The aim of this study was to determine the relationship between the size of discontinuities in lines of ablation and wavefront propagation. Discontinuities in linear radiofrequency lesions used for the treatment of atrial fibrillation may be proarrhythmic and a major clinical problem. A better understanding of the electrophysiological properties of these discontinuities (isthmuses) may assist in their detection and treatment. Linear lesions were made in the right atrial free wall using a Nd:YAG laser in 12 dogs. Conduction properties across the discontinuities were studied by pacing from either side of the lesion. Two of the three isthmuses less than 0.8 mm(2) in cross section (smallest 0.2 mm(2)) conducted at extrastimulus intervals of 300 ms. All three failed to conduct at cycle lengths close to the atrial effective refractory period. Isthmuses above 0.8 mm(2) (n¿=8) conducted at all cycle lengths. Conduction slowing (mean slowest conduction 0.5 +/- 0.3 m/s) occurred in the region of the isthmus but the overall delay was only 6 +/- 6 ms where propagation through the isthmus occurred. The effect on conduction of small discontinuities in linear lesions is dependent on the size of the residual isthmus. All but the very smallest of discontinuities in linear lesions conduct and therefore have the potential to participate in reentrant arrhythmias. Efforts should be directed toward the development of ablation techniques that reliably produce continuous transmural linear lesions for cure of atrial fibrillation and flutter.