Virtual electrode effects in myocardial fibers (original) (raw)
Related papers
Virtual electrodes in cardiac tissue: a common mechanism for anodal and cathodal stimulation
Biophysical Journal, 1995
Traditional cable analyses cannot explain complex patterns of excitation in cardiac tissue with unipolar, extracellular anodal, or cathodal stimuli. Epifluorescence imaging of the transmembrane potential during and after stimulation of both refractory and excitable tissue shows distinctive regions of simultaneous depolarization and hyperpolarization during stimulation that act as virtual cathodes and anodes. The results confirm bidomain model predictions that the onset (make) of a stimulus induces propagation from the virtual cathode, whereas stimulus termination (break) induces it from the virtual anode. In make stimulation, the virtual anode can delay activation of the underlying tissue, whereas in break stimulation this occurs under the virtual cathode. Thus make and break stimulations in cardiac tissue have a common mechanism that is the result of differences in the electrical anisotropy of the intracellular and extracellular spaces and provides clear proof of the validity of the bidomain model.
Biophysical Journal, 1995
Recent theoretical models of cardiac electrical stimulation or defibrillation predict a complex spatial pattern of transmembrane potential (vm) around a stimulating electrode, resulting from the formation of virtual electrodes of reversed polarity. The pattern of membrane polarization has been attributed to the anisotropic structure of the tissue. To verify such model predictions experimentally, an optical technique using a fluorescent voltage-sensitive dye was used to map the spatial distribution of vm around a 1 50-pm-radius extracellular unipolar electrode. An S1 -S2 stimulation protocol was used, and vm was measured during an S2 pulse having an intensity equal to 1 Ox the cathodal diastolic threshold of excitation. The recordings were obtained on the endocardial surface of bullfrog atrium in directions parallel and perpendicular to the cardiac fibers. In the longitudinal fiber direction, the membrane depolarized for cathodal pulses (and hyperpolarized for anodal pulses) but only in a region within 445 + 1 12 pm (and 616 ± 78 pm for anodal pulses) from the center of the electrode (n = 9). Outside this region, Vm reversed polarity and reached a local maximum at 922 ± 136 pm (and 988 ± 117 pm for anodal pulses) (n = 9). Beyond this point vm decayed to zero over a distance of 1.5-2 mm. In the transverse fiber direction, the membrane depolarized for cathodal pulses (and hyperpolarized for anodal pulses) at all distances from the electrode. The amplitude of the response decreased with distance from the electrode with an exponential decay constant of 343 ± 1 10 pm for cathodal pulses and 253 ± 91 pm for anodal pulses (n = 7). The results were qualitatively similar in both fiber directions when the atrium was bathed in a solution containing ionic channel blockers. A two-dimensional computer model was formulated for the case of highly anisotropic cardiac tissue and qualitatively accounts for nearly all the observed spatial and temporal behavior of vm in the two fiber directions. The relationships between vm and both the "activating function" and extracellular potential gradient are discussed.
Roles of Electric Field and Fiber Structure in Cardiac Electric Stimulation
Biophysical Journal, 1999
This study investigated roles of the variation of extracellular voltage gradient (VG) over space and cardiac fibers in production of transmembrane voltage changes (⌬V m ) during shocks. Eleven isolated rabbit hearts were arterially perfused with solution containing V m -sensitive fluorescent dye (di-4-ANEPPS). The epicardium received shocks from symmetrical or asymmetrical electrodes to produce nominally uniform or nonuniform VGs. Extracellular electric field and ⌬V m produced by shocks in the absolute refractory period were measured with electrodes and a laser scanner and were simulated with a bidomain computer model that incorporated the anterior left ventricular epicardial fiber field. Measurements and simulations showed that fibers distorted extracellular voltages and influenced the ⌬V m . For both uniform and nonuniform shocks, ⌬V m depended primarily on second spatial derivatives of extracellular voltages, whereas the VGs played a smaller role. Thus, 1) fiber structure influences the extracellular electric field and the distribution of ⌬V m ; 2) the ⌬V m depend on second spatial derivatives of extracellular voltage.
1994
NEUNLIST, M., ti AL : Optical Recordings of Ventricular Excitability of Frog Heart by an Extracellular Stimulating Point Electrode. To enhance understanding of the excitability of cardiac wusde during rest, an optical technique using the fluorescent voltage sensitive dye di-4-ANEPPS was used. Unlike conventional electrical recordings, optical recordings are free from electrical artifacts and. therefore, allow the observation of the transmembrane potential not only following the stimulation pulse, but also during the pulse itself Transmembrane potentials (V,J were recorded optically from frog ventricular epicardium in calcium containing Ringer's solution directly under an extracellular stimulating point electrode. Anodal and cathodal S^ stimuli were applied at rest. As observed by previous investigators, the post-pulse excitatory responses for cathodal pulses, compared with anodal pulses were greater. Changes in transmembrane potential (AV,J during the pulse were as expected for a passive cable only for low intensity pulses (< 4 X the cathodal threshold of excitation in diastole. CTE). However, at the higher intensities necessary to produce an excitatory response (> 6-8 X CTE), an •'irregular" response in V^ was observed-a reversal of the hyperpolarization during an anodal stimulus pulse and a reversal of the depolarization during a cathodal stimulus pulse. To elucidate further the biophysical basis for this behavior, AV,,, was mapped around the stimulating electrode. During stimulation, regions could be observed having a response with opposite polarity to that under the electrode (i.e.. depolarization for an anodal pulse and hyperpolarization for a cathodal pulse). Removal of the bath solution or the addition of channel Mockers did not eliminate the occurrence of these regions. These regions appear to be the basis for the irregular behavior of AVd irectly under the electrode as well as for anodal excitation.
