  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.
