OBJECTIVETo determine whether the extent of prolongation of the transisthmus interval after ablation predicts complete bidirectional block.

METHODSSince 1996 to 2002, 30 consecutive patients underwent ablation procedures for isthmus-dependent atrial flutter. There were 23 males and 7 females [mean age (47.85 +/- 9.35) years]. With the use of fluoroscopic view of anatomy, radiofrequency ablation was performed during coronary sinus pacing at a cycle length of 600 ms.

RESULTSBidirectional block was achieved with ablation in 29 (97%) of 30 patients. The transisthmus intervals before ablation and after complete transisthmus block were (73.82 +/- 13.01) ms and (140.47 +/- 20.48) ms, respectively, in the clockwise direction (P < 0.0001), and (77.63 +/- 8.36) ms and (138.17 +/- 15.55) ms, respectively, in the counterclockwise direction (P < 0.0001). A period of incomplete isthmus block was observed during 17 (58%) of the 29 ablation procedures. The clockwise transisthmus intervals during incomplete block [(107.65 +/- 21.33) ms] were (45.5 +/- 8.7)% longer than the baseline transisthmus intervals. An increase in the transisthmus interval by > or = 50% in both directions after ablation predicted complete bidirectional block with 100.0% sensitivity and 83.3% specificity. The positive and negative predictive values were 90.6% and 100.0%, respectively. The diagnostic accuracy of a > or = 50% prolongation in the transisthmus interval was 83.3%.

CONCLUSIONThe analysis of transisthmus interval is a valuable method for determining complete bidirectional isthmus block.