Bryan Caldwell

Cardiac electrophysiology and tissue structure: bridging the scale gap with a joint measurement and modelling paradigm

Significant tissue structures exist in cardiac ventricular tissue that are of supracellular dimension. It is hypothesized that these tissue structures contribute to the discontinuous spread of electrical activation, may contribute to arrhymogenesis and also provide a substrate for effective cardioversion. However, the influences of these mesoscale tissue structures in intact ventricular tissue are difficult to understand solely on the basis of experimental measurement. Current measurement technology is able to record at both the macroscale tissue level and the microscale cellular or subcellular level, but to date it has not been possible to obtain large volume, direct measurements at the mesoscales. To bridge this scale gap in experimental measurements, we use tissue‐specific structure and mathematical modelling. Our models have enabled us to consider key hypotheses regarding discontinuous activation. We also consider the future developments of our intact tissue experimental programme.

Imaging electrical excitation inside the myocardial wall

Cardiac arrhythmias are often triggered by ectopic membrane depolarization originating deep inside the myocardial wall. Here we propose a new method utilizing a novel near-infrared voltage-sensitive fluorescent dye DI-4-ANBDQBS to determine the three-dimensional (3D) coordinates of the sources of such depolarization. We tested the method in live preparations of pig left and right ventricular myocardium (thickness 8-18 mm) and phantoms imitating the optical properties of myocardial tissue. The method utilizes an alternating transillumination approach that involves comparing pairs of simultaneously recorded broad-field epifluorescence and transillumination images produced at two alternating directions of illumination. Recordings were taken simultaneously by two CCD cameras facing the endocardial and epicardial surfaces of the heart at a frame rate up to 3 KHz. In live preparations, we were able to localize the origin of the depolarization wave with a precision of ±1.3mm in the transmural direction and 3 mm in the image plane. The accuracy of detection was independent of the depth of the source inside ventricular wall.

Probing Field-Induced Tissue Polarization Using Transillumination Fluorescent Imaging

Despite major successes of biophysical theories in predicting the effects of electrical shocks within the heart, recent optical mapping studies have revealed two major discrepancies between theory and experiment: 1), the presence of negative bulk polarization recorded during strong shocks; and 2), the unexpectedly small surface polarization under shock electrodes. There is little consensus as to whether these differences result from deficiencies of experimental techniques, artifacts of tissue damage, or deficiencies of existing theories. Here, we take advantage of recently developed near-infrared voltage-sensitive dyes and transillumination optical imaging to perform, for the first time that we know of, noninvasive probing of field effects deep inside the intact ventricular wall. This technique removes some of the limitations encountered in previous experimental studies. We explicitly demonstrate that deep inside intact myocardial tissue preparations, strong electrical shocks do produce considerable negative bulk polarization previously inferred from surface recordings. We also demonstrate that near-threshold diastolic field stimulation produces activation of deep myocardial layers 2-6 mm away from the cathodal surface, contrary to theory. Using bidomain simulations we explore factors that may improve the agreement between theory and experiment. We show that the inclusion of negative asymmetric current can qualitatively explain negative bulk polarization in a discontinuous bidomain model.

Cardiac Response to Low-Energy Field Pacing Challenges the Standard Theory of Defibrillation

Background—The electric response of myocardial tissue to periodic field stimuli has attracted significant attention as the basis for low-energy antifibrillation pacing, potentially more effective than traditional single high-energy shocks. In conventional models, an electric field produces a highly nonuniform response of the myocardial wall, with discrete excitations, or hot spots (HS), occurring at cathodal tissue surfaces or large coronary vessels. We test this prediction using novel 3-dimensional tomographic optical imaging. Methods and Results—Experiments were performed in isolated coronary perfused pig ventricular wall preparations stained with near-infrared voltage-sensitive fluorescent dye DI-4-ANBDQBS. The 3-dimensional coordinates of HS were determined using alternating transillumination. To relate HS formation with myocardial structures, we used ultradeep confocal imaging (interrogation depths, >4 mm). The peak HS distribution is located deep inside the heart wall, and the depth is not significantly affected by field polarity. We did not observe the strong colocalization of HS with major coronary vessels anticipated from theory. Yet, we observed considerable lateral displacement of HS with field polarity reversal. Models that de-emphasized lateral intracellular coupling and accounted for resistive heterogeneity in the extracellular space showed similar HS distributions to the experimental observations. Conclusions—The HS distributions within the myocardial wall and the significant lateral displacements with field polarity reversal are inconsistent with standard theories of defibrillation. Extended theories based on enhanced descriptions of cellular scale electric mechanisms may be necessary. The considerable lateral displacement of HS with field polarity reversal supports the hypothesis of biphasic stimuli in low-energy antifibrillation pacing being advantageous.

Three-Dimensional Cardiac Tissue Image Registration for Analysis of In Vivo Electrical Mapping

A method is presented for registering 3D cardiac tissue images to reference data, for the purpose of analyzing recorded electrical activity. Following left-ventricular in vivo electrical mapping studies in pig hearts, MRI is used to define a reference geometry in the tissue segment around the recording electrodes. The segment is then imaged in 3D using a high-resolution serial imaging microscopy technique. The tissue processing required for this introduces segment-wide distortion. Piecewise-smooth maps are used to correct the tissue distortion and register the 3D images with the reference MRI data. The methods are validated and techniques for identifying the preferred maps are proposed. Recorded electrical activation is shown to map reliably onto cardiac tissue structure using this registration method.