744 Atrial Tachycardias Originating from the Atrial Septum: Electrophysiologic Characteristics and Radiofrequency Ablation CHIEN-CHENG CHEN, M.D,,* CHING-TAI TAI, M,D., CHERN-EN CHIANG, M,D., WEN-CHUNG YU, M.D.. SHIH HUANG LEE, M.D.. YI-JEN CHEN, M.D., MING-HSIUNG HSIEH, M.D., CHIN-FENG TSAI. M,D,, KUNG-WEI LEE, M.D.,* YU-AN DING, M.D,, MAU SONG CHANG, M.D., and SHIH ANN CHEN. M.D. Frotn the Division of Cardiology, National Yang-Ming Univet^ity. School of Medicine, and Taipei Veterans General Hospital, Taipei, and *Show-Chwan Memorial Hospital, Changhua. Taiwan, Republic of China Atrial Tachycardia. Introduction: The characteristics of atrial tachycardia (AT) have varied widely among different reporLs, The anatomic locations of ATs may bias the results. We propose that septal ATs and free-wall ATs have different characteristics. Methods and Results: One hundred forty-one patients with AT underwent electr(»pharmacolof>ic study, endocardial mappin}^. and radiofrequency ahlation. Forty-nine O4J%) patients had septal AT originating from the anteroseptal, mid-septal, and posteroseptal areas. Tachycardia cycle length was similar hetween septal AT and free-wall AT (367 ± 46 msec vs 366 ± 58 msec, P > 0,«5l, More patients with septal .\T required isoproterenol to facilitate induction (44.9f/f vs 31.591^, P < 0,05). Septal AT was more sensitive to adenosine than free-wall AT (84.4% vs 67.8%, P < 0.05). Only posteroseptal AT showed a positive P wave in lead V, and negative P wave in all the inferior leads (II, III, aVF). Radiofrequency catheter ahlation had a comparahle success rate for septal AT and free-wall AT (96% vs 95%) without impairment of AV conduction. During follow-up of 49 ± 13 months (range 17 to 85), the recurrence rate was similar for septal AT and free-wall AT (3.2% vs 4.6%, P = 0.08). Conclusion: Septal AT has electrophysiologic characteristics that are distinct from those of free-wait AT. Catheter ahlation of the septal AT is safe and effective. (J Cardiovasc Elcctrophxsiol. Vol. II. pp. 744-749. July 2000) atrial tachycardia, ahlation Introduction The electrophysiologic mechanisms and anatomic dis- tribution of atrial tachycardia (AT) have been studied.'' However, the electrophysiologic properties of AT origi- nating from tbe atrial septum have not been well dem- onstrated. The purpose of tbis study was to investigate the 12-Iead surface ECG, electropbysiologic properties, and results of radiofrequency ablation in patients witb septal AT. Supported in part by Grants NSC 88-2314-B-0I0-094, 88-23I4-B-075- 0(i5. 88-2314.B-075 090. 88-2314-B-OIO.()93 from the National Sci- ence Council, and Granis VTY88-P5-45. VGH-23. 36, 47. 61, 63. 64, 65. 2.'i4. 301 from Tzou's Medical Foundation. Taipei, Taiwan, Re- public of China. Address for correspondence: Shih-Ann Chen, M.D., Division of Car- diology. Depanment of Medicine. Veterans General Hospital-Taipei. 201 Section 2. Shih-Pai Road, Taipei, Taiwan. Republic of China. Fax: 886-2-2R73-5656; E-mail: sachen@vghlpe.gov.tw M;iiiuscript received 28 December IW): Accepted for publication 28 March 2000. Methods Patient Characteristics From November 1992 to July 1998, 141 patients (75 men and 66 women, mean age 53 ± 13 years) witb AT were referred to this institution for electrophysiologic study and radiofrequency catbeter ablation. Tbe histories of AT ranged from 2 to 18 years (mean 3 ± I). Fotiy- nine patients bad AT foci in tbe atrial septum, and 92 patients had AT foci in the free wall. Tbese patients were refractory to. or intolerant of, 2 t: 1 antlarrbytbmic drugs. Baseline Electrophysiologic Study As described previously," patients were studied in the postabsorptive, nonsedated state. AI! antiarrhytbmic drugs were discontinued for at least five balf-Hves. Tbree niullipuiar, closely spaced (2 mm) electrode catheters (Mansfield Scientific, Mansfield, MA, USA) were in- serted percutaneously into the rigbt and left femoral