Left Atrial Appendage Thrombus and Long-Term Clinical Outcomes in Anticoagulated Patients with Atrial Fibrillation and Atrial Flutter Referred for a Rhythm-Control Strategy: A Prospective Observational Cohort Study

Background/Objectives: Whether long-term prognosis differs between patients with atrial fibrillation (AF) and those with atrial flutter (AFL) after adjustment for baseline cardiovascular characteristics remains unclear. Methods: In this prospective observational cohort study, we enrolled 656 patients meeting predefined anticoagulation eligibility criteria with AF (n = 558) or AFL (n = 98) who underwent transesophageal echocardiography (TEE) before a planned rhythm-control intervention. The primary endpoint was left atrial appendage thrombus (LAAT) detected at the index TEE. Long-term outcomes included cardiovascular death, heart failure (HF) hospitalization, ischemic stroke, transient ischemic attack, myocardial infarction, and systemic thromboembolism. Results: LAAT was detected in 14.2% of patients with AF and 10.2% of patients with AFL (p = 0.29). After multivariable adjustment, AFL versus AF was not independently associated with LAAT (adjusted odds ratio [aOR], 0.53; 95% confidence interval [CI], 0.25–1.11; p = 0.09). Median follow-up was 6.85 (3.64–8.53) years. Cardiovascular death occurred in 96 patients (14.6%; AF: n = 78, AFL: n = 18), and HF hospitalization occurred in 91 patients (13.9%; AF: n = 78, AFL: n = 13). Log-rank comparisons were not statistically significant for cardiovascular death (log-rank p = 0.42) or HF hospitalization (log-rank p = 0.73). In extended Cox models that accounted for nonproportional hazards, AFL versus AF was not independently associated with cardiovascular death (adjusted hazard ratio [aHR], 1.09; 95% CI, 0.64–1.85; p = 0.75) or HF hospitalization (aHR, 0.69; 95% CI, 0.37–1.27; p = 0.23). Conclusions: Catheter ablation was recorded in 83.7% of AFL versus 24.6% of AF patients, including index-hospitalization ablation in 46.9% versus 0%, respectively. In this selected cohort meeting predefined anticoagulation eligibility criteria referred for TEE-guided rhythm control, no statistically significant independent association was detected between index arrhythmia type (AF versus AFL) and LAAT, cardiovascular death, or HF hospitalization. Importantly, these findings describe associations under markedly different subsequent rhythm-control treatment pathways and do not establish equivalent intrinsic risks for AF and AFL. The smaller AFL group (n = 98) and wide confidence intervals limit precision; clinically meaningful differences cannot be excluded. These findings should not be extrapolated to unselected AF/AFL populations, including asymptomatic patients, those managed exclusively with rate control, or patients not receiving anticoagulation. The comparison is based on presenting arrhythmia, not a rhythm phenotype confirmed to persist throughout follow-up; incident AF among patients initially presenting with AFL was not systematically ascertained.