Waveform Optimization

The efficacy of different defibrillation waveforms has also been determined with the virtual electrode hypothesis. It has been widely accepted that biphasic shocks have a lower defibrillation threshold than monophasic shocks,81'82 but this phenomenon has its roots in the virtual electrode theory. Monophasic shocks must be greater than the ULV in order to avoid creation of a shock-induced phase singularity, which may reinduce reentry. However, the second phase of biphasic shocks acts to reverse the first phase polarization, thus eliminating the substrate for postshock reentry.67 This phenomenon is illustrated in Fig. 9. The three maps in Fig. 9a show the postshock polarization in response to monophasic (+100 V), optimal biphasic (+100/—50 V), and nonoptimal biphasic

Figure 9: Homogenization of virtual electrode polarization (VEP) by the second phase of biphasic shocks. (a) Maps of polarization produced by monophasic (+100 V, 8ms), optimal biphasic (+100/—50 V, 8/8ms), and nonoptimal biphasic (+100/—200 V, 8/8ms) defibrillation shocks. The area of recording is indicated by the red box. ICD, implantable cardioverter defibrillator; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle (Efimov et al. 1998)67 (b) Asymmetric reversal of first-phase polarization. Plot shows gradient of transmembrane potential after second phase of biphasic shocks in which the first phase voltage was held constant and the second phase voltage was varied. Positive polarization produced by anodal first phase was fully reversed by approximately 70 V or more, whereas negative polarization produced by cathodal first phase required only 40 V to

Figure 9: Homogenization of virtual electrode polarization (VEP) by the second phase of biphasic shocks. (a) Maps of polarization produced by monophasic (+100 V, 8ms), optimal biphasic (+100/—50 V, 8/8ms), and nonoptimal biphasic (+100/—200 V, 8/8ms) defibrillation shocks. The area of recording is indicated by the red box. ICD, implantable cardioverter defibrillator; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle (Efimov et al. 1998)67 (b) Asymmetric reversal of first-phase polarization. Plot shows gradient of transmembrane potential after second phase of biphasic shocks in which the first phase voltage was held constant and the second phase voltage was varied. Positive polarization produced by anodal first phase was fully reversed by approximately 70 V or more, whereas negative polarization produced by cathodal first phase required only 40 V to reverse

(+100/—200 V) defibrillation shocks. The optimal biphasic shock does not result in reentry due to the homogeneous pattern of VEP at shock end, whereas the large gradient of VEP produced by the monophasic and nonoptimal biphasic waveforms provides the substrate for reentry.

The "homogenization" of VEP by the second phase of a biphasic shock occurs in a nonlinear fashion. After the first phase, the deexcited hyperpolarized region is easily reexcited and completely depolarized, whereas the depolarized regions are only partially deexcited.67 Therefore, not every biphasic shock will be able to produce this homogenization (Fig. 9a, right panel). If the energy of the second phase is below a certain threshold, it will not be able to reverse the hyperpolarization. If the energy of the second phase is above a certain level, it will reverse both the positive and negative polarization, creating a mirrored VEP pattern similar to a monophasic shock. Efimov et al.67 found a ratio of between 0.2 and 0.7 of second- versus first-phase voltage for optimal biphasic shocks. This agrees with clinical observations of optimal biphasic waveforms.82 Figure 9b illustrates these findings and suggests that optimal biphasic waveforms result in total positive polarization with no excitable hyperpolarized tissue remaining to provide the substrate for shock-induced arrhythmias.

Monophasic ascending defibrillation waveforms have also been shown to be superior to descending waveforms.83 As shown in Fig. 10c, d, ascending waveforms produce maximum polarization at the end of the shock. Therefore, break excitation resulting from these shocks is likely to produce faster propagation into the deexcited regions and will not form reentry (Fig. 10f). However, descending waveforms tend to reach maximum polarization before the end of the shock (Fig. 10c) and typically have lower magnitude polarization at shock end (Fig. 10d), which contributes to slower conduction and provides the substrate for shock-induced reentry via the VEIPS mechanism (Fig. 10f).67

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