Paroxysmal Starting and Stopping of Circulating Waves in Excitable Media

Levels of intracellular ${\mathrm{Ca}}^{2+}$ were monitored using fluorescence from ${\mathrm{Ca}}^{2+}$-sensitive dyes in chick embryonic heart cells cultured in an annular geometry. There was spontaneous starting and stopping of reentrant waves of activity. The results are modeled using modified FitzHugh-Nagumo equations representing pacemakers embedded in a conducting medium. These results provide a potential mechanism for spontaneous abnormal cardiac rhythms in which there are rapid heart beats (tachycardias) that repetitively start and stop.