Existence of bistability and correlation with arrhythmogenesis in paced sheep atria

Introduction: Studies of the electrical dynamics of cardiac tissue are impo rtant for understanding the mechanisms of arrhythmias. This study uses high -frequency pacing to investigate the dynamics of sheep atria, Methods and Results: A 504-electrode mapping plaque was affixed to the righ t atrium in six sheep. Cathodal pacing stimuli were delivered to the center of the plaque, Pacing period (T-p) was decreased from 275 +/- 25 msec to 7 5 +/- 25 msec and then increased to 230 +/- 70 msec in steps of either 5 or 10 msec. In all 21 trials in six sheep, the atrium responded 1:1 at longer T(p)s and 2:1 at shorter T(p)s. As T-p was decreased, the response switche d to 2:1 at a particular T-p. Conversely, as T-p was increased, the respons e switched back to 1:1 at a particular T-p. Over 21 trials, the 1:1-to-2:1 and 2:1-to-1:1 transitions occurred at 119.5 +/- 18.8 msec and 130.0 +/- 19 .1 msec, respectively. This hysteretic behavior yielded bistability windows , 10.5 +/- 7.2 msec wide, wherein 1:1 and 2:1 responses existed at the same T-p. In 15 trials and in all animals, idiopathic wavefronts emanating from outside the mapped region passed through the mapped region, In 13 of those trials, the idiopathic wavefronts occurred at T(p)s within the bistability window or within 35 msec of its upper or lower limit, Conclusion: Bistability windows and idiopathic wavefronts were observed and found to be correlated with each other, suggesting a connection between bi stability and arrhythmogenesis.