Background & Objectives: Selection of appropriate patients for successful cardioversion(CV) of atrial fibrillation(AF) is important. However, there have been no determined predictors for it. Left atrial appendage emptying velocity(LAAEV) and filling velocity(LAAFV) have been suggested to reflect the left atrial function, which is possibly responsible for outcomes of CV and the maintenance of sinus rhythm(SR). But their role and implications still have controversies. We aimed to examine whether the LA functional reserve presented by changes of LAA flow velocities during isoproterenol infusion before CV could be used as a predictor for CV and maintenance of SR. Methods & Results: 36 chronic AF patients were studied (M:F=32:4,mean age=59짹3,mean AF duration=52짹7months). Trnasthoraric echocardiography was done before CV. LAAEV and LAAFV were assessed using transesophageal echocardiography(TEE) at baseline and during isoproterenol infusion with increasing doses to the target heart rate(HR). 3 hours after discountinuing isoproterenol, CV was performed with step-up protocol(70J,100J,150J). 31 patients restored SR(Group 1) and 5 patients failed(Group 2). Baseline echocardiographic and clinical characteristics were similar between 2 groups except that left ventricular ejection fraction(LVEF) was lower in Group 2 than in Group 1(Table). In both groups, diminished LAA flow velocities at baseline increased significantly with the increase of HR during isoproterenol infusion(Table). Among Group 1, 22 patients recurred AF and 8 patients remained SR at 6 month follow up. In this subgroup analysis, there were no differences in changes of LAA flow velocities by isoproterenol between them. Conclusion: Assessment of LA functional reserve by changes of LAA flow velocities during isoproterenol infusion before CV has limited value. Using it as a predictor for successful CV and maintenance of SR remained challenging.