Hysteresis in calcium rhythm patterns during electric field stimulation of cardiac tissue
Cardiac tissue subjected to fast pacing via electric field stimulation revealed hysteresis in calcium instability patterns (stimulus:response patterns) beyond departure and return to 1:1 response. The pacing frequency at which the first appearance of instabilities occurred (Fa) was higher than the frequency of ultimate disappearance (Fd) upon rate deceleration. Furthermore, hysteresis was observed in multiple pattern transitions. In the spatially extended system studied here, 2:2 alternans were the preferred starting point (Fa) in calcium instability development, while 2:1 blocks were more common in the return to Fa from higher pacing rates. Recovery of 1:1 patterns was preceded mostly by 2:2 alternans at Fd. In addition to previously reported hysteresis in action potential duration during 1:1 rhythm and alternans magnitude hysteresis (in 2:2 rhythm), our data reveal hysteresis in rhythm pattern transitions not just away from and return to 1:1, but also between different instability patterns, and thus provide insight into the rules of such transitions in electrically stimulated cardiac tissue.