Ventricular premature pacing to reveal slow pathway conduction: A case of dual ventricular response with ventriculoatrial block

Figures

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  Figure 1. A: Twelve-lead surface electrocardiogram of the apparent clinical arrhythmia. Every other sinus P wave was associated with 2 subsequent QRS complexes. B: Magnified view of lead V1. The 3-beat repetitive pattern consists of sinus P waves with normal PR followed by a junctional beat. Second sinus P waves were conducted with longer PR, which were not followed by junctional beats.

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  Figure 2. During atrial pacing at 900 ms cycle length (A1), a ventricular premature beat is delivered 700 ms after the last atrial stimulation (V2), followed by an atrial stimulation (A2). A2 then invariably conducted via a long slow pathway (SP) to the ventricle (V3). Upper panel: Surface electrocardiogram and intracardiac electrograms. Lower panel: Laddergram. Black and red arrows with large and small dotted lines depict each pathway (antegrade fast pathway [FP] and SP, retrograde FP and SP). Blue bar shows effective refractory periods (ERPs) of FP. Yellow bars show ERPs of SP. Conduction via the antegrade FP by a stimulus from high right atrium (HRA) (a). Conduction via the antegrade SP collided with (or interfered by) conduction via the retrograde SP distally (b). Conduction via the retrograde FP after V2 collided with conduction from the antegrade FP by a stimulus from HRA (A2) (c). Since the conduction via the retrograde SP by V2 terminated distally, ERP of the SP ended before the SP conduction of A2 arrived at the distal site of retrograde block, thus allowing A2 to propagate to the His via the antegrade SP (d).

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  Figure 3. Upper panel: Magnification of lead V1 as shown Figure 1. Sinus P waves with short PR seemed to be accompanied by 2 subsequent QRS complexes. On the other hand, sinus P waves with long PR seemed to be accompanied by only 1 subsequent QRS complex. Lower panel: Ladder diagram depicting the conduction pattern in the upper panel. As antegrade fast pathway (FP) impulse reaches the compact node, it also penetrates the slow pathway (SP) retrogradely but blocks distally in the SP (a). Impulse via the antegrade SP can conduct to the His bundle, generating double response, since the SP has recovered from preceding retrograde penetration by the FP (b). The SP impulse also retrogradely penetrates the FP impeding the next sinus beat’s conduction via the FP and causing PR prolongation (green dotted line) (c). Owing to delay of conduction through the antegrade FP, retrograde SP penetration is also delayed, retarding its recovery and resulting in antegrade block of the SP impulse (d). Black and red arrows with large and small dotted lines depict each pathway (antegrade FP and SP, retrograde FP and SP). Green dotted arrows depict slowed FP conduction. Blue bar shows effective refractory period…