Differential diagnosis of wide-QRS (>120 ms) tachycardias

Wide-QRS tachycardias can be ventricular tachycardia (VT), supraventricular tachycardia (SVT) conducting with bundle branch block (BBB) aberration or through a bystander accessory pathway (AP), or an AP-mediated tachycardia, with reported proportions of 80%, 15%, and 5%, respectively. ^, The correct diagnosis of VT is critical to management because misdiagnosis and administration of drugs usually used for SVT can be harmful for patients in VT. Therefore the default diagnosis should be VT until proven otherwise. The differential diagnosis includes the following ( Box 11.1 ):

• 1.

  SVT with BBB. This may arise as a result of preexisting BBB or the development of aberrancy during tachycardia (so-called phase 3 block), which is more commonly a right bundle branch block (RBBB) pattern, because of the longer RP of the right bundle branch.

• 2.

  SVT with antegrade conduction over an AP (“preexcited SVT”) that participates in the circuit (antidromic atrioventricular reentrant tachycardia [AVRT]) or is a bystander during atrial fibrillation (AF), focal atrial tachycardia/flutter (AT/AFL), atrioventricular nodal reentrant tachycardia (AVNRT).

• 3.

  SVT with widening of QRS interval induced by drugs or electrolyte disturbances. Class IC and IA drugs cause use-dependent slowing of conduction, and class III drugs prolong refractoriness at His-Purkinje 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 artifacts can also mimic VT.

Electrocardiographic differential diagnosis

If the QRS morphology is identical during sinus rhythm and tachycardia, then this is most likely not a VT. However, bundle branch reentrant 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.

AV dissociation

The presence of either atrioventricular (AV) dissociation or capture/fusion beats in the 12-lead electrocardiogram (ECG) during tachycardia is a key diagnostic feature of VT ( Table 11.1 and Fig. 11.1 ). 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 modified chest lead placement (Lewis lead). The relation between atrial and ventricular events is 1:1 or greater (more atrial than ventricular beats) in most SVTs. AVNRT can be associated with 2:1 conduction, but this is rare. Although ventriculo-atrial conduction can be found in up to 50% of patients with VT and a 1:1 relation is possible, most VTs have a relation less than 1:1 (more QRS complexes than P waves).

Table 11.1

Summary of Key ECG Criteria That Suggest VT Rather Than SVT in Wide-Complex Tachycardia

Brugada J, Katritsis DG, Arbelo E, et al. 2019 ESC Guidelines for the management of patients with supraventriculartachycardia Eur Heart J. 2020;41(5):655-720.

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 lead *
QRS complex in aVR	Initial R wave Initial R or Q wave > 40 ms Presence of a notch of a predominantly negative complex
QRS axis –90 degrees to ±180 degrees	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; ECG, electrocardiogram; LBBB, left bundle branch block; RBBB, right bundle branch block; SVT, supraventricular tachycardia; VT, ventricular tachycardia.

* RS: beginning of R to deepest part of S.

QRS duration

A QRS duration greater than 140 ms with RBBB or greater than 160 ms with left BBB (LBBB) pattern suggests VT. These criteria are not helpful for differentiating VT from SVT in specific settings, such as preexcited SVT or when class IC or class IA antiarrhythmic drugs are administered.

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