The supraventricular tachycardias are classified by mechanism in Chapters 6 and 7, where the specific entities are discussed in more detail. This chapter is intended to provide a practical diagnostic and therapeutic approach to the patient presenting with supraventricular tachycardia of unknown mechanism. It will become evident that simple steps can narrow the diagnostic possibilities and suggest appropriate therapy in most patients.1-3
DEFINITION AND CLASSIFICATION
Supraventricular tachycardia may be defined as tachycardia in which the “atrium or atrioventricular junction above the bifurcation of the His bundle is the origin or a critical link in the perpetuation of tachycardia4.” The QRS morphology is normal or “aberrant,” the latter generally being functional right or left bundle branch block. A few notable caveats can be added at this point. Ventricular tachycardia can be relatively narrow depending on mechanism and site of origin. One needs to be especially cautious when only 1 lead or a limited lead array is available. Preexcited tachycardia related to atrioventricular pathways will not resemble bundle branch block but will be indistinguishable from focal VT originating near either AV ring. Preexcited tachycardia over an atriofascicular pathway will resemble relatively typical left bundle branch block because these most frequently insert into the distal right bundle branch region.
A classification based on mechanism as presented in Figure 12-1 is useful for cataloging and discussing tachycardia, but it is not adequate when a supraventricular tachycardia of unknown mechanism presents itself.
Classification of supraventricular tachycardia by mechanism. Mechanisms include AV node reentry (Panel A), AV reentry utilizing an accessory pathway (Panel B), nonparoxysmal junctional tachycardia (Panel D), atrial tachycardia due to atrial reentry or abnormal automaticity (Panel C), and nodoventricular reentry (Panel E). The latter is uncommon but utilizes a nodoventricular accessory pathway as part of the circuit, usually the retrograde limb, and may be indistinguishable from typical AVNRT electrocardiographically. Atrial tachycardia also includes rhythms such as atrial flutter and atrial fibrillation.
A useful working classification should be simple and helpful for guiding therapy. In our suggested classification, all the supraventricular tachycardias can be placed into 1 of 2 categories based on the requirement of the atrioventricular (AV) node for the perpetuation of tachycardia (Figure 12-2). The tachycardia is “independent” of the AV node when the arrhythmia “generator” is above the level of the AV node. The AV node-independent tachycardias are not influenced by AV block and comprise all forms of atrial tachycardia, including atrial reentrant tachycardia, atrial flutter, and atrial fibrillation. With rare exceptions,5 continuation of supraventricular tachycardia in the presence of AV block classifies the tachycardia as AV node-independent (Figure 12-3). On the other hand, the tachycardia is “dependent” on the AV node when the ...