Mechanisms of superiority of ascending ramp waveforms: new insights into mechanisms of shock-induced vulnerability and defibrillation

Abstract

. Mechanisms of superiority of ascending ramp waveforms: new insights into mechanisms of shock-induced vulnerability and defibrillation. Am J Physiol Heart Circ Physiol 289: H569 -H577, 2005. First published March 25, 2005; doi:10.1152/ajpheart.01117.2004.-Monophasic ascending ramp (AR) and descending ramp (DR) waveforms are known to have significantly different defibrillation thresholds. We hypothesized that this difference arises due to differences in mechanisms of arrhythmia induction for the two waveforms. Rabbit hearts (n ϭ 10) were Langendorff perfused, and AR and DR waveforms (7, 20, and 40 ms) were randomly delivered from two line electrodes placed 10 mm apart on the anterior ventricular epicardium. We optically mapped cellular responses to shocks of various strengths (5, 10, and 20 V/cm) and coupling intervals (CIs; 120, 180, and 300 ms). Optical mapping revealed that maximum virtual electrode polarization (VEP) was reached at significantly different times for AR and DR of the same duration (P Ͻ 0.05) for all tested CIs. As a result, VEP for AR were stronger than for DR at the end of the shock. Postshock break excitation resulting from AR generated faster propagation and typically could not form reentry. In contrast, partially dissipated VEP resulting from DR generated slower propagation; the wavefront was able to propagate into deexcited tissue and thus formed a shock-induced reentry circuit. Therefore, for the same delivered energy, AR was less proarrhythmic compared with DR. An active bidomain model was used to confirm the electrophysiological results. The VEP hypothesis explains differences in vulnerability associated with monophasic AR and DR waveforms and, by extension, the superior defibrillation efficacy of the AR waveform compared with the DR waveform.

Characteristic Energy Method for Evaluation of Monopolar Defibrillation Pulse Efficiency

“…At optimal pulse time (4.6 msec) the characteristic energy of the triangular descending pulse is 10.48 J (1.6 times high er than in triangular ascending pulse). This conclusion is consistent with experimental data [2,3].…”

“…It follows from this model that trian gular ascending pulses and triangular descending pulses of the same pulse time should have the same threshold amplitude of defibrillation current and the same thresh old defibrillation energy. However, it was shown experi mentally that a triangular ascending pulse required lower defibrillation energy (lower current amplitude) compared to a triangular descending pulse [2,3].The characteristic energy method suggested in this work provides theoretical assessment of monopolar defib rillation pulse efficiency. This method allows pulses of dif ferent shape to be tested and optimal defibrillation pulse time to be determined for elucidation of this difference.…”

“…It follows from this model that trian gular ascending pulses and triangular descending pulses of the same pulse time should have the same threshold amplitude of defibrillation current and the same thresh old defibrillation energy. However, it was shown experi mentally that a triangular ascending pulse required lower defibrillation energy (lower current amplitude) compared to a triangular descending pulse [2,3].…”

Cardiac optogenetics

“…The importance of the waveform of the stimulating pulse both for pacing and defibrillation has been long recognized in the cardiac literature (42,66,106,107,111,127). More specifically, energy minimization has been pursued via waveform optimization (106), and it is known that a typical rectangular monophasic pulse, for example, does not provide the most efficient stimulus (66). It can be speculated that the conceptually different optogenetic mode of stimulation (Fig.…”

The effect of pinacidil on postshock activation and ventricular defibrillation threshold in canine hearts

“…The earliest postshock focal activations might be caused by microreentry [9] , triggered activity [10] , or tunnel propaga tion [6,8] . For a strong electrical shock to reinitiate reentry, the shock should create a uni-directional block and slow conduction area, an area that usually forms near the location of the shock electrodes [11,12] . Thus, reentry is less likely to be the main mechanism for a postshock earliest activation that arises far away from the shock electrodes.…”