A LOGICAL STATE MODEL OF CIRCUS MOVEMENT ATRIAL-FLUTTER - ROLE OF ANATOMIC OBSTACLES, ANISOTROPIC CONDUCTION AND SLOW CONDUCTION ZONES ON INDUCTION, SUSTENANCE, AND OVERDRIVE PACED MODULATION OF REENTRANT CIRCUITS

Mapping studies of atrial flutter in both the canine sterile pericardi tis model and the right atrial enlargement model commonly reveal singl e loop reentrant circuits in the lower posterior part of the right atr ium. Functional bidirectional conduction block and natural anatomical obstacles comprise the central obstacle for reentrant impulse during c ircus movement atrial flutter. Because the relative roles of anatomica l obstacles, in combination with functional barriers, anisotropic cond uction, and slow conduction can not be readily assessed with current e lectrophysiological techniques, an atrial activation model was develop ed to study the mechanisms of circus movement atrial flutter. A discre te state model consisting of 4096 logically connected cardiac elements was used to simulate atrial activation; an inexcitable region simulat ing the inferior vena cava (IVC) was also incorporated in the model. A trial flutter was induced by programmed premature stimulation. Anisotr opic conduction velocity properties, regional variations in slow condu ction, regional refractory gradients and stimulation parameters were s pecified for each simulation. The reentrant circuit generally consiste d of a single reentrant impulse which circulated around a continuous l ine of functional bidirectional conduction block joined to the IVC. Ra pid pacing, 5-30 ms shorter than the spontaneous reentrant cycle lengt h, was applied to entrain and/or terminate the rhythm. The results of this study demonstrate that patterns of initiation, entrainment, termi nation and reinitiation of circus movement atrial flutter mimic result s from in vivo activation mapping studies. We find that sustained circ us movement atrial flutter circuits depend on: 1) natural anatomical o bstacles to stabilize reentrant circuits, and 2) anisotropic conductio n properties to reduce the degree of functional conduction block neede d to maintain circus movement. Rapid pacing of simulated circus moveme nt atrial flutter demonstrated that the entrainment criteria can be sa tisfied in a two-dimensional syncytium.