Phase-locked rhythms in periodically stimulated heart cell aggregates

We have studied the effect of injecting a periodic train of current pulses into spontaneously beating aggregates of embryonic chick ventricular heart cells. Over a range of stimulation frequencies around the intrinsic frequency of an aggregate we find one action potential for each stimulus with a fixed latency from each stimulus to the subsequent action potential. For a stimulation frequency higher (lower) than the intrinsic frequency, this corresponds to overdrive (underdrive). At high frequencies of stimulation dropped beats occur leading to complex rhythms analogous to various Wenckebach rhythms observed clinically. At higher stimulation frequencies one can obtain a complete suppression of action potential generation. At low frequencies of stimulation, there are rhythms containing escape beats. Almost every rhythm seen bears a striking resemblance to some cardiac arrhythmia. We present a simple classification scheme that predicts the order of appearance of all the classes of rhythms experimentally observed as one changes the stimulation frequency. We propose that this scheme can be used generally to describe the behavior of other biological oscillators.