Cardiac Nearfields in Complex Atrial Tissue

Life threatening cardiac arrhythmias are frequently caused by modification of the cardiac microstructure during the aging process. The prevalence of these diseases can be expected to increase with mean life expectancy. Basic research in cardiology therefore will focus increasingly on understanding the change of cardiac excitation spread induced by this structural remodeling. Cells in young hearts are electrically coupled in a way that the excitation wave propagates fairly smoothly. This type of continuous conduction has been studied extensively in numerous experiments, in theory and computer simulation. The aging process, however, causes an electrical uncoupling of cell groups leading to discontinuous conduction, it`s mechanisms are still poorly understood. The lack of knowledge about complex propagation of the cardiac impulse reveals difficulties in interpreting intracardiac signal waveforms and in reconstructing activation patterns in the cavities of the heart. Clinically applied techniques rely mainly on the measurement of electrical potentials with electrodes introduced in the heart by catheters. Electric field measurement very close to an intracardiac surface (cardiac near field) provide additional vector information. However, this has not been exploited up to now, neither clinically nor experimentally. The proposed work is to develop a new computer model which accounts for both, electrophysiological function (ionic currents) and cardiac microstructure (coupling and uncoupling of cells). Mainly two aspects will be addressed, the biophysical-theoretical aspect of the properties of the cardiac near field and the application-driven aspect of developing new techniques of electro-anatomical characterization of tissue and of navigating intracardiac diagnostic tools. The biophysical objective is to constitute a theoretical basis facilitating the interpretation of the electric near field caused by different activation sequences. Due to the importance of the atrium regarding arhythmogenesis a typical atrial area with complex arrangement of conducting fibers will be selected for electrophysiological experiments as well as for computer simulation. We recently found that during continuous conduction the cardiac near field describes a vector loop indicating the direction of propagation. It is completely unknown how the cardiac near field behaves at sites of branching conduction pathways or discontinuities. With a multidisciplinary approach of analysis (experiment-histology- computer simulation) during complex conduction we expect a deeper understanding of the underlying mechanisms. These insights will serve as a theoretical basis for the specification of technical requirements on instrumentation to measure cardiac near fields with four-element electrode arrays. Such measurements would provide relevant parameters of local excitation spread: the vector loop indicates the direction of propagation, complex conduction pathways can be detected since they are reflected in the loop morphology (like the fractionation of electrograms in uni- and bipolar recordings), the instant of activation and the local conduction velocity can be determined accurately as well. The understanding of cardiac near fields may open new perspectives for the intracardiac navigation of therapeutic tools. A catheter-mounted near field-sensor would allow a zoomed look at the excitation spread with a spatial resolution unequaled by current methods. This would complement other methods such as intracardiac non-contact mapping systems giving a global view of the activation pattern. The basis of such forward-looking technologies is a valid structure-related model and computer simulation proposed in this work.