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Wide complex tachycardia (WCT) is defined as a rhythm with a QRS duration ≥0.12 seconds and a rate >100 bpm. The term is deliberately imprecise, using a general description rather than specific identification of a ventricular or supraventricular origin. WCT poses a unique challenge in electrocardiography, and determining the exact diagnosis often proves difficult and is sometimes impossible. Up until now, we have interpreted the ECG using well-defined criteria. WCT requires a different approach. Here we use ECG clues that help you predict, but not guarantee the likely diagnosis.


A narrow QRS complex requires that both ventricles depolarize in synchrony, concurrently receiving their impulses via the right and left bundle branches. Conduction proceeds uninterrupted through the Purkinje fibers until reaching the ventricular myocardium. Any process that disrupts this continuous passage through the His-Purkinje system will cause asynchronous activation of the ventricles, which is the fundamental principle that results in a wide QRS complex. A WCT is a rapid cardiac rhythm that also exhibits an alteration of ventricular activation. The causes of WCT include:

  • Ventricular tachycardia (VT).

  • Supraventricular tachycardia (SVT) with:

    • Aberrant ventricular conduction.

      • – Fixed (preexisting anatomical bundle branch block).

      • – Functional (physiologic refractoriness during tachycardia).

    • Preexcitation (conduction over an accessory pathway).

    • Other causes of wide QRS (not due to bundle branch block or preexcitation):

      • – Ventricular enlargement.

      • – Congenital heart disease (eg, Ebstein anomaly).

      • – Electrolyte abnormalities (eg, hyperkalemia).

      • – Drug effect (eg, flecainide, tricyclic antidepressants).

  • Ventricular pacing:

    • Pacemaker-tracked supraventricular arrhythmia.

    • Pacemaker-mediated tachycardia.

In most series, 75 to 80% of WCT is due to ventricular tachycardia. SVT with aberrant conduction (either functional or due to preexisting BBB) is the cause in 15 to 25% of cases. Supraventricular arrhythmias associated with preexcitation and pacemaker-associated rhythms are rare, comprising only 1 to 5% of cases. This means our main task is to differentiate ventricular tachycardia from supraventricular tachycardia with aberrancy, something that greatly impacts clinical decision-making.


Electrocardiographers have long sought to find the “Holy Grail” of criteria to distinguish VT from SVT with aberrancy. Although that goal remains elusive, algorithms have been developed in an attempt to predict the likely diagnosis. Before we review the specifics of these approaches, we first need to understand why certain elements are included in the analysis and how to properly perform the measurements. The items in our WCT “toolbox” include examination of the following:

  • The atrial-ventricular relationship (AV dissociation).

  • Precordial lead QRS morphology (V1-V2 and V6).

  • Precordial lead concordance (V1-V6).

  • Limb lead morphology (aVR).

  • Frontal plane axis.

  • QRS duration.

  • Ventricular activation velocity.

Each approach to interpretation uses these elements in a variety of combinations and sequences in an attempt to suggest one or the other diagnosis. We introduced many of these concepts in Chapter ...

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