  During heart rhythm disorders, complex spatio-temporal patterns of electrical
activation can self-organize in vortices. Previously, it was assumed that at
the center of such vortex, there is a point singularity in the activation
phase. We hypothesize that there is another possibility, namely a region of
discontinuous phase that can exist during cardiac arrhythmias instead of a
point singularity. In this paper, we provide some theoretical insights into the
existence of such phase defect lines (PDL) and validate their presence in
ventricular tachycardia in the whole ex-vivo heart using optical mapping
experiments. We discuss the implications of phase defect lines and surfaces for
the theory of arrhythmias, cardiac modeling and practical cardiology.
