976-10 Fast-slow (Uncommon) Form of Atrioventricular Nodal Reentrant Tachycardia (AVNRT): New Classification Based on the Site of the Retrograde Slow AV Nodal Pathway

During fast-slow (uncommon) form of AVNRT. anterograde conduction occurs over the fast pathway (located at the anterior septum) and retrograde conduction occurs over the slow pathway (located at the posterior septum). Thus, in this variety of AVNRT retrograde atrial activation occurs first at or near the orifice of the coronary sinus (CSOS), then at the low septal right atrium (LSRA) in the His bundle recording site. However, retrograde slow pathway may possibly be located at the anterior septum; in such a case. activation of the LSRA would be earlier than that of the CSOS. To investigate the retrograde atrial activation, 12 pts with fast-slow AVNRT (cycle length; 305535 ms)were studied while recording the electrograms of the CSOS and the LSRA.ln addition, to investigate the location of the anterograde fast pathway, programmed atrial stimulation was done from the CSOS (or proximal CS) and the anterior high right atrium (HRA) during AVNRT. In all pts reentry using concealed accessory pathways with long conduction times was excluded.ResultsIn 6/12 pts CSOS activation during AVNRT preceded the LSRA activation by≥10 ms (posterior type). in 3/12 pts LSRA activation preceded the CSOS by≥0 ms (anterior type); in the remaining 3 pts (middle type) those two sites were activated simultaneously (time difference≥5 ms). In pts with posterior and middle type AVNRT. HRA stimulation during tachycardia preexcited the His bundle and the ventricle via the fast pathway without capturing the CSOS (i.e., inter-atrial dissociation) but the dissociation was not elucidated with CSOS stimulation. indicating that the anterogradely conducting fast pathway is anterior to the retrograde slow pathway. In pts with anterior AVNRT, CSOS stimulation (but not HRA stimulation) demonstrated the inter-atrial dissociation. establishing that the anterograde fast pathway is posterior to the retrograde slow pathway. In 6/6 pts with posterior and 3/3 pts with middle type AVNRT, P waves in inferior leads were deeply negative, but in 2/3 pts with anterior type AVNRT, P waves were biphasic or shallow.ConclusionsFast-slow AVNRT could be divided into 3 types based on the slow pathway location (posterior, middle Or anterior). In anterior type fastslow AVNRT, radiofrequency ablation using a posterior approach may fail to eliminate the slow pathway