We report a case of a patient with long-standing persistent atrial fibrillation (AF) who had rapid formation of spontaneous echo-contrast in the left atrium during pulmonary vein antrum isolation set off by a vagally mediated pause despite standard anticoagulation protocol. Spontaneous echo contrast resolved with ventricular pacing, representing visual evidence for dependence of some AF patients with poor atrial transport function on ventricular emptying with potential greater risk of thromboembolism related to a long ventricular pause.