They are better understood by computation than by analysis or by in-vivo experiments, at the moment. But experiments do confirm their existence and crucial role in fibrillation, which occurs in the majority of deaths. These include periodic waves radiating from space curves defined by a phase singularity. In the laboratory these form closed rings that shrink and vanish. In numerical experiments the rings can also link and knot, making the rings topologically stable.
Activity then persists indefinitely.
This has not yet been attempted in the laboratory. Numerical and chemical experiments show this can happen, but it need not. More commonly, two mechanisms of outward pressure forestall the anticipated collapse. Their action depends on rate constants in relation to bifurcation loci in a parameter space, but they are not well understood. Some interesting mathematical problems invite attention.