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Figure 4-1: Explanation

Figure 4–1A

A 35-year-old woman with a history of palpitations and supraventricular tachycardia was noted to be in this rhythm after catheters were placed in her heart. A maneuver was performed to identify the mechanism of tachycardia. Has this helped?

This is a relatively slow supraventricular tachycardia with a His electrogram preceding each QRS complex (HBED lead). The cycle length of tachycardia varied slightly from 726 to 735 milliseconds. The differential diagnosis includes an automatic junctional rhythm, slow–fast AV node reentry, or an atrial tachycardia with anterograde conduction over a slow pathway. The very short VA interval excludes AV reentry. A premature atrial complex is introduced at a time when the His bundle electrogram has already been activated and results in a shortening of the subsequent H–H interval to 657 milliseconds. There should be no change in the subsequent H–H interval if the mechanism is an automatic junctional rhythm because the junction would have already discharged and therefore the premature atrial complex could not have affected the subsequent cycle length. The shortening of the H–H interval is consistent with slow–fast AV node reentry in which the premature atrial complex engages a slow pathway earlier than anticipated and affects the next H–H interval. It could have also prolonged the next H–H interval and that would have still supported this diagnosis. Can one totally rule out an atrial tachycardia with conduction over a slow pathway? Note that the His to high rate interval remains constant even though the H–H interval changes, a finding consistent with AV node reentry but not atrial tachycardia. Indeed, this patient had relatively slow AV node reentry that was successfully ablated at a site around the coronary sinus ostium.

Figure 4-2: Explanation

Figure 4–2A

A 59-year-old woman underwent electrophysiologic evaluation for recurrent episodes of tachycardia. She had known right bundle branch block. Explain how initiation of tachycardia occurs, the most likely mechanism for it, and the tachycardia diagnosis.

A single atrial premature stimulus is introduced at 340 milliseconds during an atrial paced cycle length of 500 milliseconds. The premature complex conducts over the AV node and induces a short VA interval tachycardia that is most consistent with AV node reentry. This mode of induction is referred to as a two-for-one response and seems to break the laws of initiation for a reentrant circuit. In other words, a single premature complex is conducted over both the fast and slow AV nodal pathways to initiate tachycardia.

Classical teaching of reentry proposes three requisites to form the circuit: two pathways of conduction, initial ...

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