Commentary:
12 lead ECG showed narrow QRS tachycardia with retrograde VA wenkebaching, hence Atrio-ventricular reciprocating tachycardia (AVRT) and atrial tachycardia (AT) could be excluded. Intracardiac electrogram during spontaneous induction showed classical 1:2 AV nodal conduction followed by initiation of tachycardia (Fig 1B). Several features favor AV nodal re-entrant tachycardia (AVNRT) in this case: VA dissociation, response to adenosine, 1:2 AV nodal conduction, concentric atrial activation pattern and fixed HV interval (44ms) during tachycardia. The possibility of automatic junctional tachycardia was excluded based on response to premature atrial extrastimuli and intravenous adenosine. Intracardiac electrogram showed variation in A-A, V-V, H-H and HA intervals. Atypical AVNRT of slow – slow type with decremental conduction of upper common pathway (UCP) could explain the variation in A-A and H-A intervals (Fig 3A, B and C). The H-H and V-V interval variations could be explained by presence of multiple slow pathways as below
  1. Three slow pathways with different electrophysiological properties
  2. Initial re-entry between slow pathway-1 (S1-antegrade) and slow pathway-2 (S2-retrograde) at a cycle length of 410ms with concealed conduction into slow pathway-3 (S3) and decremental conduction in upper common pathway (fig 3A)
  3. Decremental conduction of S1 resulting in gradual prolongation of H-H interval followed by block which favored the transient re-entry between S3 (antegrade) and S2 (retrograde) (cycle length 379ms) and subsequent resumption of S1 conduction (fig 3B & C)
Fig 3: A – Atypical slow- slow AVNRT with re-entry between S1 and S2 with concealed conduction into S3. Upper common pathway showed decremental conduction. B – Transient block in S1 due to decremental conduction favored re-entry between S3 and S2 with concealed conduction into S1. C – Ladder diagram showing the same phenomenon. The decremental conduction in S1 prolonged H-H/V-V interval from 410ms to 478ms. S3 could conduct in the next beat with H-H interval of 379ms as S1 was transiently blocked. In the subsequent beat S1 conduction resumed with minimal prolongation of H-H interval due to concealed conduction. Atrial conduction occurred decrementally through upper common pathway (UCP). S1,2,3 – slow pathways 1,2 and 3, F – fast pathway, LCP -Lower common pathwayThe possibility of two antegrade slow pathway conduction has been described previously as a cause for cycle length alteration during AVNRT1. Multiple slow pathways are required for the initiation of re-entrant tachycardia after 1:2 AV response which otherwise would not have induced. In our patient the cycle length alteration was noted in both atrium (A-A) and ventricle (V-V). This could be explained by the rare combination of decremental conduction in both upper common pathway and antegradely conducting slow pathway (S1). The second pathway (S2) has different electrophysiological properties (conduction velocity and refractory period) as evidenced by the intracardiac electrogram and it is unlikely to be a fast pathway as the HA interval is long. The fourth beat in the figure 2C has slightly longer cycle length (422ms) due to concealed conduction of previous impulse into S1. These findings emphasize the fact that both atrium and the ventricle are not the part of circuit in AVNRT. Electroanatomic mapping (ENSITE Velocity, Abbott, Plymouth, MN) confirmed earliest atrial activation at coronary sinus ostium (fig 2B) Slow pathway was ablated (medium curve catheter; 60W, 60⁰C) which rendered the tachycardia non-inducible. Since there were three slow pathways involved in this patient, this rare variety could be labelled as atypical slow-slow-slow AVNRT. The mechanism of re-entry in typical as well as atypical AVNRT remains elusive2. There has been electrophysiologic evidence of multiple superior atrial inputs to the AV node3that could explain multiple sites of early atrial activation during tachycardia. Cycle length alternans can occur during AVNRT due to either antegrade conduction via two slow pathways or junctional bigeminism4. The decremental conduction properties of antegrade slow pathway (S1) and upper common pathway were the reasons for variation in A-A, H-H, H-A and V-V intervals in our case of atypical slow-slow-slow AVNRT.References
  1. Maury P, Raczka F, Piot C, Davy JM. QRS and cycle length alternans during paroxysmal supraventricular tachycardia: What is the mechanism? J Cardiovasc Electrophysiol 2002;13:92-3
  2. Katritsis DG, Camm AJ. Atriovenricular nodal reentrant tachycardia. Circulation 2010;122:831-40.
  3. Wu J, Wu J, Olgin J, Miller JM, Zipes DP. Mechanisms underlying the reentrant circuit of atrioventricular nodal re-entrant tachycardia in isolated canine atrioventricular nodal preparation using optical mapping. Circ Res 2001;88:1189-95.
  4. Surawicz B, Fisch C. Cardiac Alternans: Diverse mechanisms and clinical manifestations. J Am Coll Cardiol 1992;20:483-499