Why the atrial fibrillation causing thromboembolic phenomena while atrial flutter don't cause it?

With A-Fib: Blood pools in the left atrial appendage (auricle) because the atrium does not contract. With flutter the atrium contracts (and empties) Static blood tends to clot.

See below: Atrial fibrillation is completely uncoordinated electrical activity in the atria. This leads to virtually no atrial contraction. Blood stagnates, and you get clots. Flutter is an electrical circuit, so there is some contraction. Several studies have shown less embolic events with flutter, but it is still possible. Hope this helps!

Short answer: Atrial fib has increased risk secondary to irreg.firing of atria and valves causing damage to blood cells. In flutter there is a more regular rate and atrial contractile function offers some protection but there is still a slightly increased risk. In chronic flutter, coagulation is necessary given the risk of stroke is the same as atrial fib. Pts with chronic flutter often have a fib as well.