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.................................................................................. 9.2.1.5 Right bundle branch block morphology Lead V1: Typical RBBB aberrancy has a small initial r’, because in RBBB the high septum is activated primarily from the left septal bundle. Therefore, the following patterns are evident: rSR0, rSr0, or rR0 in lead V1. However, in VT, the activation wavefront progresses from the left ventricle (LV) to the right precordial lead V1, in a way that a prominent R wave (monophasic R, Rsr0, biphasic qR complex, or broad R >40 ms) will be more commonly seen in lead V1. Additionally, a double-peaked R wave (M pattern) in lead V1 favours VT if the left peak is taller than the right peak (the so-called ‘rabbit ear’ sign). A taller right rabbit ear characterizes the RBBB aberrancy but does not exclude VT. Lead V6: A small amount of normal right ventricular voltage is directed away from lead V6. As this is a small vector in RBBB aberrancy, the R:S ratio is >1. In VT, all of the right ventricular voltage, and some of the left, is directed away from V6, leading to an R:S ratio <1 (rS and QS patterns). An RBBB morphology with an R:S ratio in V6 of <1 is seen rarely in SVT with aberrancy, mainly when the patient has a left axis deviation during sinus rhythm. Differentiating fascicular VT from SVT with bifascicular block (RBBB and left anterior hemiblock) is very challenging. Features that indicate SVT in this context include QRS >140 ms, overall negative QRS in aVR, and an R/S ratio >1 in V6.44 9.2.1.6 Left bundle branch block morphology Lead V1: As stated above for RBBB, for the same reasons, the presence of a broad R wave, slurred or notched-down stroke of the S wave, and delayed nadir of the S wave are strong predictors of VT. Lead V6: In true LBBB, no Q wave is present in the lateral precordial leads. Therefore, the presence of any Q or QS wave in lead V6 favours VT, indicating that the activation wavefront is moving away from the LV apical site. These morphology criteria are not fulfilled in any lead in 4% of SVTs and 6% of VTs, and in one-third of cases when one lead (V1 or V6) favours one diagnosis, the other favours the opposite diagnosis (VT in one lead and SVT in the other, and vice versa).69,70 A number of algorithms have been developed to differentiate VT from SVT.69,71,72 Detailed presentation and comments are beyond the scope of these Guidelines, and can be found in the 2018 European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Sociedad Latinoamericana de Estimulacion Cardıaca y Electrofisiologıa consensus document.3 All of these criteria have limitations. Conditions such as bundle branch re-entrant tachycardia, fascicular VT, VT with exit site close to the HisPurkinje system, and wide QRS tachycardia occurring during antiarrhythmic drug treatment are difficult to diagnose using the mentioned morphological criteria. Differentiating VT from antidromic AVRT is extremely difficult for the very fact that the QRS morphology in antidromic AVRT is similar to that of a VT, with its origin at the insertion of the AP in the ventricular myocardium. An algorithm has been derived for differential diagnosis, based on the analysis of 267 wide QRS tachycardias, consisting of VT and antidromic Antidromic AVRT VT with negative concordance VT with positive concordance ©ESC 2019 Figure 3 Examples of positive and negative chest lead concordance in VT. AVRT = atrioventricular re-entrant tachycardia; VT = ventricular tachycardia. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026