Igor R. Efimov

The Role of Electroporation in Defibrillation

Electric shock is the only effective therapy against ventricular fibrillation. However, shocks are also known to cause electroporation of cell membranes. We sought to determine the impact of electroporation on ventricular conduction and defibrillation. We optically mapped electrical activity in coronary-perfused rabbit hearts during electric shocks (50 to 500 V). Electroporation was evident from transient depolarization, reduction of action potential amplitude, and upstroke dV/dt. Electroporation was voltage dependent and significantly more pronounced at the endocardium versus the epicardium, with thresholds of 229+/-81 versus 318+/-84 V, respectively (P=0.01, n=10), both being above the defibrillation threshold of 181.3+/-45.8 V. Epicardial electroporation was localized to a small area near the electrode, whereas endocardial electroporation was observed at the bundles and trabeculas throughout the entire endocardium. Higher-resolution imaging revealed that papillary muscles (n=10) were most affected. Electroporation and conduction block thresholds in papillary muscles were 281+/-64 V and 380+/-79 V, respectively. We observed no arrhythmia in association with electroporation. Further, preconditioning with high-energy shocks prevented reinduction of fibrillation by 50-V shocks, which were otherwise proarrhythmic. Endocardial bundles are the most susceptible to electroporation and the resulting conduction impairment. Electroporation is not associated with proarrhythmic effects and is associated with a reduction of vulnerability.

Optical mapping technique applied to biventricular pacing: potential mechanisms of ventricular arrhythmias occurrence.

GARRIGUE, S., et al.: Optical Mapping Technique Applied to Biventricular Pacing: Potential Mechanisms of Ventricular Arrhythmias Occurrence. Although it has been suggested that multisite ventricular pacing alleviates heart failure by restoring ventricular electrical synchronization, the respective roles of voltage output, interventricular delay, and pacing sites in the development of ventricular arrhythmias occurrence have not been studied during biventricular pacing or LV pacing. Voltage‐sensitive dye was used in eight ischemic Langerdorff‐perfused guinea pig hearts to measure ventricular activation times and examine conduction patterns during multisite pacing from three RV and four LV sites. The hearts were stained with di‐4‐ANEPPS and mapped with a 16 × 16 photodiode array at a resolution of 625 μm per diode. Isochronal maps of RV and LV activation were plotted. Ischemia was produced by gradually halving the perfusion output over 5 minutes. Pacing the RV apex and the base of the LV anterior wall was associated with the most homogeneous and rapid activation pattern ( 28 ± 9 vs 41 ± 12 ms with the other configurations, P < 0.01), and no inducible arrhythmia. In six hearts, ventricular tachycardia could be induced when pacing from the right and left free walls with 20 ms of interventricular delay, at six times the pacing threshold output. In four hearts, simultaneous RV and LV pacing at high voltage output induced ventricular fibrillation with complex three‐dimensional propagation patterns, independently of the pacing sites. During biventricular pacing with ischemia, pacing at high voltage output with a long interventricular delay is likely to induce ventricular arrhythmias, particularly when left and right pacing results in a conduction pattern orthogonal to the ventricular myocardial fibers orientation. PACE 2003; 26[Pt. II]:197–205)

Effects of Lidocaine on Shock-Induced Vulnerability

Introduction: Lidocaine is known to increase the defibrillation threshold (DFT) of monophasic shocks (MS) and have no effect on DFT of biphasic shocks (BS). The aim of this study was to enhance our understanding of the mechanisms of vulnerability and defibrillation through the investigation of this difference.

