Sharon Shen, MD, and Bradley P. Knight, MD
A 24-year-old man without significant past medical history presents to clinic for management of paroxysmal supraventricular tachycardia (PSVT). He recalls palpitations since he was a teenager, but the episodes were generally infrequent and not bothersome. Over the past few months, however, episodes have become more frequent, occurring once per week, with the last episode prompting a visit to the emergency department. At that time, a regular, narrow QRS tachycardia at 145 bpm was noted and terminated with adenosine. Baseline ECG shows normal sinus rhythm with normal intervals and absence of preexcitation. The patient elected to undergo catheter ablation.
This clinical scenario, in particular the patient’s young age, raises suspicion for a concealed accessory pathway (AP) participating in atrioventricular reentrant tachycardia (AVRT). This chapter will focus on the diagnostic maneuvers of His-refractory ventricular extrastimulus testing, otherwise known as delivering a “PVC on His” and para-Hisian pacing, two maneuvers frequently used to determine the presence of a concealed or retrogradely conducting AP.
PVC ON HIS
In this patient with a suspected AP participating in tachycardia, a useful maneuver to consider during tachycardia is to deliver a ventricular extrastimulus when the His bundle is refractory and assess its effect on the atrium (Figure 5-1).1
FIGURE 5-1 An example is shown of a PVC delivered during His bundle refractoriness during PSVT in the EP laboratory. There is preexcitation of the atrium with a His-synchronous PVC, which is evidence of an accessory pathway but is not diagnostic of AVRT. Although unlikely, an atrial tachycardia with a concealed bystander accessory pathway cannot be excluded. The His bundle is refractory as seen by the coincident antegrade His potential. Thus, retrograde conduction of the PVC must have occurred over an accessory pathway.
• Advancement of atrial activation with a PVC delivered during PSVT when the His bundle should be refractory as seen in this patient indicates that a retrogradely conducting AP is present, but it may not necessarily participate in tachycardia. Although unlikely, an atrial tachycardia with a concealed bystander AP cannot be excluded.
• A His-synchronous PVC that terminates the tachycardia without preexcitation of the atrium is diagnostic of AVRT (Figure 5-2). An atrial tachycardia would not terminate under those conditions. Note that in this example there was an attempt to entrain the atrium with ventricular pacing, but pacing terminated tachycardia. Careful examination showed that the first beat of the pacing train occurred during the His bundle refractory period and ultimately provided the diagnosis of AVRT.
FIGURE 5-2 Termination of tachycardia with a His-synchronous PVC without preexcitation of the atrium is shown. This is diagnostic of AVRT.
• The Principle: A PVC that preexcites the atrium must do so via an AP because when the His-Purkinje system (HPS) is refractory during PSVT, a PVC cannot conduct to the atrium through the HPS. Similarly, a PVC that terminates tachycardia without preexciting the atrium must have occurred as a result of causing block in the AP. When atrial activation is delayed with a PVC delivered during His refractoriness, the maneuver is also diagnostic for AVRT because an AP must be participating in the tachycardia.
• Pitfall: Preexcitation of the atrium may not occur despite the presence of an AP if the pacing site is far from the ventricular insertion of the AP; the ability of the ventricular extrastimulus to enter the reentrant circuit before ventricular activation over the normal pathway is affected by (1) the conduction time from the ventricular stimulation site to the AP, (2) the local ventricular refractory period, and (3) the tachycardia cycle length.
Keys to Performing This Maneuver
• Scan diastole with PVCs at progressively shorter coupling intervals (decrease by 10 ms).
• Confirm ventricular capture.
• Look for shortening of the A-A interval with the longest coupled PVC.
• Confirm that the PVC is delivered when the His bundle is refractory either by a manifest His potential (see Figure 5-1) or that the stimulus is less than 35 to 55 ms before the next expected His potential (see Figure 5-2).
• An alternative method is to examine the atrial response during overdrive ventricular pacing when the ventricular complexes appear fused. Fusion confirms anterograde activation of the His bundle during the initiation of overdrive pacing. Preexcitation of the atrium with a fused ventricular complex therefore indicates the presence of an AP (Figure 5-3).
FIGURE 5-3 Preexcitation of the atrium with a fused ventricular extrastimulus is shown. The fused appearance of the third ventricular extrastimulus confirms that the His bundle was activated anterogradely. Preexcitation of the atrium in this situation offers the same evidence as when a PVC is delivered when the His is refractory and confirms the presence of an accessory pathway.
• Useful maneuver performed during sinus rhythm to evaluate for the presence of a septal AP.2
• Incremental high-output pacing during sinus rhythm is performed at the site of the largest His bundle recording with various stimulation intensities to achieve (1) simultaneous ventricular and His bundle (and/or right bundle branch) capture and (2) ventricular capture alone.
• Simultaneous ventricular and His bundle capture results in both anterograde and retrograde conduction via the HPS with the anterograde conduction resulting in a relatively narrow QRS morphology.
• Nodal Response: In the absence of an AP, retrograde conduction occurs over the His bundle with a stimulus to His (SH) interval of 0, followed by atrial activation where the stimulus to atrium (SA) interval equals the His to atrium (HA) interval. With a decrease in the pacing output, His bundle capture is lost (confirmed by a widening in the QRS) and anterograde conduction occurs over ventricular myocardium. His activation is delayed (SH increases) as it now follows retrograde activation of the right bundle leading to an increase in the SA interval (Figure 5-4A).
FIGURE 5-4 Para-Hisian pacing. (A) Nodal response: High-output pacing at the His bundle (HB) captures both the HB and right ventricle (RV), whereby anterograde and retrograde activation occurs over the His-Purkinje system leading to a short stimulus to atrium (SA) interval. At a lower pacing output, only the ventricle is paced, and retrograde atrial activation is delayed (SA increases) as it now follows retrograde right bundle activation. (B) Extranodal response: As retrograde conduction occurs over the accessory pathway and not the AV node, the SA interval is unchanged upon loss of HB capture.
• Extranodal Response: As retrograde conduction occurs over the AP instead of the AV node, the SA interval remains unchanged (Figure 5-4B).
• Carefully examine that the retrograde atrial activation sequence is unchanged; a change suggests that retrograde conduction could be occurring over multiple pathways, either over multiple APs, fast and slow AV node pathways, or simultaneously over the AV node and AP.
• Pitfalls: Although an extranodal response has a positive predictive value of 83% for AVRT, the sensitivity is only 47% when assessed for all forms of APs.1 Ability to elicit an extranodal response is limited by the how far the AP is located from the pacing site and how slowly it conducts.
Keys to Performing This Maneuver
• Identify (1) simultaneous His bundle and ventricular capture and (2) ventricular capture only with varying the pacing stimulus output.
• Confirm the absence of atrial capture.
• Confirm that the retrograde atrial activation sequence is unchanged.
• Compare the SA intervals at the earliest atrial activation.
SA increases with loss of His bundle capture → nodal response (Figure 5-5A).
SA unchanged → extranodal response (Figure 5-5B).
FIGURE 5-5 Examples of nodal (A) and extranodal (B) responses to para-Hisian pacing.
1. 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(2):574-582.
2. Hirao K, Otomo K, Wang X, et al. Para-Hisian pacing. A new method for differentiating retrograde conduction over an accessory AV pathway from conduction over the AV node. Circulation. 1996;94(5):1027-1035.