A 47 year old man presents to the hospital complaining of palpitations. The patient repos that while cooking breakfast this morning, he felt his hea "racing in his chest" and was unable to catch his breath. He states that sitting down brought no relief. He called for an ambulance and he was brought to the emergency depament. The man has no significant past medical history and takes no medications regularly, other than ranitidine for occasional heaburn. On examination, the patient is quite thin, but well developed and in mild distress. His globes appear exophthalmic. His pulse is 140/min and irregularly irregular. There are no murmurs, and the lung examination is clear. A non-tender midline thyroid mass is palpable. Which of the following findings on his echocardiogram would suggest a diagnosis of long- standing atrial fibrillation?