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Chapter 5: Narrow QRS Tachycardia

CASE 5–1A

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 maneuver helped?

An ECG of 35-year-old woman with a history of palpitations and supraventricular tachycardia in a specific rhythm after catheters were placed in heart. The rhythm was a relatively slow supraventricular tachycardia with a His electrogram preceding each QRS complex, as indicated by the HBED lead. The cycle length of the tachycardia varied slightly from 726 to 735 milliseconds. A maneuver was performed to differentiate between possible mechanisms of tachycardia, including automatic junctional rhythm, slow-fast AV node reentry, or atrial tachycardia with anterograde conduction over a slow pathway. The maneuver involved introducing a premature atrial complex and observing its effects on the subsequent cycle length and His-His (H-H) interval. The results of the maneuver indicated that the premature atrial complex caused a shortening of the H-H interval to 657 milliseconds, suggesting the engagement of a slow pathway earlier than anticipated. This finding, along with the constant His to high-rate interval and the lack of change in the VA interval, supported the diagnosis of slow-fast AV node reentry.
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CASE 5–1B

An ECG of a 35-year-old woman with history of palpitations and supraventricular tachycardia in a specific rhythm after catheters were placed in heart. The rhythm observed was a relatively slow supraventricular tachycardia, evident by the presence of a His electrogram before each QRS complex, as seen in the HBED lead. The tachycardia had a variable cycle length, ranging from 726 to 735 milliseconds. To distinguish between potential mechanisms of the tachycardia, which included automatic junctional rhythm, slow-fast AV node reentry, or atrial tachycardia with conduction over a slower pathway, a specific maneuver was executed. This maneuver involved introducing a premature atrial complex and closely monitoring its impact on the subsequent cycle length and His-His (H-H) interval. The results of this maneuver revealed that the premature atrial complex led to a noticeable shortening of the H-H interval to 658 milliseconds. This shortening indicated earlier engagement of a slower pathway than initially expected. Additionally, the consistent His to high-rate interval and the absence of changes in the VA interval collectively supported the diagnosis of slow-fast AV node reentry.
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Explanation:

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.

CASE 5–2A

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

An electrophysiologic evaluation for recurrent episodes of tachycardia in a 59-year-old woman. The initiation of tachycardia occurs as follows: 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. The most likely mechanism of this tachycardia is AV node reentry. The diagnosis of AV node reentry is supported by the fact that a single premature complex is conducted over both the fast and slow AV nodal pathways to initiate tachycardia. This mode of induction is referred to as a “two-for-one response,” which is unusual and doesn’t follow the classical rules of initiation for a reentrant circuit. Classically, reentry circuits require three requisites: two pathways of conduction, an initial block in one pathway, and slowing of conduction in the second pathway to allow re-excitation of the initially blocked pathway, leading to reentry. However, in this case, neither pathway has a block since the premature atrial complex conducts anterogradely over both pathways. This unusual mechanism is best explained by the inability of the initial fast pathway conduction to conceal retrogradely into the slow pathway, allowing the slow pathway to conduct anterogradely and initiate tachycardia.
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CASE 5–2B

An electrophysiologic evaluation for recurrent episodes of tachycardia in a 59-year-old woman. The tachycardia’s onset unfolds as follows: A single atrial premature stimulus is introduced at the 340-millisecond mark during an atrial-paced cycle length of 500 milliseconds. Remarkably, this premature complex conducts through the AV node and triggers a tachycardia characterized by a short VA interval. The most likely mechanism behind this tachycardia is AV node reentry. The diagnosis of AV node reentry gains support from the fact that a single premature complex manages to traverse both the fast and slow AV nodal pathways to initiate tachycardia. This induction mode is referred to as a “two-for-one response,” a departure from the conventional rules governing reentrant circuit initiation. Traditionally, reentry circuits are governed by three requirements: the presence of two pathways for conduction, an initial block in one of these pathways, and deceleration of conduction in the second pathway enabling reexcitation of the initially blocked pathway, thus facilitating reentry. However, in this particular case, neither pathway exhibits a block, as the premature atrial complex conducts anterogradely across both pathways. This unusual mechanism can be best explained by the failure of initial fast pathway conduction to conceal itself retrogradely into the slow pathway. Consequently, this allows the slow pathway to engage in anterograde conduction, ultimately kickstarting the tachycardia.
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Explanation:

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 ...

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