EFFERENT VAGAL INNERVATION OF THE CANINE ATRIA AND SINUS AND ATRIOVENTRICULAR NODES - THE 3RD FAT PAD

Background The purpose of this study was to investigate the functional pathways of efferent vagal innervation to the atrial myocardium and s inus and atrioventricular (AV) nodes. Methods and Results Using vagall y induced atrial effective refractory period shortening, slowing of sp ontaneous sinus rate, and prolongation of AV nodal conduction time as end points of vagal effects, we determined the actions of phenol and e picardial radiofrequency catheter ablation (RFCA) applied to different sites at or near the atrial myocardium to inhibit these responses. We found that efferent vagal fibers to the atria are located both subepi cardially and intramurally or subendocardially. Most efferent vagal fi bers to the atria appear to travel through a newly described fat pad l ocated between the medial superior vena cava and aortic root (SVC-Ao f at pad), superior to the right pulmonary artery, and then project onto two previously noted fat pads at the inferior vena cava-left atrial j unction (IVC-LA fat pad) and the right pulmonary vein-atrial junction (RPV fat pad) and to both atria. A few vagal fibers may bypass the SVC -Ao fat pad and go directly to the IVC-LA or RPV fat pad and then inne rvate the atrial myocardium. Vagal fibers to the sinus and AV nodes al so converge at the SVC-Ao fat pad (a few fibers to the sinus node go d irectly to the RPV fat pad) before projecting to the RPV and IVC-LA fa t pads. Long-term vagal denervation of the atria and sinus and AV node s can be produced by RFCA of these fat pads and results in vagal dener vation supersensitivity. Vagal denervation prevents induction of atria l fibrillation in this model. Conclusions The newly described SVC-Ao f at pad receives most of the vagal fibers to the atria and sinus and AV nodes. Elimination of the fat pads with RFCA selectively vagally dene rvated the atria and sinus and AV nodes.