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 ):
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.
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).
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.
Pacemaker-related endless loop tachycardia and artifacts can also mimic VT.
Wide-QRS (>120 ms) tachycardias
Ventricular paced rhythm
Antidromic AV reentrant tachycardia
Supraventricular tachycardias with aberration/bundle branch block (preexisting or rate-dependent during tachycardia)
Atrial or junctional tachycardia with preexcitation/bystander AP
Supraventricular tachycardia with QRS widening caused by electrolyte disturbance or antiarrhythmic drugs
AF or atrial flutter or focal AT with varying AV block conducted with aberration
Antidromic AV reentrant tachycardia caused by a nodoventricular/fascicular AP with variable VA conduction
Torsades de pointes
Regular wide-QRS (>120 ms) tachycardias
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.
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).
|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
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.
Because VT circuits (especially after myocardial infarction [MI] or in cardiomyopathies) often lie outside the normal His-Purkinje network, significant axis shifts are likely to occur, enabling diagnosis. Therefore in SVT patients with aberrancy the QRS axis is confined between –60 degrees and +120 degrees. Εxtreme axis deviation (axis from –90 degrees to ±180 degrees) in particular is strongly indicative of VT in the presence of both RBBB and LBBB.
Chest lead concordance
The presence of negative chest lead concordance (all QRS complexes negative V1–V6) is almost diagnostic of VT with a specificity of more than 90% but only present in 20% of VTs ( Fig. 11.2 ). Positive concordance can be indicative of VT or an antidromic tachycardia using a left posterior or left lateral AP.
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: rSR′, rSr′, or rR′ in lead V1. However, in VT the activation wave front progresses from the left ventricle (LV) to the right precordial lead V1, in a way that a prominent R wave (monophasic R, Rsr′, 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 favors 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.
A small amount of normal right ventricular (RV) voltage is directed away from V6. Because this is a small vector in RBBB aberrancy, the R/S ratio is greater than 1. In VT, all the RV voltage, and some of the left, is directed away from V6, leading to an R/S ratio less than 1 (rS, QS patterns). An RBBB morphology with an R/S ratio in V6 of less than 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 greater than 140 ms, r′ in V1, overall negative QRS in aVR, and R/S ratio greater than 1 in V6.
As stated earlier for RBBB, for the same reasons, the presence of broad R wave, slurred or notched down stroke of the S wave, and delayed nadir of S wave is a strong predictor of VT.
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 favors VT, indicating the activation wave front is moving away from the LV apical site.
Algorithms for electrocardiographic differential diagnosis
A number of algorithms have been developed to differentiate VT from SVT. The most established are the Brugada algorithm and the Vereckei algorithm, which uses a single lead aVR.
Rs interval in precordial leads
The absence of RS complex in precordial leads (only R and S complexes are seen on ECG) is only found in VTs ( Fig. 11.3 ). An RS complex is found in all SVTs and in 74% of VTs. The longest interval from the onset of the R wave to the deepest part of the S wave longer than 100 ms, irrespective of the morphology of the tachycardia, is not observed in any SVT with aberrant conduction. About half of the VTs have an RS interval of 100 ms or less and the other half have an RS interval of more than 100 ms. The Brugada et al. algorithm has a sensitivity and specificity of 98.7% and 96.5%, respectively.