Shien Fong Lin

Virtual electrodes in cardiac tissue: a common mechanism for anodal and cathodal stimulation.

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.

Quatrefoil reentry in myocardium: an optical imaging study of the induction mechanism.

Quatrefoil Reentry in Myocardium. Introduction: The “critical point hypothesis” for induction of ventricular fibrillation has previously been extended to infer the coexistence of four critical points, and hence four simultaneous spiral reentries or a quatrefoil reentry, resulting from only one premature stimulus delivered to the same location as the pacing stimulus. An optical imaging technique was used to explore its existence and to study the induction mechanism of this peculiar reentry pattern.

Experimental and theoretical analysis of phase singularity dynamics in cardiac tissue.

Phase Singularity Dynamics. Introduction: Quantitative analysis of complex self‐excitatory wave patterns, such as cardiac fibrillation and other high‐order reentry, requires the development of new tools for identifying and tracking the most important features of the activation, such as phase singularities.

PANORAMIC OPTICAL IMAGING OF ELECTRICAL PROPAGATION IN ISOLATED HEART

Optical imaging of cardiac transmembrane potential in dye-stained tissue is an emerging technique in cardiac electrophysiology. Despite its widespread application to studies of isolated hearts, it has been applied traditionally to recording only a single view that presents the potential distribution of a fraction of the cardiac surface. This poses a significant limitation in studying whole heart electrophysiology, particularly when large-scale phenomena such as fibrillation and defibrillation are of interest. We have developed a panoramic imaging system based on a high-speed charge-coupled device camera with a maximum imaging speed of 335 frames/s at 128×64u2009pixels/frame. Our system provides one front view and two back mirror views of isolated hearts, thus extending optical imaging capabilities to record from the entire three dimensional heart surface with only one camera.

Three-dimensional surface reconstruction and fluorescent visualization of cardiac activation

Optical imaging of transmembrane potentials in cardiac tissue is a rapidly growing technique in cardiac electrophysiology. Traditional studies typically use a monocular imaging setup, thus limiting investigation to a restricted region of tissue. However, studies of large-scale wavefront dynamics, especially those during fibrillation and defibrillation, would benefit from visualization of the entire epicardial surface. To solve this problem, a panoramic cardiac visualization algorithm was developed which performs the two tasks of reconstruction of the surface geometry of the heart, and representation of the panoramic fluorescence information as a texture mapping onto the geometry that was previously created. This system permits measurement of epicardial electrodynamics over a geometrically realistic representation of the actual heart being studied. To verify the accuracy of the algorithm, the procedure was applied to synthetic images of a patterned ball; further verification was provided by application of the algorithm to a model heart placed in the experimental setup. Both sets of images produced mean registration image errors on the order of 2 pixels, corresponding to roughly 3 mm on the geometry. The authors demonstrate the algorithm by visualizing epicardial wavefronts on an isolated, perfused rabbit heart.

High-resolution high-speed synchronous epifluorescence imaging of cardiac activation

An optical imaging technique with high spatial and temporal resolution was developed to record fractional changes in laser-induced epifluorescence associated with the cardiac transmembrane potential during and after the application of monophasic point stimuli. The technique takes advantage of the repeatability of the recorded events, and uses a synchronized laser strobing mechanism to overcome the speed limitation inherent to slow-scan charge-coupled device cameras, and achieves an effective frame rate of 500 frames/s at a spatial resolution of 100×100 pixels in a single frame with a pixel resolution of 75 μm. The signal-to-noise ratio can be improved with boxcar averaging. Patterns of virtual cathode and anode with distinctive regions of simultaneous depolarization and hyperpolarization during stimulation are demonstrated with stimuli applied to the resting myocardium of an isolated rabbit heart. The technique described in this article provides a powerful tool for investigating repeatable dynamics in the...

