WAVE-FRONT CURVATURE AS A CAUSE OF SLOW CONDUCTION AND BLOCK IN ISOLATED CARDIAC-MUSCLE

We have investigated the role of wave-front curvature on propagation b y following the wave front that was diffracted through a narrow isthmu s created in a two-dimensional ionic model (Luo-Rudy) of ventricular m uscle and in a thin (0.5-mm) sheet of sheep ventricular epicardial mus cle. The electrical activity in the experimental preparations was imag ed by using a high-resolution video camera that monitored the changes in fluorescence of the potentiometric dye di-4-ANEPPS on the surface o f the tissue. Isthmuses were created both parallel and perpendicular t o the fiber orientation. In both numerical and biological experiments, when a planar wave front reached the isthmus, it was diffracted to an elliptical wave front whose pronounced curvature was very similar to that of a wave front initiated by point stimulation. In addition, the velocity of propagation was reduced in relation to that of the origina l planar wave. Furthermore, as shown by the numerical results, wave-fr ont curvature changed as a function of the distance from the isthmus. Such changes in local curvature were accompanied by corresponding chan ges in velocity of propagation. In the model, the critical isthmus wid th was 200 mu m for longitudinal propagation and 600 mu m for transver se propagation of a single planar wave initiated proximal to the isthm us. In the experiments, propagation depended on the width of the isthm us for a fixed stimulation frequency. Propagation through an isthmus o f fixed width was rate dependent both along and across fibers. Thus, t he critical isthmus width for propagation was estimated in both direct ions for different frequencies of stimulation. In the longitudinal dir ection, for cycle lengths between 200 and 500 milliseconds, the critic al width was < 1 mm; for 150 milliseconds, it was estimated to be betw een 1.3 and 2 mm; and for the maximum frequency of stimulation (117 +/ - 15 milliseconds), it was > 2.5 mm. In the transverse direction, crit ical width was between 1.78 and 2.32 mm for a basic cycle length of 20 0 milliseconds. It increased to values between 2.46 and 3.53 mm for a basic cycle length of 150 milliseconds. The overall results demonstrat e that the curvature of the wave front plays an important role in prop agation in two-dimensional cardiac muscle and that changes in curvatur e may cause slow conduction or block.