Mechanism of Discordant Alternans in Spatially Homogeneous Tissue
  Discordant alternans,the phenomenon of 2 separate cardiac tissue locations exhibiting action potential duration (APD) alternans of opposite phase, appears to be a potential mechanism for electrocardiographic T wave alternans, but its initiation mechanism is unknown. We studied behavior of one- and two- dimensional cardiac tissue spatially homogenous in all respects, including APD restitution and conduction velocity restitution, using the Beeler Reuter ion channel model. We found that discordant alternans was initiated when spatial gradients of APD arose dynamically, such as from fixed rate pacing of a cable end (sinus node scenario), or from fixed rate pacing at one site preceded by a single excitation wavefront from another site (ectopic focus scenario). In the sinus node scenario, conduction velocity restitution was necessary to initiate discordant alternans. Alternating regimes of concordant and discordant alternans arose along the length of the cable, with regimes delimited by nodes of fixed APD. The number of observable nodes depended upon pacing rate and tissue length. Differences in beat to beat conduction velocity values at steady state were small. In the ectopic focus scenario, variable conduction velocity was not required for induction of discrodant alternans. In both scenarios, stability of node position was dependent upon electrotonic coupling. Other mathematical models produced qualitatively similar results. We conclude that spatial inhomogeneities of electrical restitution are not requiredd to produce discordant alternans; rather, discordant alternans can arise dynamicaly from interaction of APD and conduction veolocity resstitution with single site pacing, or from APD restitution alone in two site pacing. 
Mari A. Watanabe, Flavio H Fenton, Steven J. Evans, Harold Hastings   and Alain Karma