The pinwheel experiment revisited.

The critical point hypothesis explains the origin of some cardiac arrhythmias, and the bidomain model describes electrical stimulation of the heart. In this paper, the critical point hypothesis is combined with the bidomain model. The result is four new predictions about the pinwheel experiment, a fundamental experiment in cardiac electrophysiology. These are: (1) The duration of the vulnerable period during cathodal S2 stimulation is longer for an S1 wavefront propagating perpendicular to the fibers than for an S1 wavefront propagating parallel to the fibers. (2) For anodal S2 stimulation with the S1 wavefront propagating parallel to the fibers, the vulnerable period splits into two periods with an "invulnerable period" between them. (3) For anodal S2 stimulation with the S1 wavefront propagating perpendicular to the fibers, the vulnerable period consists of only one period. (4) A previously suggested mechanism for the upper limit of vulnerability (S2 is so strong that the entire tissue is depolarized by an amount greater than S*) is no longer applicable.