VULNERABILITY IN AN EXCITABLE MEDIUM - ANALYTICAL AND NUMERICAL-STUDIES OF INITIATING UNIDIRECTIONAL PROPAGATION

Cardiac tissue can display unusual responses to certain stimulation pr otocols. In the wake of a conditioning wave of excitation, spiral wave s can be initiated by applying stimuli timed to occur during a period of vulnerability (VP). Although vulnerability is well known in cardiac and chemical media, the determinants of the VP and its boundaries hav e received little theoretical and analytical study. From numerical and analytical studies of reaction-diffusion equations, we have found tha t 1) vulnerability is an inherent property of Beeler-Reuter and FitzHu gh-Nagumo models of excitable media; 2) the duration of the vulnerable window (VW) the one-dimensional analog of the VP, is sensitive to the medium properties and the size of the stimulus field; and 3) the ampl itudes of the excitatory and recovery processes modulate the duration of the VW. The analytical results reveal macroscopic behavior (vulnera bility) derived from the diffusion of excitation that is not observabl e at the level of isolated cells or single reaction units.