withhold appropriate therapies for ventricular arrhythmias due to design and programming limitations. Electrophysiologists should have a thorough understanding of the SecureSense algorithm before routinely programming it and understand the implications for ventricular arrhythmia misclassification.

Intracardiac QT integral on far-field ICD electrogram predicts sustained ventricular tachyarrhythmias in ICD patients.

BACKGROUND: Prediction of sustained ventricular tachycardia (VT)/ventricular fibrillation (VF) could help to guide preventive interventions in at-risk patients. The QRST integral (∫QT) reflects intrinsic repolarization properties. OBJECTIVE: The objective of this study was to determine whether intracardiac ∫QT predicts VT/VF in the next few months in patients with implantable cardioverter defibrillators (ICDs). METHODS: Far-field (FF) and near-field (NF) right ventricular intracardiac electrograms (EGMs) were recorded via telemetry in 46 patients with structural heart disease and ICDs implanted for secondary prevention of sudden cardiac death. Epochs of 4.9 ± 0.4 minutes during sinus rhythm (mean heart rate 70.9 ± 15.2 beats/min) and ventricular pacing at 105 beats/min were analyzed. Mean ∫QT was calculated on FF and NF EGMs as the algebraic sum of areas under the QRST curve and adjusted by mean heart rate. Patients were followed up for at least 3 months. True VT/VF events treated by the ICD served as the end point. RESULTS: During a mean follow-up of 4.6 months, 22 patients (48%) were treated for VT/VF. Unadjusted and adjusted by heart rate, FF EGM ∫QT in sinus rhythm was a significant predictor of VT/VF (unadjusted ∫QT hazard ratio 1.007; 95% confidence interval 1.002 to 1.0013; P = .007; adjusted ∫QT hazard ratio 1.68; 95% confidence interval 1.19 to 2.36; P = .002). The highest quartile of intracardiac ∫QT predicted VT/VF (log-rank test P = .042) and identified patients at risk with a specificity of 86% and positive predictive value of 73%. CONCLUSION: Increased intracardiac FF EGM ∫QT predicts VT/VF in patients with structural heart disease and secondary prevention ICDs.

Intracardiac electrogram T-wave alternans/variability increases before spontaneous ventricular tachyarrhythmias in implantable cardioverter-defibrillator patients: a prospective, multi-center study.

BACKGROUND: T-wave alternans (TWA) increases before ventricular tachycardia (VT) or fibrillation (VF), suggesting that it may warn of VT/VF in implantable cardioverter-defibrillator patients. Recently, we described a method for measuring alternans and nonalternans variability (TWA/V) from electrograms (EGMs) stored in implantable cardioverter-defibrillators before VT/VF. The goal of this prospective, multicenter study was to determine whether EGM TWA/V was greater before VT/VF than at baseline. METHODS AND RESULTS: We enrolled 63 implantable cardioverter-defibrillator patients. TWA/V was computed from stored EGMs before spontaneous VT/VF and from sequential windows of 8 pairs of beats using 4 different control recordings: baseline rhythm, rapid pacing at 105 bpm, segments of ambulatory Holter EGMs matched to the time of VT/VF episodes, and EGMs before spontaneous supraventricular tachycardia. During follow-up, 28 patients had 166 episodes of VT/VF. TWA/V was greater before VT/VF (62.9 ± 3.1 µV; n = 28) than during baseline rhythm (12.8 ± 1.8 µV; P < 0.0001; n = 62), during rapid pacing (14.5 ± 2.0 µV; P < 0.0001; n = 52), before supraventricular tachycardia (27.5 ± 6.1 µV; P < 0.0001; n = 9), or during time-matched ambulatory controls (12.3 ± 3.5 µV; P < 0.0001; n = 16). By logistic regression, the odds of VT/VF increased by a factor of 2.2 for each 10-µV increment in TWA/V (P < 0.0001). CONCLUSIONS: In implantable cardioverter-defibrillator patients, EGM TWA/V is greater before spontaneous VT/VF than in control recordings. Future implantable cardioverter-defibrillators that measure EGM TWA/V continuously may warn patients and initiate pacing therapies to prevent VT/VF.

Optimizing defibrillation waveforms for ICDs.