Imaging of the atrioventricular node using optical coherence tomography

Dual-pathway theory of AV nodal (AVN) conduction recently was supported by a morphologic description of the rightward and leftward posterior nodal extensions (PNE) by Inoue and Becker (Circulation 1998) and their electrophysiologic characterization by Medkour et al. (Circulation 1998). Using x8f uorescent imaging, we recently mapped excitation during AVN reentry (Nikolski and Ex8e mov, Circ Res. 2001). Three-dimensional reentrant pathways include deep structures of AVN and PNE and a surface layer of atrial/atrial-nodal transitional cells (A/AN). The rightward and leftward extensions provide anatomic substrates for the fast and slow pathways (FP and SP), respectively. The goal of this study was to delineate x8f uorescent functional data obtained during AVN reentry with structural data obtained using optical coherence tomography (OCT) imaging in vitro. OCT is an emerging noninvasive diagnostic technology that uses back-rex8f ected infrared light (1,310 nm) to perform cross-sectional, depth-resolved imaging similar to ultrasound, yet with micron-scale resolution (Schmitt, JSTQE 1999). We used a current-generation real-time OCT system similar to one described previously, but imaging at 8 frames/sec (Rollins et al., Opt Express 1998). OCT was able to visualize the complex 3D morphology of the AVN and PNE supporting a multilayer conduction pattern in the triangle of Koch (n 3) revealed by x8f uorescent imaging. Left panel shows an image of triangle of Koch with SP exit point and isochronal activation wavefronts as determined by x8f uorescent imaging. Right panel shows OCT images obtained at the SP and AVN sites (dashed black lines in the left panel). Upper panel of the OCT image shows textural content of the endocardial A/AN layer (bright band) and the underlying interwoven compact AV node (N). These two layers supported two distinct wavefronts during normal conduction and reentry. Lower panel shows dissociation between PNE and endocardial A/AN transitional layer. SP exit (left panel) was detected at the site with no apparent dissociation between cell layers (bottom OCT image). OCT provides images of internal structures of the triangle of Koch up to the depth of 1–2 mm. We suggest that a transvenous catheter-based OCT probe can be useful for guiding an RF probe during SP ablation procedure. A combined OCT-RF transvenous catheter appears feasible. Such a catheter could be used in RF ablation treatment of WPW syndrome, AF, and VT originating in the infarction border zone. This study was supported by NSF Grant 9872829 to Dr. Izatt and Grant RO1-HL58808 to Dr. Ex8e mov.

Anode-break excitation during end-diastolic stimulation is explained by half-cell double layer discharge

The phenomenon of anodal-break excitation during end-diastolic stimulation of the heart was discovered many years ago by B. Hoffman. Yet, the existence and mechanistic explanation of this effect remain controversial. We sought to confirm its existence and to determine a possible role of half-cell potential. We used isolated Langendorff-perfused rabbit hearts (n = 6) which were stained with di-4-ANEPPS and perfused with 15-mM butanedione monoxime (BDM). Transmembrane potentials were optically recorded at the left ventricular epicardium with a high spatial and temporal resolution (200 /spl mu/m/343 /spl mu/s) near the tip of a 120-/spl mu/m platinum-iridium Teflon-coated unipolar pacing electrode to detect virtual electrode polarization and to reconstruct an activation pattern. Hearts were paced at a cycle length of 300 ms by anodal square pulses with an amplitude of 0.1-10 mA and a duration of 5-60 ms. Data revealed that the anodal-break excitation does exists and is accompanied by an overshoot in the recordings of the pacing current. Addition of a diode in the stimulation circuit eliminated both the overshoot and the break excitation. The findings suggest that a half-cell surface potential at the pacing electrode metal-saline interface may influence the pacing currents during unipolar anodal cardiac stimulation providing break-like activation. We also confirmed that the threshold of break-like excitation is lower than make-excitation. We suggest that further exploration of this effect is needed in order to design improved multiphasic pacing waveforms.

Optical Coherence Tomography Imaging of the Purkinje Network

Optical imaging of the conduction system with fluorescent potentiometric probes has significantly advanced our undersigning of impulse initiation in the sino-atrial node,1 conduction through the atrioventricular node,2 and synchronization of contraction by the Purkinje system.3 However, further advancement of our understanding of molecular and cellular mechanisms of cardiac conduction is hampered by the limited experimental ability of existing imaging techniques to correlate functional and structural information in the complex three-dimensional structure of the conduction system of the heart. We suggest that the combination of fluorescent potentiometric imaging with optical coherence tomography (OCT) can potentially provide a unique opportunity for structurefunction studies at the cellular and tissue levels. In this study, we investigated the possibility of OCT imaging the threedimensional (3D) structure of the Purkinje network. The figure shows that OCT provides high-resolution 3D images of the complex morphology of the Purkinje network. OCT is a promising new tool for 3D structure-function imaging of the Purkinje network. OCT is an emerging non-invasive diagnostic technology that uses back-reflected infrared light (1,310 nm) to perform 3D imaging at micron-scale resolution.4 The technology is based on low-coherence Michelson interferometry. Light back-scattered from the sample interferes with reference light, allowing us to detect light reflected from a specific depth determined by the reference light path length. By scan-

Fibrillation or Neurillation: Back to the Future in Our Concepts of Sudden Cardiac Death?

