Ventricular overdrive pacing during supraventricular tachycardia.

Case

An 18-year-old male with recurrent palpitations and documented regular, long RP, narrow QRS tachycardia was taken up for electrophysiology study. Four catheters were placed in the high right atrium, right ventricular apex, coronary sinus and His bundle location. Tachycardia was easily inducible with an atrial extrastimulus. During tachycardia (cycle length = 356 ms), ventricular overdrive pacing (VOP) at cycle length of 320 ms was done (Fig. 1). What is the mechanism of tachycardia?

Fig. 1

Ventricular overdrive paced supreventricular tachycardia.

Pacing cycle length = 320 ms, Tachycardia cycle length = 356 ms. CS 9–10 to CS 1–2 -coronary sinus proximal to distal, His p, His m and His d -His proximal to distal, HRA -high right atrium, RVA -right ventricular apex.

Discussion

Differential diagnosis for any regular, long RP, narrow QRS tachycardia includes atrial tachycardia (AT), orthodromic reentrant tachycardia (ORT) mediated by a slow conducting accessory pathway and atypical atrioventricular nodal reentrant tachycardia (AVNRT).

In Fig. 1, atrial cycle length (CL) is the same as the paced CL, confirming entrainment of the tachycardia. Earliest atrial activation is in the proximal CS and this is consistent with any of the diagnoses listed above. The surface QRS is relatively narrow and predominantly positive in lead aVF while the pacing catheter is placed in the RV apex. This suggests that some part of ventricle is activated through His-purkinje system and ventricular activation is a fusion between antegrade conduction over His-purkinje system and RV pacing. Stable fusion morphology of surface QRS during ventricular overdrive pacing (VOP) of supraventricular tachycardia (SVT) suggests the diagnosis of ORT. But this finding has interobserver variability and a narrow window for its observation [1].

Antegrade activation sequence in His bundle during VOP of SVT provides intracardiac evidence of fusion. This finding is diagnostic of ORT and can be seen even when there is minimal or concealed fusion in surface QRS [2,3]. It can be noted in Fig. 1 that the His activation occurs before the pacing spike and the sequence of activation in mid and distal His channels is also consistent with antegrade activation of the His bundle. Patients with AVNRT or AT will not show this finding because entrainment of AVNRT or overdrive pacing of AT requires retrograde activation of His bundle before the impulse reaches the atrium (Fig. 2). Although uncommon, it is possible to entrain ORT with retrograde activation of His bundle when collision of the wavefronts is occurring above the His bundle. Thus, while the antegrade His bundle activation during VOP confirms the diagnosis as ORT, retrograde His bundle activation makes the diagnosis of ORT unlikely but does not rule it out and needs further EP evaluation.

Fig. 2

Explanatory schematic diagram.

Panels A, B and C represent ventricular overdrive pacing during orthodromic reentrant tachycardia, atrioventricular nodal reentrant tachycardia and atrial tachycardia respectively. Red line indicates antidromic wavefront while the blue line indicates the orthodromic wavefront in panel A and B. Collision between orthodromic and antidromic wavefronts has to occur within the circuit for entrainment to happen. Blue star denotes the focus of atrial tachycardia in panel C. (A) During ventricular overdrive pacing of orthodromic reentrant tachycardia, collision between orthodromic and antidromic wavefronts can occur within the ventricle (marked X) producing manifest fusion and antegrade His bundle activation. However, site of collision may be at Y, which will produce antegrade His bundle activation, but only concealed fusion or, rarely, at Z, which will show retrograde His bundle activation. (B) During ventricular overdrive pacing of atrioventricular nodal reentrant tachycardia, pacing impulse travels retrograde in His-purkinje system to reach till tachycardia circuit thus it is not possible to entrain the atrioventricular nodal reentrant tachycardia with antegrade His bundle activation. (C) During ventricular overdrive pacing of atrial tachycardia, pacing impulse travels retrograde in His-purkinje system and AV node to reach till tachycardia circuit or site of origin thus it is not possible to entrain or overdrive pace the atrial tachycardia with antegrade His bundle activation. X -a site within ventricle below the His-purkinje system, Y -a site within His-purkinje system below the His bundle, Z -a site above the His bundle.

Rarely it is possible to get fusion in surface QRS morphology and antegrade His activation during VOP of AVNRT or AT with bystander concealed posteroseptal accessory pathway if the retrograde His-purkinje conduction is too slow or absent. Thus, after ablation of accessory pathway, non-inducibility of SVT including AVNRT or AT should be confirmed.

This patient was diagnosed as a case of ORT mediated by a slow and decremental conducting concealed posteroseptal accessory pathway. Pathway was mapped during tachycardia and successfully ablated. No tachycardia was inducible after ablation.

Funding

None.

Declaration of competing interest

None for any of the authors.