While no simple electrical descriptor provides a good measure of defibrillation efficacy, the waveform parameters that most directly influence defibrillation are voltage and duration. Voltage is a critical parameter for defibrillation because its spatial derivative defines the electrical field that interacts with the heart. Similarly, waveform duration is a critical parameter because the shock interacts with the heart for the duration of the waveform. Shock energy is the most often cited metric of shock strength and an ICD's capacity to defibrillate, but it is not a direct measure of shock effectiveness. Despite the physiological complexities of defibrillation, a simple approach in which the heart is modeled as passive resistor-capacitor (RC) network has proved useful for predicting efficient defibrillation waveforms. The model makes two assumptions: (1) The goal of both a monophasic shock and the first phase of a biphasic shock is to maximize the voltage change in the membrane at the end of the shock for a given stored energy. (2) The goal of the second phase of a biphasic shock is to discharge the membrane back to the zero potential, removing the charge deposited by the first phase. This model predicts that the optimal waveform rises in an exponential upward curve, but such an ascending waveform is difficult to generate efficiently. ICDs use electronically efficient capacitive-discharge waveforms, which require truncation for effective defibrillation. Even with optimal truncation, capacitive-discharge waveforms require more voltage and energy to achieve the same membrane voltage than do square waves and ascending waveforms. In ICDs, the value of the shock output capacitance is a key intermediary in establishing the relationship between stored energy-the key determinant of ICD size-and waveform voltage as a function of time, the key determinant of defibrillation efficacy. The RC model predicts that, for capacitive-discharge waveforms, stored energy is minimized when the ICD's system time constant taus equals the cell membrane time constant taum, where taus is the product of the output capacitance and the resistance of the defibrillation pathway. Since the goal of phase two is to reverse the membrane charging effect of phase one, there is no advantage to additional waveform phases. The voltages and capacitances used in commercial ICDs vary widely, resulting in substantial disparities in waveform parameters. The development of present biphasic waveforms in the 1990s resulted in marked improvements in defibrillation efficacy. It is unlikely that substantial improvement in defibrillation efficacy will be achieved without radical changes in waveform design.

Improved defibrillation efficacy with an ascending ramp waveform in humans.

OBJECTIVES: The purpose of this study was to compare an ascending ramp waveform (RAMP) with a standard, clinically available biphasic truncated exponential waveform (BTE) for defibrillation in humans. BACKGROUND: In animal studies, RAMP had a lower defibrillation threshold (DFT) than BTE. METHODS: We studied 63 patients at implantable cardioverter-defibrillator placement using a dual-coil lead and left pectoral active can. The subjects were divided into two groups, one with a 12-ms ascending first phase and one with a 7-ms ascending first phase. Phase 2 of RAMP for both groups was a truncated exponential decay with 65% tilt and reversed polarity. The BTE had a 50% tilt in each phase. DFT and upper limit of vulnerability (ULV) were measured for both waveforms using a binary search protocol. RESULTS: The patient population was 77% male, with a mean age of 63 +/- 10 years and ejection fraction of 33 +/- 13%. Delivered energy at DFT was lower with the 7-ms RAMP vs BTE (5.4 +/- 2.6 J vs 6.5 +/- 3.4 J; P < .01) but unchanged with the 12-ms RAMP (7.4 +/- 4.5 J vs 7.1 +/- 4.9 J). Maximal voltage at DFT was significantly lower with either RAMP compared to BTE (P < .01). There was a strong correlation between ULV and DFT for both RAMP and BTE (P < .01). CONCLUSIONS: The 7-ms ascending ramp waveform significantly reduced delivered energy (18%) and voltage (24%) at DFT, whereas the 12-ms RAMP reduced only DFT voltage. This is the first report of a waveform that is superior to a BTE for defibrillation in humans. ULV correlates with DFT for RAMP, supporting the use of ULV testing for implantation of devices.

Determination of the upper limit of vulnerability using implantable cardioverter-defibrillator electrograms.

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 (Tpeak). We hypothesized that the recovery time (TR), 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 TR. Tpeak and TR were highly correlated (Tpeak-TR=-2+/-11 ms; rho=0.80, P<0.001). At least 1 most vulnerable interval timed between -20 ms and +20 ms relative to Tpeak in all patients and between -40 ms and +20 ms relative to TR in 96% of patients. CONCLUSIONS: The recovery time of shock electrograms provides accurate information about global repolarization. TR closely approximates Tpeak. The ULV method may be automated in an ICD by timing T-wave shocks relative to TR.