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A 50-year-old patient with type IV Wolff-Parkinson-White (WPW) syndrome is shown, who presents with a crisis of atrial fibrillation (A) and atrial flutter (B) that mimics ventricular tachycardia. The diagnosis of atrial fibrillation is supported by the history (knowing that the patient has WPW syndrome) and the following characteristics of the ECG: (1) the wide complexes have a very irregular rhythm and are more or less wider (present more or less pre-excitation); and (2) the narrow complexes (the sixth and the last one on the top) are sometimes close (the last complex) and sometimes far (sixth complex) to the previous QRS. In sustained ventricular tachycardia, the QRS complexes are regular, and in the presence of narrow complexes, the QRS complexes are always close to the previous one (capture beats). B. In WPW syndrome with flutter, the differential diagnosis with sustained ventricular tachycardia based Source: Chapter 15. Surface Electrocardiography, Hurst's The Heart, 13e only on ECG is more difficult because the RR are regular. C. Patient with crisis of atrial fibrillation with a very fast response of the ventricles (>300 ×′) and, Citation: Fuster Walsh RA, Harrington RA. TheR-R Heart, 13e; 2011 Available at: http://mhmedical.com/ Accessed: Maywhich 03, 2017 sometimes, very narrow R-RV,intervals (<200 ms). After a Hurst's very short interval, a crisis of ventricular fibrillation was triggered (arrow), had to be Copyright © 2017 McGraw-Hill Education. All rights reserved resolved by electric cardioversion. D. Patient with reciprocating tachycardia. The conduction in this circuit is retrograde over the accessory AV pathway and anterograde via the normal AV conduction. The RP′ ratio is smaller than P′R ratio, which is typical for reciprocating tachycardia that involves an accessory AV pathway.