Inferior vena cava-tricuspid annulus isthmus is a critical site of unidirectorial block during the induction of common atrial flutter

Background: Until recently no clinical studies had reported precise right a trium (RA) mapping when performing induction of atrial flutter (AF1). We st udied the mode of tachycardia initiation in 16 patients (pts) referred for radiofrequency (RF) AF1 ablation. AF1 induction was performed at the beginn ing of the procedure (n = 10), or after previous AF1 termination during RF delivery (n = 6). Detailed analysis of AF1 initiation was provided by duode -capolar (Halo) and multipolar catheters positioned in the peritricuspidian region at the lateral right atrial wall (LRA), the inferior vena cavatricu spid annulus (TVC-TA) isthmus and the interatrial septum. Induction was obt ained during incremental pacing (IAP) (15 pts) or programmed stimulation (1 pt) from the proximal coronary sinus (PCS). Results: Atrial flutter with counterclockwise (CCW) RA rotation was induced in all pts by PCS pacing. During PCS IAP, at long pacing cycle lengths, im pulse propagated in a clockwise (CW) direction through the IVC-TA isthmus a nd then upward at low (L) LRA. This led to a collision at the mid LRA with another wave front propagating in a CCW direction at the septum. LAP from P CS induced a progressive delay of propagation at the TVC-TA isthmus resulti ng in a prolongation of the PCS-Mid Isthmus interval from 85 +/- 29 to 151 +/- 42 msec. At same pacing cycle lengths (CL), the PCS-HLRA interval was c omparatively less prolonged, from 75 +/- 12 to 105 +/- 18 msec, p = 0.0007. This preferential slowing of conduction between PCS and mid isthmus, durin g IAP from PCS, was associated with a displacement of the zone of collision to the Low LRA. Finally a CW functional block occurred at the TVC-TA isthm us and CCW AF1 was induced through a period of transient concealed entrainm ent. The paced CL required to initiate flutter ranged from 290 to 180 msec and the mean CL of induced atrial flutter was 254 +/- 27 msec. Conclusions: The IVC-TA isthmus has decremental properties and exhibits wen ckebach phenomenon during incremental PCS pacing. Initiation of a countercl ockwise flutter by PCS pacing is associated with appearance of a functional unidirectional block at the IVC-TA isthmus.