May 2024

Answer
Atrioventricular Re-entry Tachycardia (AVRT) 

Explanation

A lot of crucial information can be obtained from this image trace.

A continuation of the narrow complex tachycardia (NCT) can be observed from the last two beats on the right in Figure 1.

The following are the three main differential diagnosis of an NCT¹:

1. AVNRT (typical/slow-fast or atypical/fast-slow/slow-slow)
2. AVRT
3. AT

Three further observations can be made about the NCT:

Firstly, the ventriculo-atrial (VA) conduction time is long (140ms) which makes typical AVNRT (VA time <70ms) and junctional tachycardia very unlikely².

Secondly, the atrial activation pattern is eccentric (earliest in CS 5-6/3-4). As the atrial activation pattern in AVNRT is concentric (CS proximal to distal, reflecting conduction through the AV node), this narrows down the differential diagnosis to either a left-sided AVRT or a left-sided AT.

Thirdly, the NCT has a 1:1 VA relationship. If this was not the case, then an AVRT could be ruled out. However in this case, the 1:1 VA relationship did not narrow down the differential diagnosis (AVRT vs AT).

The pacing manoeuvre is ventricular overdrive pacing (VOP) from the apex of the right ventricle, which is reflected in the first 4 beats (note the broad and negative QRS in V1 and superior axis). VOP is one of the most useful pacing manoeuvres to establish the mechanism of an NCT. SVT entrainment with manifest fusion was not seen in this case but its occurrence is proof that AVRT is present3.

An atrial-atrial-ventricular (or V-A-A-V) response is diagnostic of AT². In this case, the response to VOP was V-A-V; which suggested either AVRT or atypical AVNRT (also unlikely in this case due to the aforementioned reasons). Two quantitative features of VOP can be observed which suggest AVRT:

1. The difference between the post-pacing interval (PPI) and the TCL was 97ms (i.e PPI-TCL <115ms³ or corrected PPI-TCL<110ms)
2. The difference between the stimulus to atrial electrogram (SA) interval and the tachycardia VA interval was 70ms (i.e SA-VA <85ms)

A corrected PPI-TCL <110ms and SA-VA <85ms suggests AVRT (as the ventricular pacing site is close to the tachycardia circuit)¹.

In order to accurately interpret the response to VOP, it is important to confirm that the tachycardia was entrained in the first place. Entrainment is confirmed if the re-entrant tachycardia was reset by the series of consecutive beats of a pacing train (i.e VOP) with resumption of the tachycardia when VOP was discontinued4. In this case, the atrial cycle length during VOP matched the paced cycle length (360ms, which was faster than the TCL) and the tachycardia resumed upon discontinuation of VOP. It is also worth noting that the atrial activation during VOP was similar to that of the NCT (i.e eccentric which suggests retrograde atrial activation via a left-sided accessory pathway).

When the patient was back in sinus rhythm, a retrograde curve was performed (by pacing from the right ventricle, similar to that of VOP but at a slower rate), which showed eccentric atrial activation (Figure 2). Putting it all together, the mechanism of this patient’s NCT is most likely to be an AVRT from a concealed left accessory pathway.

Further recommended reading can be found in the references.

Figure 2. Retrograde curve of the EP study, which showed eccentric atrial activation suggesting the presence of a concealed left accessory pathway.

Electrocardiogram labels from top to bottom: Leads II, III, V1, V4, V5. His 3-4 (proximal), His 1-2 (distal), CS 9-10 (proximal) to CS 1-2 (distal), RVA proximal, RVA distal.

References:

1. Veenhuyzen GD, Quinn FR, Wilton SB, Clegg R, Mitchell LB. Diagnostic pacing maneuvers for supraventricular tachycardia: part 1. Pacing Clin Electrophysiol. 2011; 34 (6):767-782.
2. Veenhuyzen GD, Quinn FR. Principles of entrainment: Diagnostic utility for supraventricular tachycardia. Indian Pacing Electrophysiol J. 2008; 8: 51-65.
3. Michaud GF, Tada H, Chough S, Baker R, Wasmer K, Sticherling C, Oral H, Pelosi F Jr, Knight BP, Strickberger SA, Morady F. Differentiation of atypical atrioventricular node re-entrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pac- ing. J Am Coll Cardiol 2001;38:1163–1167.
4. Knight BP, Ebinger M, Oral H, Kim MH, Sticherling C, Pelosi F, Michaud G, Strickberger SA, Morady F. Diagnostic value of tachycardia features and pacing maneuvers during paroxysmal supraventricular tachycardia. J Am Coll Cardiol. 2000; 36(2):574-582.