Efferent Vagal Innervation of the Canine Atria and Sinus and Atrioventricular Nodes: The Third Fat Pad

Abstract

Background The purpose of this study was to investigate the functional pathways of efferent vagal innervation to the atrial myocardium and sinus and atrioventricular (AV) nodes. Methods and Results Using vagally induced atrial effective refractory period shortening, slowing of spontaneous sinus rate, and prolongation of AV nodal conduction time as end points of vagal effects, we determined the actions of phenol and epicardial 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 subepicardially and intramurally or subendocardially. Most efferent vagal fibers to the atria appear to travel through a newly described fat pad located between the medial superior vena cava and aortic root (SVC-Ao fat pad), superior to the right pulmonary artery, and then project onto two previously noted fat pads at the inferior vena cava–left atrial junction (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 innervate the atrial myocardium. Vagal fibers to the sinus and AV nodes also converge at the SVC-Ao fat pad (a few fibers to the sinus node go directly to the RPV fat pad) before projecting to the RPV and IVC-LA fat pads. Long-term vagal denervation of the atria and sinus and AV nodes can be produced by RFCA of these fat pads and results in vagal denervation supersensitivity. Vagal denervation prevents induction of atrial fibrillation in this model. Conclusions The newly described SVC-Ao fat pad receives most of the vagal fibers to the atria and sinus and AV nodes. Elimination of the fat pads with RFCA selectively vagally denervated the atria and sinus and AV nodes.