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E

The ECG displayed shows a narrow complex irregular rhythm, and no two R-R intervals in the lead II rhythm strip at the bottom of the image are the same (hence the term "irregularly irregular" that is often used of atrial fibrillation). Inspection of leads III and V1, in particular, shows that the atrial activity is very disorganised and irregular. P waves are not seen in atrial fibrillation; instead there are irregular fibrillatory waves at a rate of 400 to 600 per minute, only a proportion of which are conducted to the ventricles, to produce a rapid irregular ventricular rate (in the presence of normal AV conduction).