Three-Dimensional Visualization of Phase Singularities on the Isolated Rabbit Heart

We observed the three-dimensional electrodynamic behavior of a Langendorff-perfused rabbit heart stained with the voltage-sensitive dye di-4-ANEPPS during endocardial pacing and x8e brillation, using panoramic epicardial imaging to provide catadioptric visualization of the anterior and posterior regions of the heart with one CCD camera and two mirrors at a frame rate of 335 frames/sec in a 128 3 64 pixel format.1 The Ca channel blocker D600 eliminated motion artifacts. We then created phase maps2 from the normalized x8f uorescence (F) using an algorithm based on the Hilbert transform3 and generated singularity maps by computing topologic charge.4 In contrast to previous studies, with a limited x8e eld of view, we herein report panoramic visualization not only of Vm but also of cardiac phase and phase singularity distributions over the entire ventricular epicardium. Whole-heart geometric reconstructions are shown in panels a through c of the Figure for both pacing and x8e brillatory activity, with the atrial pixels removed (white circle indicates endocardial pacing site). The ventricular dF/dt mapping is illustrated in panel a, showing the wavefronts. Panel b depicts the phase mapping for the behavior in panel a, whereas panel c shows the phase singularity distribution nt computed from panel b. Note the expected absence of singularities during pacing and their presence during x8e brillation. This procedure illustrates that x8e brillatory wholeheart electrodynamic behavior can be described in terms of phase singularities by observing the singularities’ location and dynamics relative to ventricular epicardial anatomy.

Stable Bound Pair of Spiral Waves in Rabbit Ventricles

Ventricular x8e brillation (VF) was induced by burst pacing in Langendorff-perfused rabbit hearts (n 10). Wavefront propagation was observed in the frontal view of the heart with optical imaging, using a CCD camera at a frame resolution of 100 100 pixels and a temporal resolution of 3.75 msec. After adding D600 of 0.5 to 2.5 mg/L to the perfusate, baseline VF converted to ventricular tachycardia with a variety of stable reentrant pathways. Phase maps were constructed by phase plane analysis (Gray et al., Nature 1998;392:75-78) to facilitate the identix8e cation of spiral cores. For each pixel, the state of the action potential during a cycle is mapped as an angular value between p and p . Of a total of 28 episodes of stable reentry ( 20 cycles), 21 were single spirals and 7 were double spirals. All of the double spirals but one showed x8e gure-of-eight reentry with the two spirals rotating in opposite directions. In the remaining one as shown in the x8e gure of phase maps (recorded at 98, 116, and 131 msec), both of these two spirals rotated in a counterclockwise direction and had an identical cycle length of approximately 49 msec. Two cores (white circles in the x8e gure) were identix8e ed as spatial points where the encircling pixels complete a 2p phase transition. These cores meandered slightly on the heart surface with an average distance of 6.4 mm. Dissection of the heart with pins inserted at these cores revealed that each spiral anchored to one papillary muscle insertion respectively in the right ventricle. Double spirals with opposite direction of rotation have been observed previously, such as in x8e gure-of-eight and quatrefoil reentries. Here we demonstrate the existence of a novel, phase-locked spiral pair with the same chirality in Langendorff-perfused rabbit ventricles. Formation of this spiral pair appears to be associated with the underlying anatomic structures, i.e., papillary muscle insertions.

A Phased-Array Stimulator System for Studying Planar and Curved Cardiac Activation Wavefronts

Wavefront propagation in cardiac tissue is affected greatly by the geometry of the wavefront. We describe a computer-controlled stimulator system that creates reproducible wavefronts of a predetermined shape and orientation for the investigation of the effects of wavefront geometry. We conducted demonstration experiments on isolated perfused rabbit hearts, which were stained with the voltage-sensitive dye, di-4-ANEPPS. The wavefronts were imaged using a laser and a charge-coupled device (CCD) camera. The stimulator and imaging systems have been used to characterize the relationship between wavefront velocity and fiber orientation. This approach has potential applications in investigating curvature effects, testing numerical models of cardiac tissue, and creating complex wavefronts using one-, two-, or three-dimensional electrode arrays.

Three-dimensional visualization of epifluorescent electrodynamics

Fluorescence imaging of transmembrane potentials is used to observe cardiac activation during fibrillation and defibrillation, but spatial visualization is limited by current techniques. The authors have developed a panoramic cardiac visualization algorithm which reconstructs the geometry of the heart using the fluorescence information as a texture map. This system permits measurement of epicardial electrodynamics over a geometrically realistic representation of the actual heart being studied.