Therapy Efficacy Defibrillation

The defibrillation shocks delivered by ICDs were optimized through acute experiments, while diagnostics had an insignificant role. The most important improvement in ICD therapy was the development of the biphasic defibrillation shock waveform. Biphasic waveforms reduced the energy requirements for successful defibrillation, allowing pectoral implant of the ICD and the use of transvenous shocking electrodes. This was a vast improvement over abdominally implanted ICDs with epicardial patch electrodes. ICD diagnostics confirmed the system's defibrillation efficacy during ambulatory use.

The first pectoral-endocardial ICD system delivered successful defibrillation therapy in the ambulatory setting for 99% of the detected spontaneous fibrillation episodes.27 The high rate of successful defibrillation has been maintained across the different patient populations, including the typical ICD patient population, comprising both primary and secondary prevention patients.28 Although the overall efficacy of defibrillation by ICDs is high, reviewing ICD recordings shows that the ability to provide multiple shocks in an episode of VF is of vital importance for overall defibrillation efficacy. For example, in the PainFREE Rx II trial the first defibrillation shock was set to 10 J more than the defibrillation threshold measured at implant. This shock energy succeeded in 87% of the first shocks for VF.29 The subsequent shocks were at the full output capability of the device; an example of a multiple-shock episode is shown in Fig. 3. None of the episodes where the first shock failed required more than three additional shocks to convert. The overall shock efficacy was 100%. The most recent shock performance reporting is from the Low Energy Safety Study (LESS).28 The full-output capability of the device resulted in a first shock defibrillation success rate of 97%. This was compared to programming the first shock of the device to a rigorous defibrillation threshold measurement (DFT++). DFT++ also resulted in 97% first shock success. In LESS, the overall efficacy of defibrillation, including multiple attempts, was 100%.

In addition to verifying programming strategies and corroborating acute study results, ICD diagnostics motivated important improvements for automatic shock delivery algorithms. Initial ICD designs were committed to shock delivery once VF was identified because of concerns that EGM amplitude during VF might become too small for accurate device detection. However, diagnostic data showed high rates of spontaneous termination of detected episodes during the capacitor-charging phase of therapy delivery.30 EGM amplitude was also robust enough for accurate device rhythm classification during extended durations of VF. This combination of factors has lead to extending the amount of time required for tachycardia detection before classifying a rhythm and changing shock delivery algorithms to reconfirm the presence of the tachyarrhythmia after charging.

Figure 3: Example of multiple shocks to convert spontaneous ventricular fibrillation (VF). The three sections proceed from top to bottom.. The upper electrogram (EGM) trace is measured from the shocking coil to the implantable cardioverter-defibrillator (ICD) can; the lower trace is the near-field sensing EGM. The intervals measured by the device from the Vtip-Vring EGM are given in milliseconds. "CD" markers indicate shock delivery with delivered energy measured by the device below the marker. The successful second shock was at the 30 J full-output capability of the device

Figure 3: Example of multiple shocks to convert spontaneous ventricular fibrillation (VF). The three sections proceed from top to bottom.. The upper electrogram (EGM) trace is measured from the shocking coil to the implantable cardioverter-defibrillator (ICD) can; the lower trace is the near-field sensing EGM. The intervals measured by the device from the Vtip-Vring EGM are given in milliseconds. "CD" markers indicate shock delivery with delivered energy measured by the device below the marker. The successful second shock was at the 30 J full-output capability of the device

When it comes to the survival of patients with ICDs, its high efficacy makes it difficult for any single study to reflect aspects of ICD therapy needing improvement. An analysis by Mitchell et al.31 used patient data from a number of trials over a 5-year period to determine why some ICD patients still suffer sudden cardiac death. There were 90 deaths categorized as "sudden cardiac," of which the cause could be assigned for 68. The majority (81%) of these sudden deaths began with VT or VF. The reasons for therapy failure were a mix of technical failures or clinical conditions. Technical failures were VT or VF falling into the therapeutic programming of the device but with no device response or persistence of VT/VF after a therapy delivery that was not recognized by the device. The clinical scenarios included exhaustion of the maximum number of shocks without terminating VT/VF or incessant VT/VF that was appropriately detected and effectively treated by the ICD but immediately returned to arrhythmia (Fig. 4 is an example). A unique clinical failure is post-shock electromechanical dissociation (EMD), in which the device restores a normal electrical rhythm but not sufficient mechanical function. The dominant mode of sudden cardiac death was post-shock EMD (20 cases), followed by the inability to terminate the arrhythmia (17 cases), and incessantly recurring VT/VF (9 cases). It may be just as important for future device therapies to avoid or correct post-shock EMD as it is for improved shock efficacy when looking to increase the ability of devices to reduce sudden cardiac death.32

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