Sharon Shen, MD, and Bradley P. Knight, MD
A 55-year-old man without significant past medical history presents to clinic for consultation regarding the management of paroxysmal supraventricular tachycardia (PSVT). He states that he first noted rapid palpitations approximately 6 months ago shortly after hip surgery. Palpitations were rapid and regular with abrupt onset and offset. Several episodes lasted 3 to 6 hours. He endorsed about 8 to 10 episodes over the past few months. LV function by echocardiogram was normal. He had previously seen another physician a few months prior who inserted an implantable loop recorder. Interrogation of his ILR showed a regular tachycardia at 120 bpm.
This is an example of a typical patient who will undergo catheter ablation for symptomatic SVT. To choose a target for ablation, however, the mechanism of tachycardia must first be determined. This chapter will focus on the diagnostic pacing maneuvers of ventricular and atrial overdrive pacing. For the purposes of this chapter, the term “entrainment” refers to acceleration of the atrial or ventricular electrograms to the pacing cycle length (CL) during overdrive pacing from the ventricle or atrium, respectively, with resumption of the original tachycardia upon cessation of pacing.
VENTRICULAR OVERDRIVE PACING
A useful first pacing maneuver during PSVT induced in the electrophysiology laboratory is to entrain the tachycardia from the ventricle. In this patient, a regular tachycardia with a 1:1 AV relationship, CL of 580 ms, and a septal VA time of 130 ms is induced in the EP lab. The differential diagnosis includes atypical atrioventricular nodal reentry tachycardia (AVNRT), orthodromic atrioventricular reentry tachycardia (AVRT), and atrial tachycardia (AT). Ventricular overdrive pacing at a CL 10 ms shorter than the tachycardia CL produces the response shown in Figure 4-1A.
FIGURE 4-1A An example of ventricular overdrive pacing during PSVT is shown. During tachycardia, ventricular overdrive pacing was performed at 10 ms shorter than the tachycardia CL. After confirming acceleration of the atrial CL to the pacing CL, ventricular pacing was stopped, and the response to cessation of pacing was observed to be VAV.
VAV OR VAAV
• Assessment of the electrogram sequence following the last paced ventricular complex can be used to differentiate between AT and an AV node-dependent arrhythmia (AVNRT or AVRT).1,2
• The principle: Overdrive ventricular pacing in tachycardia often results in 1:1 retrograde conduction whereby the atrial rate accelerates to the ventricular pacing rate. With either AVNRT or AVRT, the last entrained beat conducts via the retrograde limb of the circuit (AV nodal pathway or accessory pathway [AP]) and then anterograde down the AV node displaying an “atrial-ventricular” response (VAV) as is the case in this patient (see Figures 4-1A and 4-1B).
FIGURE 4-1B Ladder diagram for VAV response.
• Conversely, during entrainment in AT, both retrograde and anterograde conduction occurs via the AV node. Therefore, upon cessation of pacing, the last entrained atrial beat finds the AV node refractory displaying an “atrial-atrial-ventricular” response described as VAAV (Figures 4-2A and 4-2B).
FIGURE 4-2A An example of a “VAAV response” to entrainment pacing from the ventricle during a long RP tachycardia. After cessation of overdrive pacing, the electrogram response is VAAV consistent with the patient’s diagnosis of AT. Note also the change in the atrial activation sequence during ventricular pacing consistent with AT.
FIGURE 4-2B Ladder diagram of VAAV response.
• Assessment of the retrograde atrial activation sequence (RAAS) during entrainment is useful; if the RAAS of the entrained complex is different from that in tachycardia, a diagnosis of AT or bystander AP is suggested (see Figure 4-2A).
• Pseudo VAAV : Caution should be exercised in identifying the last entrained atrial beat as prolonged retrograde conduction can give the false appearance of a VAAV response (Figure 4-3).
FIGURE 4-3 “Pseudo VAAV” in a patient with atypical AVNRT.
