Abstract-During failure of the sinoatrial node, the heart can be driven by an atrioventricular (AV) junctional pacemaker.The position of the leading pacemaker site during AV junctional rhythm is debated. In this study, we present evidence from high-resolution fluorescent imaging of electrical activity in rabbit isolated atrioventricular node (AVN) preparations that, in the majority of cases (11 out of 14), the AV junctional rhythm originates in the region extending from the AVN toward the coronary sinus along the tricuspid valve (posterior nodal extension, PNE). Histological and immunohistochemical investigation showed that the PNE has the same morphology and unique pattern of expression of neurofilament160 (NF160) and connexins (Cx40, Cx43, and Cx45) as the AVN itself. Block of the pacemaker current, I f , by 2 mmol/L Cs ϩ increased the AV junctional rhythm cycle length from 611Ϯ84 to 949Ϯ120 ms (meanϮSD, nϭ6, PϽ0.001). Immunohistochemical investigation showed that the principal I f channel protein, HCN4, is abundant in the PNE. As well as the AV junctional rhythm, the PNE described in this study may also be involved in the slow pathway of conduction into the AVN as well as AVN reentry, and the predominant lack of expression of Cx43 as well as the presence of Cx45 in the PNE shown could help explain its slow conduction. Key Words: ablation Ⅲ electrophysiology Ⅲ surgery Ⅲ arrhythmia Ⅲ imaging S ince Tawara's discovery of the atrioventricular node (AVN) nearly a century ago, 1 anatomists and electrophysiologists have established that the AVN is the only conduction pathway between the atria and ventricles in the normal heart. 2 The AVN has unique slow and frequencydependent conduction properties. 2 Under normal physiological conditions, the AVN determines the appropriate frequency-dependent delay of conduction between the atria and ventricles and, during atrial fibrillation, the AVN filters high-frequency excitation, thus protecting the ventricular myocardium. 3 The AVN has dual inputs (fast and slow pathways) from the atrial myocardium and this may be the substrate for AVN reentry. 4,5 The AVN also has pacemaking ability: during failure of the sinoatrial node, the heart can be driven by an atrioventricular (AV) junctional pacemaker, although the position of the leading pacemaker site is debated. 6,7 Recent application of fluorescent imaging with voltagesensitive dyes 5,8 -12 has provided new insights into the electrophysiology of the AV junction. With fluorescent imaging, we have recently shown how the fast and slow pathways of conduction support normal conduction, 9 AVN echo, 5 and AVN reentry. 12 Application of immunohistochemical imaging has shown that the expression of ion channels 13 and gap junction channel isoforms 14,15 can explain the electrophysiology of the AVN. In particular, a lack of or a low density of Na ϩ channels in the compact node (CN) can explain the slow upstroke and low amplitude of the action potential in CN. 13 Similarly, in CN, a lack of or a low density of low impedance isoforms of gap...

The outcome of defibrillation shocks is determined by the nonlinear transmembrane potential (DeltaVm) response induced by a strong external electrical field in cardiac cells. We investigated the contribution of electroporation to DeltaVm transients during high-intensity shocks using optical mapping. Rectangular and ramp stimuli (10-20 ms) of different polarities and intensities were applied to the rabbit heart epicardium during the plateau phase of the action potential (AP). DeltaVm were optically recorded under a custom 6-mm-diameter electrode using a voltage-sensitive dye. A gradual increase of cathodal and well as anodal stimulus strength was associated with 1) saturation and subsequent reduction of DeltaVm; 2) postshock diastolic resting potential (RP) elevation; and 3) postshock AP amplitude (APA) reduction. Weak stimuli induced a monotonic DeltaVm response and did not affect the RP level. Strong shocks produced a nonmonotonic DeltaVm response and caused RP elevation and a reduction of postshock APA. The maximum positive and maximum negative DeltaVm were recorded at 170 +/- 20 mA/cm2 for cathodal stimuli and at 240 +/- 30 mA/cm2 for anodal stimuli, respectively (means +/- SE, n = 8, P = 0.003). RP elevation reached 10% of APA at a stimulus strength of 320 +/- 40 mA/cm2 for both polarities. Strong ramp stimuli (20 ms, 600 mA/cm2) induced a nonmonotonic DeltaVm response, reaching the same largest positive and negative values as for rectangular shocks. The transition from monotonic to nonmonotonic morphology correlates with RP elevation and APA reduction, which is consistent with cell membrane electroporation. Strong shocks resulted in propidium iodide uptake, suggesting sarcolemma electroporation. In conclusion, electroporation is a likely explanation of the saturation and nonmonotonic nature of cellular responses reported for strong electric stimuli.

