Optical Transmembrane Potential Recordings During Intracardiac Defibrillation-Strength Shocks
Optical recording using voltage-sensitive dyes has been used to investigate the mechanisms of defibrillation because it (1) is immune to the artifacts produced by high-voltage shocks, (2) provides the time course of the membrane action potential, and (3) can be used to make simultaneous recordings at many sites. The authors used the laser scanning technique to optically record action potentials from 100 sites with 1-ms resolution on the surface of the isolated, perfused rabbit heart during defibrillation. The data were typically analyzed by constructing maps of impulse propagation and examining individual recordings from sites of interest. Described here is a new analysis method that creates millisecond-by-millisecond images of the spatial distribution of membrane potentials. The experimental protocol applied a test shock to the fibrillating heart, followed by a rescue shock and a paced beat. Optical recordings were calibrated to yield membrane voltage as a percentage of the resting and overshoot levels of the postrescue stimulated action potential. The positions of the recording sites and the membrane voltage levels for all 100 sites during a single 1-ms interval were used to interpolate membrane voltage levels at points within a 128 x 128 pixel frame using the biharmonic interpolation method. The level of membrane potential was encoded by pixel color and surface elevation. Sequential frames were viewed as a face-on two dimensional or as a three-dimensional perspective of the colored surface. Animation of membrane voltage distributions enabled the visualization of the interaction between the shock-induced electrophysiologic response and the propagation of electrical activity preceding and following a defibrillation shock. Successful defibrillation shocks synchronized repolarization across the surface of the heart following the shock.