Role of conduction velocity restitution and short-term memory in the development of action potential duration alternans in isolated rabbit hearts

Background - Spatially discordant alternans (SDA) has been linked to life-threatening arrhythmias. The mechanisms underlying SDA development in cardiac tissue remain unclear. Methods and results - We investigated the role of conduction velocity (CV) restitution and short-term memory in the organization and evolution of alternans in action potential duration using high-resolution optical mapping of the epicardial surface in 8 isolated, Langendorff-perfused rabbit hearts. To assess the spatial organization of alternans, we tracked the evolution of nodal lines that separate out-of-phase regions of SDA. We measured the action potential duration heterogeneity index and maximal slope of CV restitution and estimated the effects of short-term memory by calculating time constant of action potential duration accommodation (τ). We found that 2 mechanisms underlie the development of SDA in the heart, leading to 2 distinct behaviors of nodal lines. The first mechanism is based on steep CV restitution and is associated with small τ and stable nodal lines. The second mechanism is associated with short-term memory (large τ) and is characterized by shallow CV restitution and unstable behavior of nodal lines. The maximum slope of the CV restitution was steeper (18.16±3.34 m/s) and τ was smaller (τ=4.31±0.33 stimuli) for areas with stable nodal lines than for areas with unstable nodal lines (6.32±0.96 m/s and τ=10.3±1.84 stimuli; P<0.01). Conlusions - Our results provide new insight into the mechanisms underlying SDA formation in the rabbit heart. Specifically, our results suggest that a new mechanism associated with short-term memory underlies SDA formation in the heart, in addition to steep CV restitution.