“ If the heart trembles, has little power and sinks, the disease is advancing … and death is near … .” —The Papyrus Ebers (circa 3500 BCE) nnNumerous concepts of cardiac electrophysiology have been advanced, enthroned, and then laid to rest. Other theories have withstood the test of time despite the restless energy of inquisitive doubt of future generations. Until recently, one such concept has been the foundation of the mechanisms of fibrillation, which was imprinted in the very name to emphasize its fibrillar or myogenic nature.nnIt was apparently known to ancient Egyptians and Chinese that an irregular heartbeat is associated with death. However, scientifically rigorous description of a causal relationship was presented only in the middle of the 19th century. Erichsen described in 1842 that coronary artery ligation led to “tumultuous,” “tremulous,” and “irregular” behavior of the ventricles.1 First documentation of the onset of ventricular fibrillation (VF) during electrical stimulation was recorded in 1849 using Ludwig’s “kymographion” by his associate Hoffa.2 Interestingly, at the time, Hoffa was assigned to investigate autonomic nervous system effects on cardiac activity, which had been discovered a year earlier by Ludwig himself.3 Hoffa described irregular contractions induced by “faradization” (electrical stimulation), which persisted even after the termination of electrical stimulation and resulted in cardiac arrest that could not be checked by vagal stimulation.nnIntensive investigation of the newly described phenomenon led to the introduction of numerous terms, which aimed to capture the mechanistic and/or anatomic nature of the irregular contractions and resulting cardiac arrest.4 The main disagreement gravitated toward one question: is the phenomenon neurogenic or myogenic in nature? In other words, is irregular activity due to abnormal …

Diastolic Shocking Experience

Despite efforts of several generations of clinical and basic researchers, mechanisms of defibrillation remain highly controversial. The original theory of defibrillation, proposed by Prevost and Betelli,1 was based on their observations of complete secession of cardiac electrical and mechanical activity following strong defibrillation shock. They proposed the socalled “incapacitation” theory, which suggested that defibrillation is achieved through a complete electromechanical incapacitation of myocardium by electric shock. Incapacitation of the myocardium stops any electrical activity, including fibrillation. However, it also will prevent resumption of normal sinus rhythm immediately following defibrillation. Therefore, direct heart massage was suggested to sustain circulation until the defibrillated heart recovered from incapacitation and was capable of regaining normal excitation-contraction coupling. In particular, this method was used by Beck et al.2 during their pioneering clinical defibrillation work. Introduction of capacitor discharge monophasic and biphasic defibrillation waveforms allowed reduction of myocardial injury and resulting incapacitation, while achieving successful defibrillation.3,4 These advances allowed Gurvich5 to introduce his “excitatory” theory of defibrillation, which suggested that fibrillation is extinguished by direct excitation of myocardium, thus preventing propagation of fibrillatory wavefronts without preventing resumption of normal sinus rhythm. This theory was extended to the socalled “critical mass of fibrillation” theory.6,7 Experimental evidence presented by Kwaku and Dillon8 provided direct support to this theory. However, the work of Kwaku and Dillon was carried out in intact Langendorff-perfused rabbit hearts, by epicardial imaging of defibrillation with voltagesensitive dye. Therefore, the question remained as to whether or not these results could be extended to mid-myocardium. Classic cable theory suggests that the effect of externally applied fields is limited by the space constant and confined to surfaces of myocardium,9 which are responsible for polarization produced by “virtual electrodes”10 or “secondary sources.”11 However, the recent introduction of the bidomain model suggested that tissue could be polarized at tissue heterogeneities, such as fiber curvature, rotation.12 Thus, theories offer controversial predictions regarding tissue bulk response. Experimental testing is required. The article by Sharifov and Fast13 in this issue of the Journal presents evidence supporting deep tissue penetration dur-

Effects of electroporation on cellular responses to high-intensity electrical shocks

The response of cardiac cells to the external electrical field is a crucial factor contributing to the success and failure of defibrillation. We aimed to determine a possible contribution of electroporation to optically recorded response during high-intensity shocks. We applied 10 ms anodal and cathodal stimuli of different intensities to the rabbit heart epicardium during plateau phase of the action potential with 6-mm-diameter electrode. Transmembrane potential changes were monitored directly under the electrode by an optical technique using voltage-sensitive dye. As stimulus current was increased from 0 to 160 mA/cm/sup 2/, the shock-induced transmembrane potential changes (AV,) also increased while the resting potential after the stimulation remains constant. At higher current densities (> 160 mA/cm/sup 2/) effects of electroporation were observed: (a) after the initial rapid change at the stimulus onset AV. quickly declined resulting in gradual decreasing of the final /spl Delta/Vm with increasing of the stimulus strength; (b) post-repolarization elevation of resting potential gradually increased as the stimulus strength increased. These results demonstrate that during strong electrical shocks electroporation changes in transmembrane potential traces are present for hyperpolarized as well as depolarized stimuli of a similar strength. This can explain the multiphasic nature of cellular responses reported for strong electric stimuli.

