Three-dimensional organisation of re-entrant propagation during experimental ventricular fibrillation

Ventricular fibrillation is believed to be produced by the breakdown of re-entrant propagation waves of excitation into multiple re-entrant sources. These re-entrant waves may be idealised as spiral waves in two-dimensional, and scroll waves in three-dimensional excitable media. Optically monitored, simultaneously recorded endocardial and epicardial patterns of activation on the ventricular wall do not always show spiral waves. Analysis of optically recorded irregular electrical wave activity on the surface of the heart during experimentally induced fibrillation reveals a strong local temporal periodicity. The spatial distribution of the dominant temporal frequencies of excitation has a domain organisation. The domains are large (≈1 cm2) and they persist for minutes. We show that numerical simulations, even with a simple homogeneous excitable medium, can reproduce the key features of the simultaneous endo- and epicardial visualisations of propagating activity, and so these recordings may be interpreted in terms of scroll waves within the ventricular wall. The domain structure can be reproduced in a two-dimensional excitable medium governed by the FitzHugh–Nagumo equations with a spatial inhomogeneity. We identified two potential mechanisms that may contribute to the observed experimental dynamics: coexistence of stable spiral waves with non-commensurate frequencies of rotation, and Wenckebach-like frequency division from a single spiral source due to inhomogeneity. Both mechanisms reproduce the uniformity of the dominant frequency within individual domains and sharp boundaries between domains. The possibility of distinguishing between different mechanisms using Lissajous figures is discussed.