Unfortunately, I can't remember who the author of this ECG is. Perhaps he/she will get in touch with me. However, the ECG is very nice (despite the less than optimal recording quality) and that's why I want to post it. I don't know the history.
In the first section of the ECG we see coarse fibrillation waves and QRS complexes with very different RR intervals, which is why this is atrial fibrillation. It is not atrial flutter because the fibrillation waves are of different size and polarity (depending on the lead) and the intervals between the atrial actions are irregular.
The atrial fibrillation ends, after a short pre-automatic pause (less than 1500 ms) a sinus node beat follows, then a PAC. After another short pause, another sinus node beat follows, then most probably atrial fibrillation again