THE POSTPACING INTERVAL
• Useful for differentiating atypical AVNRT from AVRT using a septal AP.3
• Postpacing interval minus tachycardia CL (PPI-TCL) >115 ms suggests atypical AVNRT (Figure 4-4).
FIGURE 4-4 PPI-TCL and SA-VA example of a patient with AVNRT. PPI-TCL >115 ms and SA-VA >85 ms are both consistent with a diagnosis of AVNRT.
• PPI-TCL <115 ms suggests a septal AP-mediated AVRT (Figure 4-5).
FIGURE 4-5 PPI-TCL and SA-VA example of a patient with a septal AP-mediated AVRT. As the ventricle is a part of the AVRT circuit, RV pacing as a result is near the circuit lending to a shorter PPI and shorter SA interval. PPI-TCL <115 ms and SA-VA <85 ms are consistent with this patient’s diagnosis of a septal AP-mediated AVRT.
• The Principle: As the ventricle is an obligate member of the AVRT circuit, namely the RV pacing site is near the circuit, entrainment from the ventricle results in a shorter PPI compared to that in AVNRT.
• Pitfall: This calculation includes anterograde AV conduction. If the first AH interval after pacing is stopped is longer than the AH during SVT, it can falsely prolong the PPI.
• Alternatively, a second calculation that only measures the difference in VA conduction is the stimulus to A interval minus VA interval during tachycardia (SA-VA) where >85 ms suggests atypical AVNRT (see Figure 4-4) and <85 ms suggests a septal AP-mediated AVRT (see Figure 4-5).
• Exception to both calculations exists in very slowly conducting accessory pathways whereby marked cycle length-dependent conduction delay can increase the PPI and SA values.4
Keys to Performing This Maneuver
• Pace the ventricle at a CL 10 to 40 ms shorter than the tachycardia CL.
• Confirm ventricular capture.
• Confirm that the atrial CL equals the pacing CL.
• Identify the last entrained atrial depolarization.
• Identify the next ventricular depolarization.
• Characterize the response as VAV or VAAV.
• If the mechanism is now between atypical AVNRT and AVRT, calculate PPI-TCL or SA-VA.
ATRIAL OVERDRIVE PACING
Another useful pacing maneuver in tachycardia is to entrain from the atrium. This pacing maneuver can be used to assess for VA linking and effectively exclude AT.
• Pace the atrium 10 to 40 ms shorter than the tachycardia CL to entrain the ventricle.
• Compare the VA interval of the first return beat after cessation of pacing to the VA interval during tachycardia.
• If the VA interval is unchanged, this suggests that atrial activation is linked to ventricular activation (Figure 4-6) thus excluding AT as the mechanism.
FIGURE 4-6 VA linking in a patient with AVNRT. Upon cessation of atrial overdrive pacing, the last entrained ventricular beat has the same VA time as that in tachycardia, suggesting that the timing of atrial activation is linked to the timing of ventricular activation.
• VA linking is not 100% predictive as coincident events can rarely result in apparent VA linking and 3% of AVNRT do not show VA linking.2
1. Knight BP, Zivin A, Souza J, et al. A technique for the rapid diagnosis of atrial tachycardia in the electrophysiology laboratory. J Am Coll Cardiol. 1999;33(3):775-781.
2. Knight BP, Ebinger M, Oral H, et al. Diagnostic value of tachycardia features and pacing maneuvers during paroxysmal supraventricular tachycardia. J Am Coll Cardiol. 2000;36:574-582.
3. Michaud GF, Tada H, Chough S, et al. Differentiation of atypical atrioventricular node reentrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pacing. J Am Coll Cardiol. 2001;38:1163-1167.
4. Bennett MT, Leong-Sit P, Gula LJ, et al. Entrainment for distinguishing atypical atrioventricular node reentrant tachycardia from atrioventricular reentrant tachycardia over septal accessory pathways with long-rp [corrected] tachycardia. Circ Arrhythm Electrophysiol. 2011;4:506-509.