Regular and chaotic behaviour of cardiac cells stimulated at frequencies between 2 and 20 Hz.

We measured the non-linear dynamics of cardiac action potentials by varying the stimulation frequency from 2 to 20 Hz and obtained the following results: (i) When the fast Na+ current initiated the action potentials ('fast action potentials') periodicity was maintained, i.e. the pattern of action potentials repeated after a finite number of stimuli. Chaotic sequences were absent. The transition from one sequence to the next occurred as a devil's staircase. (ii) When, however, the slow Ca2+ current initiated the action potentials ('slow action potentials'), we observed chaos, i.e. fully irregular behaviour, as well as bifurcations. (iii) Our results confirm the supposition that the global dynamics of cardiac cells can be well described by simple one-dimensional maps which predict these two kinds of behaviour.