Aims Mitral isthmus (MI) ablation is technically challenging, requiring long endocardial ablation times and frequently coronary sinus (CS) ablation. The circumflex artery lies in the epicardium in close proximity to the CS and the mitral annulus and may potentially be injured during radiofrequency ablation. Methods and results Fifty-four patients underwent catheter ablation procedures that included MI ablation for treatment of atrial fibrillation. Irrigated ablation catheters were used with the

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... following settings: endocardial surface (max power: 40/50 W at the annular end; max temperature: 488C); CS (max power: 25/30 W; max temperature: 488C). Coronary angiography was performed pre-and post-ablation and analysed by two cardiologists with quantitative coronary angiography. Mitral isthmus block was achieved in 89% of patients (60% required CS ablation). Fifteen patients (28%) had angiographic changes following ablation: eight had mid-circumflex narrowing only, one had circumflex and obtuse marginal (OM) artery narrowing, one had OM narrowing only, and five had distal circumflex occlusion/narrowing. Five patients had significant narrowing (50-84%), which resolved with intracoronary glycerine trinitrate. Fourteen (93%) of the patients with circumflex 'injury' had CS ablation and a longer mean CS ablation time (5.0 + 3.0 vs. 2.6 + 3.3 min, P ¼ 0.03). Patients with distal circumflex occlusion had significantly smaller vessel diameter (1.0 + 0.1 vs. 2.1 + 0.2 mm, P ¼ 0.03). A shorter distance between the circumflex and the CS was also associated with circumflex 'injury' (3.2 + 1.9 vs. 5.6 + 3.2 mm, P ¼ 0.04). There were no electrocardiographic or echocardiographic abnormalities and no angina symptoms during follow-up. Conclusion Acute sub-clinical circumflex 'injury' following MI ablation is not uncommon. Ablation within the CS, proximity of the circumflex and the CS, and a small distal circumflex were risk factors for 'injury'. ---