Optical transmembrane potential recordings during intracardiac defibrillation-strength shocks

Asymmetry in membrane responses to electric shocks: insights from bidomain simulations

Models of myocardial membrane dynamics have not been able to reproduce the experimentally observed negative bias in the asymmetry of transmembrane potential changes (DeltaVm) induced by strong electric shocks delivered during the action potential plateau. The goal of this study is to determine what membrane model modifications can bridge this gap between simulation and experiment. We conducted simulations of shocks in bidomain fibers and sheets with membrane dynamics represented by the LRd'2000 model. We found that in the fiber, the negative bias in DeltaVm asymmetry could not be reproduced by addition of electroporation only, but by further addition of hypothetical outward current, Ia, activated upon strong shock-induced depolarization. Furthermore, the experimentally observed rectangularly shaped positive DeltaVm, negative-to-positive DeltaVm ratio (asymmetry ratio) = approximately 2, electroporation occurring at the anode only, and the increase in positive DeltaVm caused by L-type Ca2+-channel blockade were reproduced in the strand only if Ia was assumed to be a part of K+ flow through the L-type Ca2+-channel. In the sheet, Ia not only contributed to the negative bias in DeltaVm asymmetry at sites polarized by physical and virtual electrodes, but also restricted positive DeltaVm. Inclusion of Ia and electroporation is thus the bridge between experiment and simulation.

Optical imaging of the heart

Optical techniques have revolutionized the investigation of cardiac cellular physiology and advanced our understanding of basic mechanisms of electrical activity, calcium homeostasis, and metabolism. Although optical methods are widely accepted and have been at the forefront of scientific discoveries, they have been primarily applied at cellular and subcellular levels and considerably less to whole heart organ physiology. Numerous technical difficulties had to be overcome to dynamically map physiological processes in intact hearts by optical methods. Problems of contraction artifacts, cellular heterogeneities, spatial and temporal resolution, limitations of surface images, depth-of-field, and need for large fields of view (ranging from 2x2 mm2 to 3x3 cm2) have all led to the development of new devices and optical probes to monitor physiological parameters in intact hearts. This review aims to provide a critical overview of current approaches, their contributions to the field of cardiac electrophysiology, and future directions of various optical imaging modalities as applied to cardiac physiology at organ and tissue levels.

Synthesis of voltage-sensitive fluorescence signals from three-dimensional myocardial activation patterns

Voltage-sensitive fluorescent dyes are commonly used to measure cardiac electrical activity. Recent studies indicate, however, that optical action potentials (OAPs) recorded from the myocardial surface originate from a widely distributed volume beneath the surface and may contain useful information regarding intramural activation. The first step toward obtaining this information is to predict OAPs from known patterns of three-dimensional (3-D) electrical activity. To achieve this goal, we developed a two-stage model in which the output of a 3-D ionic model of electrical excitation serves as the input to an optical model of light scattering and absorption inside heart tissue. The two-stage model permits unique optical signatures to be obtained for given 3-D patterns of electrical activity for direct comparison with experimental data, thus yielding information about intramural electrical activity. To illustrate applications of the model, we simulated surface fluorescence signals produced by 3-D electrical activity during epicardial and endocardial pacing. We discovered that OAP upstroke morphology was highly sensitive to the transmural component of wave front velocity and could be used to predict wave front orientation with respect to the surface. These findings demonstrate the potential of the model for obtaining useful 3-D information about intramural propagation.