A 67-year-old woman without structural heart disease was referred for evaluation and treatment of incessant palpitations. The 12-lead ECG was recorded at the beginning of the scheduled invasive electrophysiological examination (Figure 1). What is the underlying mechanism of the arrhythmia, and what are the differential diagnostic considerations?

Figure 1. Surface 12-lead ECG recorded during electrophysiological study.

Please turn the page to read the diagnosis.

The surface 12-lead ECG shows a narrow-QRS complex tachycardia with left axis deviation and ventricular cycles varying between 470 and 680 ms. At first glance, the irregularity of the ventricular rhythm may mislead one to the diagnosis of atrial fibrillation. However, careful examination reveals a regularly irregular rhythm with group beating pattern. Group beating of narrow-QRS complexes is a typical feature of atrioventricular Wenckebach conduction and atrial premature beats, but may also reflect junctional beats, double ventricular response, or reentrant beats involving dual atrioventricular nodal (AVN) pathways or an accessory connection. To differentiate between these potential arrhythmia mechanisms, it is important to carefully look for the presence of P waves and analyze their relation to the QRS complexes.

In fact, sinus P waves can be discerned throughout the tracing with an apparent cycle length of 1760 ms (Figure 2A, asterisks). The first 4 sinus P waves are each followed by 3 QRS complexes; the fifth sinus P wave is just followed by 2. The absence of any discernible P wave between the first and second QRS complex of the beating groups rules out atrial premature beats and atrial or supraventricular tachycardia with Wenckebach conduction. It is important to note that the second QRS complex reveals a pseudo-r′ wave in lead V₁, which represents a superimposed retrograde P wave (Figure 2A, arrows). This presumed AVN echo beat subsequently conducts down the slow pathway (SP) to generate the third QRS complex, which in turn is not followed by a retrograde P wave because of conduction block in the fast pathway. Because the basic sinus cycle is actually 880 ms and not reset by the AVN echo, the subsequent sinus P waves appear on time and repeatedly initiate this particular pattern of group beating until conduction block occurs in the SP.

Figure 2. Selected surface ECG leads and explanatory ladder diagram of the arrhythmia.A, Shown are surface ECG leads I, II, III, aVF, and V₁. Note the repetitive group beating pattern headed by sinus P waves (asterisks), leading to a regularly irregular narrow-QRS complex tachycardia. The second QRS complex of each group beating pattern reveals a pseudo-r′ wave in lead V₁ (arrows), which reflects a superimposed retrograde P wave. B, Explanatory ladder diagram and corresponding surface lead V₁. The underlying arrhythmia mechanism is double (1:2) AVN ventricular response. Each manifest sinus beat is conducted simultaneously down the fast and slow AVN pathway, producing 2 ventricular responses followed by an AVNE beat with subsequent anterograde SP conduction, which results in a combined nonreentrant and reentrant nodal tachycardia during sinus rhythm. A indicates atrium; AV, atrioventricular; AVN, atrioventricular nodal; AVNE, AVN echo; FP, fast pathway (solid blue arrows); SN, sinus node; SP, slow pathway (dashed red arrows); and V, ventricle. *Sinus node depolarization.

The basic underlying arrhythmia mechanism is double (1:2) AVN ventricular response.¹ Each manifest sinus beat is conducted simultaneously down the fast and slow AVN pathway, producing 2 ventricular responses followed by an associated fast–slow AVN reentrant sequence, which results in a combined nonreentrant and reentrant nodal tachycardia during sinus rhythm (Figure 2B). The key determinant for this unique variant of dual AVN nonreentrant tachycardia is a sufficiently prolonged anterograde SP conduction in the presence of rather poor retrograde fast pathway conduction. The very slow second AVN response enables both the His-Purkinje system and fast pathway to timely recovery excitability and conduct the impulse simultaneously down to the ventricles and back to the atria, resulting in a double ventricular response with an AVN echo. The subsequent anterograde SP conduction finds the fast pathway refractory, and retrograde conduction block occurs. Explanatory intracardiac electrograms are provided in Figure 3.

Figure 3. Explanatory intracardiac electrograms recorded during double response with associated fast–slow AVN reentrant sequence. Shown are surface ECG leads I, II, V₁, and V₆; intracardiac recordings of the His-bundle electrogram (HBE), from the proximal (9/10) to distal (1/2) coronary sinus (CS) and right ventricular apex (RVA). Each sinus beat (A) is followed by 2 His-bundle deflections (H1 and H2), 2 ventricular responses (V1 and V2), and a subsequent AVNE (Ae), which in turn conducts the impulse down the SP and His bundle (He) to the ventricles (Ve). This results in the third QRS complex of the group beating pattern, which terminates with anterograde conduction block in the SP after the fifth AVNE. Note that the A–H2 intervals are markedly prolonged (558–572 ms). A indicates sinus beat; Ae, AVNE beat; AVN, atrioventricular nodal; AVNE, AVN echo; FP, fast pathway; H1, FP-mediated HBE; H2, SP-mediated HBE; He, AVNE-mediated HBE; SP, slow pathway; V1, FP-mediated ventricular response; V2, SP-mediated ventricular response; and Ve, AVNE-mediated ventricular response.

It is noteworthy to mention that these manifestations of dual AVN pathway physiology may occasionally create quite interesting and confusing ECG phenomena in the presence of functional infrahisian conduction delay or block (Figure 4).² Of clinical importance, the resulting aberrant conduction patterns of the apparently atrioventricular dissociated double-response beats may mimic arrhythmias of ventricular origin.

Figure 4. Concealed and overt manifestations of double response associated with functional infrahisian conduction block. The electrograms are from the same patient and organized as in Figure 3. A through C, each sinus beat is followed by 2 His-bundle deflections and 2 ventricular responses with and without left bundle-branch block (LBBB) aberration attributable to infrahisian conduction delay (prolongation in H2–V2 interval) and associated AVNE (arrows). Note that the retrograde P wave in C precedes the QRS complex (arrow). D, Concealed 1:2 AVN conduction with associated AVNE results in an isolated retrograde P wave on the surface ECG, mimicking a nonconducted atrial or His extrasystole. E, Concealed 1:2 AVN conduction without associated AVNE mimicking normal sinus rhythm. Note that the third dual AVN response conducts to the ventricle with LBBB and markedly prolonged H2-V2 interval (198 ms). Of clinical importance, the apparently atrioventricular-dissociated beats with LBBB mimic an arrhythmia of ventricular origin. A indicates sinus beat; Ae, AVNE beat; AVN, atrioventricular nodal; AVNE, AVN echo; FP, fast pathway; H1, FP-mediated HBE; H2, SP-mediated HBE; He, AVNE-mediated HBE; SP, slow pathway; V1, FP-mediated ventricular response; V2, SP-mediated ventricular response; and Ve, AVNE-mediated ventricular response.

As expected in patients with dual AVN nonreentrant tachycardia,³ intravenous administration of orciprenaline effectively suppressed the double response in our patient but could not improve the poor baseline retrograde conduction or facilitate the induction of sustained AVN reentrant tachycardia. Radiofrequency catheter ablation completely abolished SP conduction and rendered the patient free of any arrhythmia.

This was the end of an exceptional never-ending story, which, to the best of our knowledge, has not yet been told in this way.

None.

https://www.ahajournals.org/journal/circ