CIRCUS MOVEMENT ATRIAL-FLUTTER IN THE CANINE STERILE PERICARDITIS MODEL - CRYOTHERMAL TERMINATION FROM THE EPICARDIAL SITE OF THE SLOW ZONEOF THE REENTRANT CIRCUIT

Background. We have shown that atrial flutter (AF) in dogs with steril e pericarditis is commonly due to a single-loop reentrant circuit in t he lower right atrium comprised of a functional or functional/anatomic obstacle and a slow zone of conduction (SZ) between the central obsta cle and the atrioventricular (AV) ring. The goals of the present study were 1) to establish that the epicardial SZ is the critical component of circus movement AF and 2) to identify the optimal site within the epicardial SZ at which interruption of circus movement can be accompli shed by ablative techniques. Methods and Results. We analyzed the atri al activation patterns during epicardial cooling of the SZ with as N2O -cooled probe in eight dogs (five with clockwise [CW] reentrant circui t, one with counterclockwise [CCW] reentrant circuit, and two with bot h CW and CCW reentrant circuits around the same pathway). In all eight dogs, cooling (-5 to +5-degrees-C for 5-20 seconds) the narrow isthmu s at the inferoposterior part of the SZ between the central obstacle a nd the AV ring reversibly terminated the reentrant circuit, whereas co oling outside this area failed to terminate the reentrant circuit. The circus movement was not observed to continue along alternate pathways when conduction in this critical zone was interrupted. Both CW and CC W reentrant circuits could be terminated from the same site within the SZ. Cooling resulted in slowing of conduction in the SZ (55+/-15 msec ) in both CW and CCW reentrant circuits. Cooling-induced termination o f CW reentrant circuits was characteristically associated with oscilla tions of conduction in the cooled zone of the last three cycles before termination and conduction block occurred within the cooled zone. The last ''manifest'' reentrant cycle was associated with the longest con duction delay in the cooled zone. However, this delay was not necessar ily reflected in the length of the last reentrant cycle because of com pensatory acceleration of conduction in the rest of the pathway. On th e other hand, in CCW reentrant circuits, conduction block occurred abr uptly at the distal border of the SZ and without significant oscillati ons of conduction. Conclusions. The present study provides convincing evidence that single-loop circus movement in this model is critically dependent on an obligatory conduction in a SZ in the inferoposterior p ortion of the free right atrial wall between a functional obstacle and the AV ring. Because the atrial myocardium behaves electrophysiologic ally as a two-dimensional surface, the results of this study may help to guide the endocardial electrode catheter ablative technique for tre atment of clinical AF.