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INTRODUCTION

Wide QRS tachycardia (WCT) is traditionally defined as tachycardia having QRS duration greater than 120 ms, the upper limit of normal QRS duration.1 Tachycardia fitting this general description may be broadly classified by mechanism into 3 groups2,3 as depicted in Figure 13-1 These include ventricular tachycardia (VT), supraventricular tachycardia (SVT) with aberration, and preexcited tachycardia. To these 3, it is useful to consider and rule out paced rhythm (with rate response, tracking of atrial rhythm, or pacemaker-mediated tachycardia) and artifactual tachycardia (Figure 13-2), which may at times look deceptively like VT. Finally, sinus or supraventricular tachycardia with gross ST elevation may occasionally be mistaken for VT. SVT may be defined for this discussion as any tachycardia utilizing the normal atrioventricular (AV) conduction system for ventricular excitation, with the tachycardia either originating in the atria or AV node or requiring 1 of the latter structures for their perpetuation. These tachycardias include AV node reentry, orthodromic atrioventricular reentry, atrial flutter, atrial fibrillation, sinoatrial node reentry, and atrial tachycardia.

FIGURE 13-1

Depiction of wide QRS tachycardia by mechanism. The figure is self-explanatory although pseudo tachycardia due to artefact is not included. Panel A represents VT, Panel B SVT with bundle branch block, Panel C preexcited tachycardia, and Panel D paced rhythm.

FIGURE 13-2

This WCT looks deceptively like VT, which was the reason for referral in this patient asymptomatic at the time the tracing was recorded. The clues to artefact include the broad nonphysiological signals obviously noise that usher the WCT in and out and further nonphysiological behavior such as seen in the second row, next to last arrow where a PVC like complex precedes the normal QRS at an apparent coupling interval that defies reasonable ventricular refractoriness. It is clinched by the observation that the normal QRS complexes (arrows) “march” through the WCT.

Aberration of conduction or aberrancy has been variously defined2,4 but may be simply defined as conduction delay or block in the His-Purkinje system during anterograde conduction of impulses over the normal AV conduction system. Conduction block may be fixed or functional. A sudden acceleration of rate, as with the onset of SVT, is a frequent cause of aberration of conduction that may be perpetuated by continuous concealed retrograde conduction into the blocked pathway. Although any part of the His-Purkinje system may be involved in this functional block, right bundle branch block and left bundle branch block are by far the most frequent aberration patterns.5-8 The term preexcited tachycardia refers to any tachycardia where the ventricles are activated anterogradely over an accessory pathway. The accessory pathway may be a bystander to the tachycardia mechanism as in atrial fibrillation or a part of the circuit or mechanism as ...

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