Rate-dependent block in the sinus venosa of the swine heart during transverse right atrial activation: Correlation between electrophysiologic and anatomic findings

Introduction: Whether the crista terminalis or the sinus venosa result in rate-dependent block during transverse activation of the right atrial activation remains unknown. In the present study, right atrial activation at different cycle lengths was studied in the swine heart using high-resolution noncontact mapping (Endocardial Solutions). The location of the block was tagged and correlated with postmortem anatomical findings. Methods and Results: Eight pigs were studied using noncontact mapping to obtain right atrial geometry and detailed sequence of activation using noncontact endocardial mapping. During sinus rhythm, activation proceeded uninterrupted craniocaudally along the sinus venosa and crista terminalis with similar conduction velocities (1.08 ± 0.17 and 1.17 ± 0.14 m/sec, respectively). Proximal coronary sinus stimulation was used to create transverse activation of the posterior right atrial wall. A rate-dependent decrease in conduction velocity occurred in the sinus venosa region (0.93 ± 0.21, 0.82 ± 0.14, and 0.52 ± 0.09 m/sec at 500, 400, and 300 ms, respectively; P < 0.05). The line of block verified by isopotential mapping and double potentials was obtained at cycle lengths of 240 ± 30 ms. This line of the block was tagged with radiofrequency current lesions. Postmortem, all lesions were located in the sinus venosa region, 9.8 ± 4.1 mm from the posteromedial edge of the crista terminalis. This region showed abrupt changes in muscle fiber thickness and orientation as well as in collagen content. Conclusions: The sinus venosa and not the crista terminalis results in a rate-dependent line of block during transverse right atrial activation. The morphologic characteristics of the sinus venosa appear to facilitate block in this region.