Premature ventricular extrastimulus without His or ventricular capture. An unexpected response during AV nodal reentrant tachycardia

A 52-year-old male was referred to our center for catheter ablation of recurrent episodes of paroxysmal supraventricular tachycardia. No relevant abnormalities were observed at his basal ECG and echocar- diogram. Electrophysiological study was performed, and multipolar diagnostic catheters were introduced via the femoral veins. Typical atrioventricular (AV) nodal reentrant tachycardia with a medium rate of 200 bpm was reproducibly induced and diagnosed according to the standard criteria. During ventricular resetting maneuver using the His catheter, an expected phenomenon was observed after applying an early premature ventricular extrastimulus (PVE) (Figure 1). At a first look, the stimulus occurred during the ventricular refractory period without any capture. However, a phase of right bundle branch block (RBBB) occurred immediately after this apparently noncapturing PVE and without any subsequent change in the tachycardia cycle length making rate-dependent RBBB unlikely as a mechanism. The PVE did not capture ventricular myocardium, nor the His bundle (H-H intervals remained unchanged), and the tachycardia was not reset (Figure 1A). The induced RBBB was likely due to local concealed capture of the proximal RBB during its relative refractory period by the applied PVE. The stimulus could generate only an attenuated action potential with slow conduction in the RBB (the asterisk, Figure 1B) unable to advance the next QRS. Nevertheless, the local capture of proximal RBB rendered it unexcitable by the antegrade activation of the ongoing tachycardia. Another possible mechanism is local electrotonus after PVE causing local loss of membrane potential adjacent to the distal RBB and resulting in conduction block of the advancing wave coming down the RBB. Subsequently, RBBB was likely maintained through a linking effect and repetitive retrograde penetration of the RBB by impulses propagating antegradely over the contralateral left bundle (Figure 1C). This case highlights intriguing electrophysiological phenomena that can still be observed during a classical pacing maneuver of a common reentry circuit including concealed capture of the His-Purkinje system, electrotonus, linking effect, and functional aberrancy.