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

BACKGROUND: Until recently no clinical studies had reported precise right atrium (RA) mapping when performing induction of atrial flutter (AFl). We studied the mode of tachycardia initiation in 16 patients (pts) referred for radiofrequency (RF) AFl ablation. AFl induction was performed at the beginning of the procedure (n = 10), or after previous AFl termination during RF delivery (n = 6). Detailed analysis of AFl initiation was provided by duodecapolar (Halo) and multipolar catheters positioned in the peritricuspidian region at the lateral right atrial wall (LRA), the inferior vena cavatricuspid annulus (IVC-TA) isthmus and the interatrial septum. Induction was obtained 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, impulse propagated in a clockwise (CW) direction through the IVC-TA isthmus and 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. IAP from PCS induced a progressive delay of propagation at the IVC-TA isthmus resulting 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 comparatively less prolonged, from 75 +/- 12 to 105 +/- 18 msec, p = 0.0007. This preferential slowing of conduction between PCS and mid isthmus, during 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 IVC-TA isthmus and CCW AFl was induced through a period of transient concealed entrainment. 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 wenckebach phenomenon during incremental PCS pacing. Initiation of a counterclockwise flutter by PCS pacing is associated with appearance of a functional unidirectional block at the IVC-TA isthmus.