What could this rhythm be? And what is going on after it breaks?

Written by Pendell Meyers, edits by Steve SmithA man in his 30s with no prior medical history presented with palpitations. His vitals and exam were within normal limits with the exception of his heart rate.Here is his initial ECG:What do you think? What is the differential?This shows a regular narrow complex tachycardia, thus the differential is sinus tach, SVT (including AVNRT most commonly, AVRT [which is SVT in the setting of WPW with an accessory pathway] next most common, then atrial tachycardia, etc.) or atrial flutter.It it were atrial flutter, and because it is regular at a rate of 210, the flutter would have to be conducting 1:1 to the ventricle. There are no definite atrial waves, although the negative deflections in V1 before the QRS complexes could be atrial in origin, or the entire baseline may appear to have a sawtooth pattern in lead II - it is not clear in my opinion. 1:1 atrial flutter seems unlikely because, as it would have to be 1:1 conduction, the atrial flutter rate would have to be a very slow 210 bpm (normal would be 300). The ventricular rate of roughly 210 bpm makes sinus tachycardia unlikely in a 30 year old (max sinus rate theoretically 190ish according to 220-Age rule of thumb).Aside: if the atrial flutter rate were 300 bpm, it could not be conducted by the AV node, and the ventricular rate could only be 300 if there were an accessory pathway or a peri-AV node bypass tract that inserts into the bundle of His. In the former case, there wo...
Source: Dr. Smith's ECG Blog - Category: Cardiology Authors: Source Type: blogs