After seeing this video till will be clear why there is no 'p' wave in ECG in patients with Atrial fibrillation. In Atrial fibrillation, atrium doesn't contract it just ripples like waves....so no 'p' waves. On the other hand in Atrial flutter, atrium contracts but at a very fast rate, out of sync with ventricle to produce a series of 'p' waves called Flutter waves.