Termination of Reentrant Propagation By a Single Stimulus: A Model Study WEILUN QUAN and YORAM RUDY From the Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio QUAN, W., ETAL.: Termination of Reentrant Propagation By a Single Stimulus: A Model Study. A com- puter model of a ring-shaped one-dimensional cardiac fiber was used to examine responses of reentrant propagation to premature stimuli appJied under di//eren( degrees of head-tail interaction. Two different types (type 1 and type 11) of termination window fTW) were identified. The type 1 TW was generated by functional inhomogeneity created by reentrant propagation. The width of the type 1 TW was proportional to the degree of cellular uncoupling. In contrast, uniform reduction in sodium channel conductance decreased the width of type 1 TW. The type U TW was generated by electrical alternans created by the head-tail interaction of the reentrant action potential. It was demonstrated that electricaJ alternans were most significant in medium degree head-taii interaction. Forstronger or weaker head-tail interaction, the electrical alternans tended to decrease. The type U TW was located in excitable gaps following reentrant action potentials of short durotion. Its size was proportional to the degree of electrical alternans. The type II TW was usually much larger than the type I TW. A premature conditioning stimulus induced alternans and created a type II TW. This response implies that a conditioning stimulus could facilitate greatiy the termination of clinical reentrant arrhythmias by programmed electrical stimulation. (PACE, Vol. 14, November, Part 11 1991} reentry, unidirectional block, excitable gap, electrical alternans, action potential duration, termination of reentry Introduction Reentrant tachycardia is caused by continu- ous circulating electrical activity. It is common practice in electrophysiological study to terminate reentrant tachycardia using a critically timed sin- gle premature stimulus coupled to the tachycardia complex.^ However, the time interval for success- ful termination (termination window, TW) and its relationship to underlying mechanisms have not This study was supported by a Whitaker Foundation Award (WQ) and a National Institutes of Health grant HL 33343 (YR). Computer time was provided by Pittsburgh Supercomputing Center grant PSCA 29 through the NIH Division of Research Resources cooperative agreement 1 P41 RR060009-01 and through a grant from NSF cooperative agreement ASC- 8500650. Address for reprints: Wellun Quan, Department of Biomedical Engineering, Case Western Reseirve University, Cleveland, OH 44106. Fax: (216) 368-4969. been characterized in a systematic way. During reentrant propagation, a partially or fully excitable gap may exist between the revolving repoiariza- tion phase and the advancing depolarization phase. Previously, we have sbown tbat reducing tbe size of tbe reentrant pathway resulted in an increased degree of head-tail interaction tbat, in turn, changed tbe kinetics of the slow ionic cban- ncls. bringing about shortening and alternans in action potential duration (APD).^ However, tbe electrical alternans as a function of tbe degree of bead-tail interaction and tbe relationsbip between tbe electrical alternans and tbe ability of a single premature stimulus to terminate reentry bave not been fully elucidated. Tbe main objectives of tbis article are: (1) to characterize electrical alternans during reentry under different degrees of head-tail interaction; and (2) to investigate tbe relationsbip between tbese alternans and tbe ability of a single premature stimulus to terminate reentry. 1700 November 1991, Part II PACE, Vol. 14