Measurement And Analysis Of Cardiac Tissue During Electrical Stimulation
Holcomb, Mark Raymond
Over 300,000 deaths in the United States each year are caused by sudden cardiac death. The most common cause of sudden cardiac death is ventricular fibrillation. Once initiated, ventricular fibrillation in humans almost never terminates spontaneously. If allowed to persist, death usually occurs within minutes. The application of strong electric shocks, termed defibrillation, is a very effective and well-established method of terminating fibrillation, but the underlying dynamics are still not well understood. The goal of this research was to better understand the response of cardiac tissue to applied electric fields. The primary probes used in these studies are fluorescent dyes, whose spectra shift with changing transmembrane potential or intracellular calcium concentration. The fluorescence emitted by the dyes is collected via optical imaging techniques that yield data with high temporal spatial resolutions. Before the desired studies could be conducted, several instrumentation needs were addressed: 1. A flexible software library that can be used to control multiple high-speed CCD cameras, illumination, and electrical stimulation was developed. 2. Three microcontroller-based constant-current electrical stimulators capable of delivering complex biphasic stimulation sequences were designed and constructed. 3. A two-camera imaging system with camera calibration ability was assembled which allow simultaneous measurement of two dynamic quantities, Vm (Transmembrane Potential) and [Ca2+]i, (Intracellular Calcium Concentration) from the same spatial region. 4. A three-camera panoramic cardiac imaging system was designed, constructed, and validated to enable imaging of Vm over the entire epicardium. Many of these technological advances were used for a comprehensive study of Vm dynamics of an isolated and flattened right ventricle under field-shock conditions which produced a number of interesting observations. We observed a striking symmetric reversal of virtual anodes and virtual cathodes upon reversal of shock polarity. Clear relationships between field strength, electroporation, and virtual anodes were observed. We discovered a previously unreported dye artifact. We attempted the first reported correlation between Vm response to field shock and fiber direction obtained by diffusion tensor imaging.