Anatomical characteristics of the phrenic nerve and the spontaneous conduction block between the right atrium and the superior vena cava

Abstract Introduction Although the superior vena cava isolation (SVCI) with minimal radiofrequency application using a spontaneous right atrium – superior vena cava (RA-SVC) conduction block has been reported, the anatomical characteristics of the spontaneous RA-SVC conduction block is not clear. Purpose The aim of this study to evaluate the anatomical relationship between the spontaneous RA-SVC conduction block and the phrenic nerve (PN). Methods This study consisted of 73 patients who attempted to undergo SVCI using a RA-SVC conduction block. The RA-SVC conduction block was visualized using the extended early meets late tool with the CARTO3 system. We set the first lower threshold at 1800/total local activation time to optimize the visualization of spontaneous RA-SVC conduction block. If the SVC was not isolated by ablation to the gap of the visualized conduction block line, the lower threshold was increased by 5%, and the new gap was ablated each time. The course of the right PN was detected by pacing with 20 mA /2 ms. The RA-SVC conduction block and the course of the PN were measured with reference to the height of the left atrial roof. The extend of the RA-SVC conduction block and PN were measured in angles with respect to the ventral side (Figure 1). These anatomical parameters were compared between the RA-SVC block line and the PN. Results Sixty-one patients (83.6%) had spontaneous RA-SVC conduction block, and the remaining 12 patients (16.4%) did not. Anatomical characteristics were evaluated in 61 patients with spontaneous RA-SVC conduction block. Compared with the course of the PN, RA-SVC conduction block extended more posteriorly (110 degrees vs 128 degrees, P = 0.013) and more caudally (9.5 mm vs 16.1 mm, P<0.0001) (Figure 2). In the patients who underwent SVCI using a RA-SVC conduction block, any patients did not undergo radiofrequency application on the course of the PN. Conclusion Since the caudal end of the RA-SVC conduction block is posterior to the caudal end of the phrenic nerve, SVCI using the RA-SVC conduction block may be able to avoid ablation near the PN.Figure1Figure2