Atypical accessory fibers as a lone or additional substrate for 1 to 2 response phenomenon

The phenomenon of a “double fire,” double atrioventricular nodal (AVN) response or dual AVN nonreentrant tachycardia (DAVNNT) first described in 1975 and published until the present mainly as isolated case reports, is a rare clinically manifested arrhythmia. Specific electrocardiographic sign of this infrequent phenomenon is that 1 sinus beat is followed by 2 narrow or wide (as a result of aberrancy) QRS complexes1,2 owing to the simultaneous anterograde conduction via fast and slow pathways (FP, SP) of the functionally dissociated AVN. Atypical bypass fibers with decremental conduction is another rare clinical entity. Electrocardiographic recognition of those structures during sinus rhythm (SR) may be difficult owing to the presence of subtle or no preexcitation. Premature ventricular complexes (PVC) ablated from Koch’s triangle is also an infrequent entity.3 Crossing key points for all of them might be QRS linking, atypical accessory fibers, or crux cordis, making differential diagnosis challenging. We describe a case of idiopathic frequent monomorphic PVCs from the basal ventricular septum with a possible diagnosis as 1:2 response phenomenon over atypical accessory fibers. Key Teaching Points • “Double fire” might be a heterogenous group of arrhythmias with different substrates, mechanisms, and electrocardiogram (ECG) manifestation in which 1:2 atrioventricular nodal (AVN) response composes only some part. Anatomical and physiological substrates for 1:2 response phenomenon are functional dissociation of the AVN (with specific conditions) as a solo entity, accessory Mahaim fibers as another distinct entity, or a combination of both. This may explain the presence of 1:2 response phenomenon in similar proportion of male and female patients compared to AVN reentrant tachycardia. • In our case it was impossible to manifest anterograde conduction over bypass structures during atrial pacing, but the accurate preliminary analysis of electrocardiogram (ECG) allowed us to suspect Mahaim fibers and carry out mapping of Koch’s triangle. As a consequence, registered M-potential, along with the results of pacing (selective capture) and radiofrequency ablation, confirm the diagnosis. Based on this we made the following conclusion: while it is impossible, with atrial pacing maneuvers, to validate conduction down the Mahaim fibers during the electrophysiology study, this fact does not exclude the origin of atypical accessory fibers above the AV ring. • ECG and electrophysiological signs of 1:2 response over Mahaim fibers resembles premature ventricular contraction with exit site at the para-Hisian space. High burden and ineffective antiarrhythmic drug treatment promote ablation therapy as a treatment of choice. The described specific properties and manifestations should allow one to suspect atypical accessory fiber as a substrate in similar cases and carry out correct planning of cardiac mapping procedures, avoiding “unnecessary” high-risk application at the para-Hisian space.