Research Article Characterizing Spatial Dynamics of Bifurcation to Alternans in Isolated Whole Rabbit Hearts Based on Alternate Pacing Kanchan Kulkarni, 1 Ramjay Visweswaran, 1 Xiaopeng Zhao, 2 and Elena G. Tolkacheva 1 1 Department of Biomedical Engineering, University of Minnesota, 312 Church Street SE, Minneapolis, MN 55455, USA 2 Department of Mechanical, Aerospace, and Biomedical Engineering, University of Tennessee, 1506 Middle Drive, Knoxville, TN 37996, USA Correspondence should be addressed to Elena G. Tolkacheva; talkacal@umn.edu Received 10 November 2014; Accepted 6 February 2015 Academic Editor: Sergio Alonso Copyright © 2015 Kanchan Kulkarni et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Sudden cardiac death instigated by ventricular fbrillation (VF) is the largest cause of natural death in the USA. Alternans, a beat- to-beat alternation in the action potential duration, has been implicated as being proarrhythmic. Te onset of alternans is mediated via a bifurcation, which may occur through either a smooth or a border-collision mechanism. Te objective of this study was to characterize the mechanism of bifurcation to alternans based on experiments in isolated whole rabbit hearts. High resolution optical mapping was performed and the electrical activity was recorded from the lef ventricle (LV) epicardial surface of the heart. Each heart was paced using an “alternate pacing protocol,” where the basic cycle length (BCL) was alternatively perturbed by ±. Local onset of alternans in the heart, BCL start , was measured in the absence of perturbations (=0) and was defned as the BCL at which 10% of LV exhibited alternans. Te infuences of perturbation size were investigated at two BCLs: one prior to BCL start (BCL prior = BCL start + 20 ms) and one preceding BCL prior (BCL far = BCL start + 40 ms). Our results demonstrate signifcant spatial correlation of the region exhibiting alternans with smooth bifurcation characteristics, indicating that transition to alternans in isolated rabbit hearts occurs predominantly through smooth bifurcation. 1. Introduction Ventricular fbrillation (VF), manifesting as chaotic unsyn- chronized electrical activity in the heart, is known to cause sudden cardiac death (SCD). SCD is one of the largest natural causes of death, killing more than 300,000 people annually in the United States [1, 2]. Alternans, which is a beat-to-beat alternation in the action potential duration (APD), has been implicated as being proarrhythmic and a potential source of cardiac instability [3–6]. When the heart is paced at progressively increasing rates, electrical restitution, which is a fundamental property of cardiac myocytes, causes the heart to bifurcate from a constant APD response at lower frequencies to an alternating long-short APD pattern (alternans) at higher frequencies [7–11]. Characterizing this bifurcation can provide useful insights into understanding the dynamics of the cardiac system. In particular, knowledge of the bifurcation type that governs the transition to alternans can potentially be used as a means to predict the formation of alternans prior to their onset [12], which may be useful in preventing arrhythmias. Previous research has implicated both smooth and bor- der-collision bifurcations when trying to characterize the tra- nsition to alternans in small cardiac tissue. Initially, Nolasco and Dahlen modeled transition to alternans as a smooth bifurcation [7]; however, later studies reported a border- collision type of bifurcation to alternans [13]. More recently, it was shown that a more complex bifurcation model involving the coexistence of both smooth and border-collision charac- teristics may exist in the heart. Based on experiments on adult bullfrog ventricular tissue samples, Berger et al. suggested the existence of a smooth-like behavior close to the bifurcation point and a border-collision type behavior further away from the bifurcation point [14]. Although the presence of diferent bifurcation characteristics in small tissue samples suggests very interesting dynamical behavior, their study was limited Hindawi Publishing Corporation BioMed Research International Volume 2015, Article ID 170768, 8 pages http://dx.doi.org/10.1155/2015/170768