SPIRAL WAVES IN 2-DIMENSIONAL MODELS OF VENTRICULAR MUSCLE - FORMATION OF A STATIONARY CORE

Previous experimental studies have clearly demonstrated the existence of drifting and stationary electrical spiral waves in cardiac muscle a nd their involvement in cardiac arrhythmias. Here we present results o f a study of reentrant excitation in computer simulations based on a m embrane model of the ventricular cell. We have explored in detail thf parameter space of the model, using tools derived from previous numeri cal studies in excitation-dynamics models. We have found appropriate p arametric conditions for sustained stable spiral wave dynamics (1 s of activity or similar to 10 rotations) in simulations of an anisotropic (ratio in velocity 4:1) cardiac sheet of 2 cm x 2 cm. Initially, we u sed a model that reproduced well the characteristics of planar electri cal waves exhibited by thin sheets of sheep ventricular epicardial mus cle during rapid pacing at a cycle length of 300 ms. Under these condi tions, the refractory period was 147 ms; the action potential duration (APD) was 120 ms; the propagation velocity along fibers was 33 cm/s; and the wavelength along fibers was 4.85 cm. Using cross-field stimula tion in this model, we obtained a stable self-sustaining spiral wave r otating around an unexcited core of 1.75 mm x 7 mm at a period of 115 ms, which reproduced well the experimental results. Thus the data demo nstrate that stable spiral wave activity can occur in small cardiac sh eets whose wavelength during planar wave excitation in the longitudina l direction is larger than the size of the sheet. Analysis of the mech anism of this observation demonstrates that, during rotating activity, the core exerts a strong electrotonic influence that effectively abbr eviates APD (and thus wavelength) in its immediate surroundings and is responsible for the stabilization and perpetuation of the activity. W e conclude that appropriate adjustments in the kinetics of the activat ion front (i.e., threshold for activation and upstroke velocity of the initiating beat) of currently available models of the cardiac cell al low accurate reproduction of experimentally observed self-sustaining s piral wave activity. As such, the results set the stage for an underst anding of functional reentry in terms of ionic mechanisms.