BACKGROUND: Several different patterns of wavebreak have been described by mapping of the tissue surface during fibrillation. However, it is not clear whether these surface patterns are caused by multiple distinct mechanisms or by a single mechanism. METHODS: To determine the mechanism by which wavebreaks are generated during ventricular fibrillation, we conducted optical mapping studies and single cell transmembrane potential recording in 6 isolated swine right ventricles. RESULTS: Among 763 episodes of wavebreak (0.75 times/sec/cm2), optical maps showed 3 patterns: 80% due to a wavefront encountering the refractory waveback of another wave, 11.5% due to wavefronts passing perpendicularly each other and 8.5% due to a new (target) wave arising just beyond the refractory tail of a previous wave. Computer simulations of scroll waves in 3-D tissue showed that these surface patterns could be attributed to two fundamental mechanisms: head-to-tail interactions and filament break. CONCLUSION: We conclude that during sustained ventricular fibrillation in swine RV, surface patterns of wavebreak are produced by two fundamental mechanisms: head-to-tail interaction between waves and filament break.