ENDOCARDIAL MAPPING OF REENTRY AROUND AN ANATOMICAL BARRIER IN THE CANINE RIGHT ATRIUM - OBSERVATIONS DURING THE ACTION OF THE CLASS IC AGENT, FLECAINIDE

Introduction: Flecainide is effective in terminating stable atrial flu tter in the conscious dog with a Y-shaped right atrial lesion. In this model, flutter is due to circus movement of the impulse around a fixe d anatomical barrier. Methods and Results: To investigate the mechanis m of flecainide-induced termination of this type of reentry, we determ ined the pattern of endocardial activation of the right and left atria before and during administration of flecainide by recording simultane ously from 192 electrode pairs in the isolated blood perfused heart. A t least five consecutive flutter beats were analyzed before and during flecainide for each of eight termination episodes in five hearts. In all, flecainide increased flutter cycle length (164 +/- 24 msec) by 89 % to 309 +/- 17 msec (P < 0.05) before termination. Atrial refractory period and conduction time during paced beats were also increased by f lecainide. In five episodes, termination was due to conduction block o f the impulse at critical sites within the reentrant circuit (mode 1). Cycle length oscillations (+/- 30 msec) at sites proximal to site of block preceded termination in three of these episodes. In three other episodes, interruption of the original circuit occurred when there was failure of a lateral boundary, giving rise to an impulse that reset t he original circuit (mode 2). In these episodes, long-short cycle leng th oscillations led to return reexcitation by the impulse within the p rimary path and subsequent termination. Conclusion: In summary, simila r to our previous findings with the Class III agent, d-sotalol, two di fferent modes of termination of atrial reentry were observed with flec ainide.