A patient complaining of chest discomfort. What is the rhythm?
This is an irregular wide ("bizarre") complex tachycardia at at a rate of about 250 bpm.
"The presence of WPW syndrome is suspected from a rhythm strip the ventricular rate during atrial fibrillation is during AF exceeds 200 bpm. Such a rapid ventricular response is unusual if the impulses were conducted via the normal AV conduction system".
Reference: Surawicz B and Knilans TK. 2008. Chou’s Electrocardiography in Clinical Practice. 6th ed. PA. Saunders-Elseiver
This is the 12L during sinus rhythm showing the hallmarks of WPW: short PRI, delta wave and widening of the QRS
Here is also a comment courtesy of Jason Roediger:
"Atrial fibrillation predominantly conducted with preexcitation. Wolff-Parkinson- White (W-P-W) syndrome. Based on the negative polarity of the QRS complex in lead V1, the accessory pathway is probably situated on the right-side of the heart. Only the 8th, 9th, and 10th beats are conducted normally whereas the remainder of them have Delta waves. It is highly unlikely that you would see this level of irregularity in ventricular tachycardia. Note that the longest RR interval, between the 13th and 14th beats, is greater than twice the shortest RR interval between the 4th and 5th beats. That's more than 100% difference between the two RR intervals. Superficially, this is mimicking LBBB, however what looks like fat little r-waves in the anteroseptal leads are actually Delta waves. When LBBB is seen with little r-waves in these leads, they are usually very narrow."