Excitation Wave Dynamics and their Interaction with External Fields

Abstract

Rotating spiral waves of excitation are common in many physical, chemical and biological systems. In physiological systems like the heart, such waves anchor to unexcitable tissue (an obstacle), become stable pinned waves and cause life-threatening cardiac arrhythmias. The traditional high voltage defibrillation techniques used to treat arrhythmias are known to have pro-arrhythmic effects. Therefore, it is crucial to develop low energy methods to unpin and eliminate them. This thesis investigates two kinds of low voltage electric fields to unpin the pinned spiral waves. In the first method using pulsed electric fields, the spiral wave will be unpinned only when the pulse is delivered inside a narrow time interval called the unpinning window of the spiral. In experiments with cardiac monolayers, we found that other obstacles situated near the spiral’s pinning centre can facilitate unpinning. In numerical simulations, we found that the unpinning window can change depending on the location, orientation and distance between the pinning centre and the obstacle. The second method involves unpinning the spiral using circularly polarised electric fields (CPEF). Here, we show that the spiral can always be unpinned below a threshold time period of CPEF for a given obstacle size. Our analytical formulation accurately predicts the threshold and explains the absence of the traditional unpinning window. We also show that the unpinning always happens within the first rotation of the electric field. Previous unpinning studies using two-dimensional experimental and numerical models show that the width of the unpinning window is very narrow. This could be due to the presence of multiple obstacles as our results suggests. The absence of unpinning window with CPEF eliminates the problem of timing the pulses and guarantees unpinning of the spiral below a certain threshold time period. We hope that the results discussed in this thesis regarding the spatial arrangement of the obstacles and its interactions with the electric fields will open new ways towards low-energy therapies of the cardiac arrhythmias.