Aging and disease cause changes in the composition of cardiac tissue that affect the conduction of electrical signals through the heart.  Life threatening arrhythmias, disruptions of the normal activation sequences of the heart, may result if cardiac conduction is sufficiently disrupted.  In clinical catheterization labs physicians analyze electrograms recorded directly from the interior surface of the heart to identify these diseased tissues.  New multielectrode recording devices with the ability to track the position of each electrode in the body have enhanced our ability to rapidly identify these regions and to direct therapy.  However, these signals are dependent on the pattern of tissue activation, an uncontrolled parameter from the clinicians perspective, and may mask abnormalities that would arise under a different activation pattern.  We have begun to explore a new strategy of tissue characterization with these multielectrode catheters in which activation is stimulated from one site on the array while electrograms are recorded from the other electrodes.  This approach gives us control over the activation sequence of heart and allows use to robustly characterize the conduction properties. We hope to extend this approach to allow tomographic reconstruction of cardiac conduction in order to better identify the tissues responsible for arrhythmias.