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Narrow QRS complex tachycardia with and without ventriculoatrial block: What is the mechanism?

Published:August 04, 2022DOI:https://doi.org/10.1016/j.hrthm.2022.07.023
      A 39-year-old woman with no structural heart disease underwent an electrophysiology study (EPS) to assess recurrent supraventricular tachycardia (SVT). The electrocardiogram did not show any preexcitation. The baseline atrio-His and His-ventricular intervals were 108 and 43 ms. Ventricular pacing demonstrated concentric, decremental ventriculoatrial (VA) conduction, and the single jump changed the earliest atrial activation site (EAAS) from the His bundle region to the proximal coronary sinus (CS) region. Programmed atrial pacing with a single atrial extrastimulus demonstrated 2 slow pathways (SPs) and induction of a narrow QRS complex tachycardia with VA block (SVT1) after a jump to the second SP (Online Supplemental Figure 1). During SVT1, the His-His (H-H) intervals varied, and an alternating EAAS between the His bundle and proximal CS regions and VA block were documented (Figure 1A ). After isoproterenol administration, a narrow QRS complex tachycardia with the EAAS in the His bundle region (SVT2; tachycardia cycle length [TCL] 355 ms) but without VA block was induced by atrial burst pacing. A single His-refractory premature ventricular complex from the right ventricular (RV) apex delayed the subsequent atrial and His bundle timing with a change in the EAAS to the proximal CS region (Figure 1B). Figure 1C presents the response to RV overdrive pacing. What is the likely mechanism of the 2 types of SVT and how can the responses to the maneuvers be explained?
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      Figure 1A: Intracardiac electrograms revealing a narrow QRS complex tachycardia with ventriculoatrial (VA) block. The His-His intervals vary, and an alternating earliest atrial activation site (EAAS) in the His and proximal coronary sinus regions and VA block are recorded. B: A single His-refractory premature ventricular complex (PVC) during the narrow QRS complex tachycardia with the EAAS in the His bundle region. PVC resets the tachycardia and delayed the subsequent atrial and His bundle electrogram. Note that the EAAS immediately after PVC moved from the His bundle region to the proximal coronary sinus region. C: Right ventricular overdrive pacing revealed antidromic His bundle capture, a postpacing interval minus the tachycardia cycle length of 145 ms, and a resumption of the tachycardia with a V-A-V response. I, II, and V1 = surface electrocardiographic leads; HRA, His, CS, and RV = intracardiac electrograms recorded from the high right atrium, His bundle region, coronary sinus, and right ventricular apex, respectively; p and d = proximal and distal electrodes; A, H, V, and S = atrium, His bundle, ventricle, and stimulation. The numbers are the intervals in milliseconds.

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