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Figure 5-1: Explanation

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Figure 5–1A

A 33-year-old man with a history of recurrent palpitations and documented wide QRS tachycardia undergoes electrophysiologic evaluation. His 12-lead ECG is normal. The following was observed (figure). What is the most likely mechanism of tachycardia?

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The initial four QRS complexes have an atypical right bundle branch block morphology suggesting either ventricular tachycardia (VT) or a preexcited tachycardia. There is 1:1 AV relationship with the earliest atrial electrogram in the septum. In such a situation, the differential diagnosis includes VT with 1:1 VA conduction, atrial tachycardia or AV node reentry with anterograde conduction over an accessory pathway (AP), and antidromic tachycardia with anterograde conduction over an AP and retrograde conduction over either a second septal AP or the normal VA conduction system.

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An atrial stimulus (S2) introduced during tachycardia at the MCS electrode site terminates tachycardia without conduction to the ventricle. This eliminates VT as a diagnosis. Termination with a relatively late-coupled PAC also favors a macroreentrant tachycardia with close access of the PAC to the excitable gap, that is, antidromic reentry in this case. Additional pacing maneuvers during the study proved the mechanism to be antidromic tachycardia with retrograde conduction over the normal VA conduction system.

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The last two QRS complexes show no evidence of anterograde AP conduction. In fact, there is not even local evidence of preexcitation on the coronary sinus leads as demonstrated by a long conduction time between the local A and V (arrow). In a patient with antidromic reciprocating tachycardia (ART) there is typically some evidence of anterograde AP activation during sinus rhythm. This was a very unusual AP that only became manifest with multiple premature atrial extrastimuli that resulted in block over the AV node and a very long conduction time over the AP.

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Figure 5-2: Explanation

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Figure 5–2A

A 46-year-old man with a history of recurrent palpitations and documented wide complex QRS tachycardia undergoes electrophysiologic study. His 12-lead ECG during tachycardia is shown in the figure. What are the diagnostic possibilities?

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The tachycardia exhibits an atypical left bundle branch block morphology that is less likely aberrancy and more consistent with either VT or preexcited tachycardia. If it is a preexcited tachycardia, it is conducting anterogradely over a right free wall AP. There appears to be a negative P wave before each QRS complex in ECG leads II, III, and aVF. If this is VT, 1:1 retrograde conduction is most likely over a slow AV nodal pathway. A preexcited tachycardia ...

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