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9.1.3 Electrophysiology study Several electrophysiological techniques and manoeuvres can be employed in the electrophysiology laboratory for differential diagnosis of regular narrow QRS tachycardia.45 A detailed discussion is beyond the scope of these Guidelines.

9.2 Wide QRS (>120 ms) tachycardias

Wide QRS tachycardias can be VT, SVT conducting with BBB aberration, or antegrade conduction over an AP, with reported proportions of 80, 15, and 5%, respectively.63 The correct diagnosis of a VT is critical to management, as misdiagnosis and administration of drugs usually utilized for SVT can be harmful for patients in VT.64 Therefore, the default diagnosis should be VT until proven otherwise. The differential diagnosis includes:65

(1) SVT with BBB. This may arise due to pre-existing BBB or the development of aberrancy during tachycardia (so-called phase 3 block), which is more commonly, although not invariably, a right BBB (RBBB) pattern due to the longer RP of the right bundle branch. (2) SVT with antegrade conduction over an AP, ‘pre-excited SVT’, which participates in the circuit (antidromic AVRT) or is a bystander during AF, focal AT/atrial flutter, or AVNRT. (3) SVT with widening of QRS interval induced by drugs or electrolyte disturbances. Class IA and IC drugs cause use-dependent slowing of conduction, and class III drugs prolong refractoriness at HisPurkinje tissue more than in the ventricular myocardium. They can both result in atypical BBB morphologies during SVT that mimics VT. (4) Pacemaker-related endless loop tachycardia and artefacts, which can also mimic VT.

9.2.1 Electrocardiographic differential diagnosis If the 12 lead ECG is available in sinus rhythm, it can provide useful diagnostic information. Specifically, if the QRS morphology is identical during sinus rhythm and tachycardia, the arrhythmia is most likely not a VT. However, bundle branch re-entrant VTs and high septal VTs exiting close to the conduction system can have similar morphologies to sinus rhythm. The presence of a contralateral BBB pattern in sinus rhythm is more indicative of VT.

9.2.1.1 Atrioventricular dissociation The presence of either AV dissociation or capture/fusion beats in the 12 lead ECG during tachycardia are key diagnostic features of VT. AV dissociation may be difficult to recognize because P waves are often hidden by wide QRS and T waves during a wide QRS tachycardia. P waves are usually more prominent in inferior leads and in modified chest lead placement (Lewis lead).63 The relationship between atrial and ventricular events is 1:1 or greater (more atrial than ventricular beats) in most SVTs (Table 9). AVNRT can be associated with 2:1 conduction,66 but this is rare. Although VA conduction can be found in <_50% of patients with VT and a 1:1 relationship is possible, most VTs have a relationship <1:1 (more QRS complexes than P waves). 9.2.1.2 QRS duration A QRS duration >140 ms with RBBB or >160 ms with left BBB (LBBB) pattern suggests VT. These criteria are not helpful for differentiating VT from SVT in specific settings, such as pre-excited SVT, or when class IC or class IA antiarrhythmic drugs are administered.67

9.2.1.3 QRS axis As VT circuits (especially post-MI or in cardiomyopathies) frequently lie outside the normal HisPurkinje network, significant axis shifts are likely to occur, enabling diagnosis. Therefore, in patients with SVT and aberrancy, the QRS axis is confined between -60 and þ120. In particular, extreme axis deviation (axis from -90 to ±180) is strongly indicative of VT, both in the presence of RBBB and LBBB.65

9.2.1.4 Chest lead concordance The presence of negative chest lead concordance (all QRS complexes negative V1V6) (Figure 3) is almost diagnostic of VT, with a specificity >90%, but is only present in 20% of VTs. Positive concordance can be indicative of VT or an antidromic tachycardia utilizing a left posterior or left lateral AP.68

Table 9 Summary of key electrocardiographic criteria that suggest ventricular tachycardia rather than supraventricular tachycardia in wide complex tachycardia

AV dissociation Ventricular rate > atrial rate Fusion/capture beats Different QRS morphology from that

of tachycardia

Chest lead negative concordance All precordial chest leads negative RS in precordial leads - Absence of RS in precordial leads

- RS >100 ms in any leada

QRS complex in aVR • Initial R wave • Initial R or Q wave >40 ms • Presence of a notch of a predomi-

nantly negative complex

QRS axis 290 to 6180 Both in the presence of RBBB and LBBB morphology R wave peak time in lead II R wave peak time >_50 ms RBBB morphology Lead V1: Monophasic R, Rsr’, biphasic qR complex, broad R (>40 ms), and a

double-peaked R wave with the left peak taller than the right (the so-called ‘rabbit ear’ sign) Lead V6: R:S ratio <1 (rS, QS patterns)

LBBB morphology Lead V1: Broad R wave, slurred or notched-down stroke of the S wave,

and delayed nadir of S wave Lead V6: Q or QS wave

AV = atrioventricular; LBBB = left bundle branch block; RBBB = right bundle branch block. aRS: beginning of R to deepest part of S. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026