Intramural Virtual Electrodes in Ventricular Wall: Effects on Epicardial Polarizations
Circulation, 2004
Background— Intramural virtual electrodes (IVEs) are believed to play an important role in defibrillation, but their existence in intact myocardium remains unproven. Here, IVEs were detected by use of optical recordings of shock-induced transmembrane potential (V m ) changes (ΔV m ) measured from the intact epicardial heart surface. Methods and Results— To detect IVEs, isolated porcine left ventricles were sequentially stained with a V m -sensitive dye by 2 methods: (1) surface staining (SS) and (2) global staining (GS) via coronary perfusion. Shocks (2 to 50 V/cm) were applied across the ventricular wall in an epicardial-to-endocardial direction during the action potential plateau via transparent mesh electrodes, and shock-induced ΔV m were measured optically from the same epicardial locations after SS and GS. Optical recordings revealed significant differences between ΔV m of 2 types that became more prominent with increasing shock strength: (1) for weak shocks, SS-ΔV m were large...
Electrotonic Inhibition and Active Facilitation of Excitability in Ventricular Muscle
Journal of Cardiovascular Electrophysiology, 1994
Inhibition and Facilitation in Cardiac Muscle. Introduction: The effects of subthreshold electrical pulses on the response to subsequent stimulation have heen described previously in experimental animal studies as well as in the human heart. In addition, previous studies in eardiac Purkinje fibers have shown that diastolic excitability may decrease after activity (active tnhihition) and, to a lesser extent, following .subthreshold responses (electrotonic inhihition). However, such dynamic changes in excitability bave not been explored in isolated ventricular muscle, and it is uncertain whether similar phenomena may play any role in the activation patterns associated with propagation abnormalities in the myocardium.
Examination of stimulation mechanism and strength-interval curve in cardiac tissue
American Journal of Physiology-Heart and Circulatory Physiology, 2005
Understanding the basic mechanisms of excitability through the cardiac cycle is critical to both the development of new implantable cardiac stimulators and improvement of the pacing protocol. Although numerous works have examined excitability in different phases of the cardiac cycle, no systematic experimental research has been conducted to elucidate the correlation among the virtual electrode polarization pattern, stimulation mechanism, and excitability under unipolar cathodal and anodal stimulation. We used a high-resolution imaging system to study the spatial and temporal stimulation patterns in 20 Langendorff-perfused rabbit hearts. The potential-sensitive dye di-4-ANEPPS was utilized to record the electrical activity using epifluorescence. We delivered S1-S2 unipolar point stimuli with durations of 2–20 ms. The anodal S-I curves displayed a more complex shape in comparison with the cathodal curves. The descent from refractoriness for anodal stimulation was extremely steep, and ...
The effect of plunge electrodes during electrical stimulation of cardiac tissue
IEEE Transactions on Biomedical Engineering, 2001
The mechanism for far-field stimulation of cardiac tissue is not known, although many hypotheses have been suggested. This paper explores a new hypothesis: the insulated plunge electrodes used in experiments to map the extracellular potential may affect the transmembrane potential when an electric field is applied to cardiac tissue. Our calculation simulates a 10-mm-diameter sheet of passive tissue with a circular insulated plunge electrode in the middle of it, ranging in diameter from 0.05 to 2 mm. We calculate the transmembrane potential induced by a 500-V/m electric field. Our results show that a transmembrane potential is induced around the electrode in alternating areas of depolarization and hyperpolarization. If the electric field is oriented parallel to the myocardial fibers, the maximum transmembrane potential is 89 mV. A layer of fluid around the electrode increases the transmembrane potential. We conclude that plunge electrodes may introduce artifacts during experiments designed to study the response of the heart to strong electric shocks.
Potential Distributions Generated By Point Stimulation in a Myocardial Volume
Journal of Cardiovascular Electrophysiology, 1993
Potential Patterns in a 3-D Cardiac Depolarization Model, introduction: We present simultilions of extracellular potential patterns elicited by delivering eetopic stimuli to a puriillelepipcdal slab of ventricular tissue represented as an anisutropic bidomain incorporating epi-endocardial Kber rotation. Methods and Results: Simulations were based on an eikonal model tbat determines wavefront shapes tbrou^bout the slab at every time instant during tbe depolarization phase, coupled with an approximate model of the action potential profile. The endocardial face of tbe slab was in contact witb blood and tbe composite volume was surrounded by an insulating medium. Tbe elTect of a simplilied Purkinje network was also studied. Results: (I) For all pacing depths, except endocardial pacing, a central negative area and two potential maxima were ob.served at QRS onset in all intramural planes parallel to tbe epicardium. In all planes, the axis joining tbe two maxima was approximately aligned with the direction of fibers in the plane of pacing. Endocardial pacing generated a different pattern, but only wben blood was present; (2) During later stages of excitation, outflowing currents U'roni tbe wavefront toward tbe resting tissue) were always emitted, at all intramural deptbs, only from those portions of tbe wavefront tbat spread along fibers. At any given instant, the position of tbe two potential maxima in a series of planes parallel to tbe epicardium and intersecting tbe wavefront rotated as a function of deptb, following tbe rotating direction of intramural fibers. Purkinje involvement modified tbe above patterns. Conclusion: Kpicardial and endocardial potential maps provided information on pacing site and depth and on subsequent intramural propagation by reflecting the clockwise or counterclockwise rotation of the deep positivity. Results may be applicable to epicardial and endocardial potential maps recorded at surgery or from endocavitary probes. (J Cardiovasc Electrophysiol. Vol. 4. pp. 4JS-^5H, August 1993} three-dimensional computer simulations, cardiac depolarization process, potential distributions, anisotropic cardiac tissue Niii^ionalL-delle Ricerche under contracts n. 90.