Therapy Efficacy Cardioversion

The use of cardioversion therapy, where the shock is synchronized to the depolarization of the ventricles, to terminate ventricular tachycardia provides efficacy similar to defibrillation therapy.33 There are notable differences in the factors affecting efficacy. During fibrillation or polymorphic VT (PVT) the preshock state of the heart is more complex than in tachycardia. This deemphasizes the role that synchronization of shock delivery plays in defibrillation efficacy and increases the amount of energy required for effective therapy. However, for monomorphic VT, the state of the heart is simpler, and timing effects influence both the necessary energy to terminate the rhythm and the incidence of unfavorable shock failures such as acceleration or disruption of the VT to VF (shown in Fig. 4).34'35 Shocks delivered late in relation to the onset of the surface ECG R wave have a better efficacy and lower acceleration rate than those delivered early.36 This pronounced role for timing may explain the inability to terminate VT with full-output cardioversion observed in some cases. Cardioversion shock delivery by ICDs is synchronized to an intracardiac bipolar EGM. This will have a variable relationship to ECG-based shock timing and will not be the optimal time for all VTs.

Shock therapy in the ambulatory setting, where patient circumstances are uncontrollable, can have other unexpected results. Energy-dependent effects have also included a situation where higher energy shocks (24 J) would consistently convert a fast VT to a slower VT, but treating the fast rhythm with a 3 J shock reproducibly converted to sinus rhythm.37 Additionally, high-energy shocks will sometimes result in either a brief self-terminating tachycardia or a new sustained ventricular tachycardia.38-40

Figure 4: Patient death recorded by implantable cardioverter-defibrillator (ICD) diagnostics. The top signal is the atrial near-field electrogram (EGM). Atrial intervals for intrinsic events ("AS" or "AR") and paces "AP" are above the marker line. The second signal is the shock coil to ICD can EGM. Ventricular intervals are measured from the rate-sensing bipolar EGM (not shown) and are below the marker line. Markers ending in "P" are pace events, while the remainder are intrinsic events except for "CE" for end of charging, and "CD" for charge delivered (i.e., a shock). A number of individual episodes were recorded by the device, which demonstrates continued reinitiation of the tachyarrhythmia. In this series, a cardioversion therapy accelerates ventricular tachycardia (VT) to ventricular fibrillation (VF) (a). The VF was successfully terminated, but arrhythmias continued to reinitiate after successful therapies. In (b) a spontaneous VF is converted but resumes after a few pacing cycles. The recording ends, as the EGM amplitude is too small to be detected by the device.

Figure 4: Patient death recorded by implantable cardioverter-defibrillator (ICD) diagnostics. The top signal is the atrial near-field electrogram (EGM). Atrial intervals for intrinsic events ("AS" or "AR") and paces "AP" are above the marker line. The second signal is the shock coil to ICD can EGM. Ventricular intervals are measured from the rate-sensing bipolar EGM (not shown) and are below the marker line. Markers ending in "P" are pace events, while the remainder are intrinsic events except for "CE" for end of charging, and "CD" for charge delivered (i.e., a shock). A number of individual episodes were recorded by the device, which demonstrates continued reinitiation of the tachyarrhythmia. In this series, a cardioversion therapy accelerates ventricular tachycardia (VT) to ventricular fibrillation (VF) (a). The VF was successfully terminated, but arrhythmias continued to reinitiate after successful therapies. In (b) a spontaneous VF is converted but resumes after a few pacing cycles. The recording ends, as the EGM amplitude is too small to be detected by the device.

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