Virtual Electrodes in Virtual Reality of Defibrillation

Shock-induced arrhythmia was discovered by Hoffa and Ludwig 150 years ago. This literally shocking discovery stimulated many generations of physiologists to study the effects of “faradization” on the heart, including such giants as Vulpian,2 who coined the term x8e brillation; MacWilliam,3 who was the x8e rst to recognize the clinical signix8e cance of ventricular x8e brillation; Prevost and Battelli,4 who discovered dex8e brillation; and Wiggers,5 whose mentorship and research not only shaped American cardiac physiology in most of the 20th century but also led to ground-breaking clinical success of dex8e brillation achieved by his colleagues Beck, Pritchard, and Feil at Western Reserve University. Dex8e brillation was discovered in 1899. However, unlike the discovery of the ECG, which celebrates its centenary this year, dex8e brillation did not enjoy the same immediate fame, acceptance, and widespread application. Mechanisms of dex8e brillation remain subjects of intense investigation in both basic and clinical electrophysiology (EP) laboratories. Evolution of our understanding of dex8e brillation and vulnerability was shaped by tortuous paths of discovery spanning many countries and generations of the turbulent 20th century. Prevost and Battelli discovered that an increase in the intensity of stimulation of the myocardium leads to cessation of electrical activity. They demonstrated that this incapacitation of the myocardium could be used to arrest ventricular x8e brillation, which led them to postulate that dex8e brillation is achieved through incapacitation of the myocardium. This theory dominated the x8e eld until Schtern, a former trainee of Prevost, assigned her graduate student Gurvich to continue work on dex8e brillation. Interestingly, this decision saved her life when aging dictator Joseph Stalin, in his anti-Semitic paranoia, ordered persecution and execution of many doctors and scientists, but he spared her life because he believed that she knew the secret of bringing people back from the dead. Gurvich worked on the assignment his entire life and made numerous important discoveries. In particular, he suggested that dex8e brillation can be achieved not only through incapacitation of myocardium, as was postulated by his teachers, but also via direct stimulation of myocardium.7 A few years later, Wiener and Rosenblueth presented a hypothetical mechanism that explained vulnerability by stimulus-induced reentry mechanism. This hypothesis later was reformulated as stimulus-induced phase singularity or critical point.1 0 However, experimental and theoretical investigations of basic mechanisms of dex8e brillation were hampered by two difx8e culties: (1) the lack of experimental techniques capable of measuring electrical activity during stimulation without overwhelming shock-induced artifact; and (2) the lack of an adequate theoretical framework that does not contradict empirically known facts. These experimental limitations restricted methods of investigation to empirical and black box approaches, because it was impossible to see what actually happens during a few critically important milliseconds of shock application and what happens immediately thereafter. The recent introduction into dex8e brillation research of x8f uorescent imaging with voltage-sensitive dyes and the bidomain model x8e nally delivered required methodologies. Used together, these two methodologies resolved several century-old puzzles, such as the mechanisms of pacing,1 3 x8e eld stimulation,1 4 and shock-induced vulnerability.1 5 Voltage-sensitive dye imaging recently has allowed the direct imaging of shock-induced reentry and dex8e brillation.1 6 ,1 9 These experiments conx8e rmed the original prediction of Wiener and Rosenblueth that vulnerability is due to stimulus-induced reentry, but via a different mechanism, which we termed virtual electrode-induced phase singularity.1 6 Imaging not only revealed that the stimulus depolarizes tissue directly stimulating excitable gaps or extending refractoriness elsewhere,2 0 but it also presented evidence of de-excitation,16 or stimulus-induced recovery of excitability in refractory myocardium, which creates excitable gaps for postshock arrhythmia. Despite impressive progress in imaging, we are still awaiting a truly three-dimensional mapping system that maps electrical activity from the entire heart during shocks. Only such complete information can bridge the numerous basic EP laboratory discoveries on the mechanisms of arrhythmogenesis and dex8e brillation with the wealth of clinical EP empirical x8e ndings. The bidomain model approach, reported by Eason and Trayanova in this issue of the Journal, offers a unique opportunity to bridge the complexities of preshock ventricular arrhythmia, shock-induced virtual electrode polarization, and postshock phase singularities with the statistical nature of clinical x8e brillation and dex8e brillation. This is just the x8e rst step, because the model lacks accurate ionic characteristics of ventricular cells and a profoundly complex three-dimensional x8e ber structure of the heart. Despite its simplicity and two-dimensionality, the model shows for the x8e rst time the direct relationship between deterministic mechanisms of arrhythmogenesis via a virtual electrode-induced phase singularity mechanism and the arguably stochastic nature of x8e brillation and dex8e brillation, which rex8f ects interaction of the shock with preshock phase singularities and survival of postshock phase singularities. Eason and Trayanova have opened a new and exciting avenue of exploration of the shock-induced phenomena in the heart that will unfold in the near future. These future studies will unravel: (1) the relationship between dex8e brillation waveforms and ion channel kinetics under normal and pathophysiologicconditions; (2) the physiologic correlation J Cardiovasc Electrophysiol, Vol. 13, pp. 680-681, July 2002.