First, you are right that before cardioversion, one must be certain that no clots have formed in the atria. Actually, the risk
is not highest while being in atrial fibrillation
, but when converting to sinus rhythm (naturally or by cardioversion). When the left atrium is not beating (during a-fib), the clots usually just stay where they are, but when the atria are beating again, clots may break off the walls and follow the blood stream.