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Aims Radiofrequency catheter ablation of atrial flutter and fibrillation may be favoured by large, elongated lesions. We compared bipolar ablation with unipolar ablation from one or two electrodes in the porcine heart. Methods and Results In vitro, confluent lesions were reliably created by a ‘dielectrode’ catheter (energy delivered simultaneously (in parallel) from two 4mm electrodes spaced 1mm apart, towards an indifferent electrode), and a ‘bipolar’ catheter (energy delivered (in series) between two 4mm electrodes spaced 5mm apart). Sixteen anaesthetized pigs were randomized to standard unipolar (4), dielectrode (6) or bipolar (6) ablation. Two radiofrequency current deliveries of 30s duration (70°C) were administered to the inferior vena cava–tricuspid valve isthmus and two to the right atrial free wall in all animals. After 4h, the lesions were examined macroscopically and histologically. Mean (SD) endocardial lesion length×width×depth measured 7·4 (2·4)×5·4 (2·2)×2·8 (0·8)mm in the standard unipolar mode, 10·2 (1·4)×6·3 (0·7)×3·3 (1·1)mm in the dielectrode mode and 14·0 (3·6)×6·0 (1·7)×3·8 (1·2)mm in the bipolar mode. Thus lesion length increased significantly through the three groups (P<0·001), while width and depth did not. Conclusion Both dielectrode and bipolar ablation were feasible in porcine right atrial ablation, and created longer lesions than the standard unipolar mode. By allowing a larger interelectrode distance, bipolar ablation created the longest lesions and may be favourable when linear lesions are necessary. [ABSTRACT FROM PUBLISHER]