The virtual electrode polarization (VEP) effect is believed to play a key role in electrical stimulation of heart muscle. However, under certain conditions, including clinically, its existence and importance remain unknown. We investigated the influence of acute tissue damage produced by continuous pacing with strong current (40-mA, 4-ms biphasic pulses with 4-Hz frequency for 5 min) on stimulus-generated VEPs and pacing thresholds. A fluorescent optical mapping technique was used to obtain stimulus-induced transmembrane potential distribution around a pacing electrode applied to the ventricular surface of a Langendorff-perfused rabbit heart ( n = 5). Maps and pacing thresholds were recorded before and after tissue damage. Spatial extents of electroporation and cell uncoupling were assessed by propidium iodide ( n = 2) and connexin43 ( n = 3) antibody staining, respectively. On the basis of these data, passive and active three-dimensional bidomain models were built to determine VEP patterns and thresholds for different-sized areas of the damaged region. Electrophysiological results showed that acute tissue damage led to disappearance of the VEP with an associated significant increase in pacing thresholds. Damage was expressed in electroporation and cell uncoupling within a ∼1.0-mm-diameter area around the tip of the electrode. According to computer simulations, cell uncoupling, rather than electroporation, might be the direct cause of VEP elimination and threshold increase, which was nonlinearly dependent on the size of the damaged region. Fiber rotation with depth did not substantially affect the numerical results. The study explains failure to stimulate damaged tissue within the concepts of the VEP theory.

Nikolski, Vladimir P., Aleksandre T. Sambelashvili, and Igor R. Efimov. Mechanisms of make and break excitation revisited: paradoxical break excitation during diastolic stimulation. Am J Physiol Heart Circ Physiol 282: H565-H575, 2002; 10.1152/ajpheart.00544.2001. Onset and termination of electric stimulation may result in "make" and "break" excitation of the heart tissue. Wikswo et al. (30) explained both types of stimulations by virtual electrode polarization. Make excitation propagates from depolarized regions (virtual cathodes). Break excitation propagates from hyperpolarized regions (virtual anodes). However, these studies were limited to strong stimulus intensities. We examined excitation during weak near-threshold diastolic stimulation. We optically mapped electrical activity from a 4 ϫ 4-mm area of epicardium of Langendorff-perfused rabbit hearts (n ϭ 12) around the pacing electrode in the presence (n ϭ 12) and absence (n ϭ 2) of 15 mM 2,3-butanedione monoxime. Anodal and cathodal 2-ms stimuli of various intensities were applied. We imaged an excitation wavefront with 528-s resolution. We found that strong stimuli (ϫ5 threshold) result in make excitation, starting from the virtual cathodes. In contrast, near-threshold stimulation resulted in break excitation, originating from the virtual anodes. Characteristic biphasic upstrokes in the virtual cathode area were observed. Break and make excitation represent two extreme cases of near-threshold and far-above-threshold stimulations, respectively. Both mechanisms are likely to contribute during intermediate clinically relevant strengths. optical mapping; virtual electrode; pacing THE ABILITY OF ELECTRIC POINT STIMULATION to produce a response in excitable tissues (11, 28) and induce (12) or terminate (17) arrhythmia in the heart is well known. However, the exact mechanisms of electric stimulation have been obscure until the recent discovery of virtual electrode polarization (VEP) produced by point stimulation (13,15,24,30,31). It results in a characteristic "dogbone" pattern of positive and negative polarizations. These polarizations of opposite sign are thought to be induced by a so-called virtual cathode and virtual anode. These virtual electrodes represent the driving force, which can be mathematically expressed as an activating function (19,27), which is also referred to as secondary sources (10). The activating function is governed by two major parameters: the gradient of extracellular electric field and structural heterogeneity of the heart, contributing to the polarization of the cellular membrane during stimulus. Active ionic properties of the heart, particularly calcium channels, modulate these polarizations (2,18,20,22).Dekker (4) demonstrated that both the onset (make) and termination (break) of stimulation of appropriate intensity and duration could produce a propagated response. Roth (20) and Wikswo et al. (30) provided the first mechanistic explanation of the "make" and "break" stimulation based on the VEP phenomenon.According to their theory...