Long RP tachycardia with fusion QRS during ventricular overdrive pacing: Diagnostic challenge of a bystander nodoventricular pathway

Figures

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  Figure 1. Intracardiac electrograms during tachycardia and the response to a His-bundle–refractory premature ventricular complex (PVC). A, 12-lead electrocardiogram during the electrophysiology study showing a narrow QRS tachycardia with a long RP interval (tachycardia cycle length 410 ms). B, Intracardiac electrograms demonstrating that the earliest atrial activation was recorded at the proximal coronary sinus. A His-bundle–refractory PVC delayed both A–A and H–H intervals without paradoxical reset.

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  Figure 2. Ventricular overdrive pacing and fusion QRS. A, Ventricular overdrive pacing (VOP) from the right ventricular apex produced a V–A–V response with fusion QRS complexes, and the post-pacing interval minus tachycardia cycle length (PPI-TCL) was 292 ms. StimV→A intervals used for DCE were obtained during VOP: 564 ms from the high right atrium (HRA) and 488 ms from the coronary sinus (CS). These values, in combination with StimA→V and PPIa obtained from AOP (Figure 3), were used to calculate corrected post-pacing interval from the ventricle (PPIv)-TCL values. B, 12-lead electrocardiogram during VOP showing fusion QRS morphology. As the pacing rate was increased, the QRS complexes progressively approached the fully paced morphology.

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  Figure 3. Atrial overdrive pacing (AOP). A, AOP from the high right atrium produced an A–H–A sequence without ventriculoatrial linking. The measured intervals were StimV→A 564 ms, StimA→V 290 ms, and post-pacing interval from the atrium (PPIA) 610 ms, yielding a calculated corrected post-pacing interval from the ventricle (PPIv)-TCL of 244 ms. B, AOP from the proximal coronary sinus also demonstrated an A–H–A response without VA linking. StimV→A was 488 ms, StimA→V 270 ms, and PPIA 442 ms, resulting in a corrected PPIv-TCL of 316 ms.

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  Figure 4. Schematic illustration of the proposed mechanism for QRS fusion during ventricular overdrive pacing in the presence of a bystander concealed nodoventricular pathway. A, When the pacing stimulus solely activates the ventricle via direct myocardial capture (or when the cNVP-HPS route is delayed), a fully paced QRS morphology results. B, Fusion QRS is produced by the collision of 2 distinct wavefronts originating from the same pacing stimulus (n). The first wavefront is from direct, slow myocardial activation from the pacing site. The second, concurrent wavefront is generated when the pacing stimulus propagates retrogradely through the bystander cNVP, accesses the His-Purkinje system (HPS), and then activates the ventricle rapidly and anterogradely. The parallel activation of the ventricle by these 2 wavefronts results in the fusion QRS morphology.